WO1984003570A1

WO1984003570A1 - Method for obtaining and observing color and relief images

Info

Publication number: WO1984003570A1
Application number: PCT/FR1984/000050
Authority: WO
Inventors: Emmanuel Jean Raymond Malifaud
Original assignee: Emmanuel Jean Raymond Malifaud
Priority date: 1983-03-03
Filing date: 1984-03-02
Publication date: 1984-09-13
Also published as: FR2542107A1; EP0135573A1

Abstract

The process enabling to obtain both color and relief images comprises the use of two preexisting images representing two points of view of a subject and the selection of one of the trichromatic components of one image and the other two trichromatic components of the other image, and the combination of those three components on a same medium to form a unique composite image. Thereafter said unique composite image is observed with ocular filters so as to benefit both by the depth effect due to the binocular vision and by the colors. The defects, particularly the so-called "ghosts", are eliminated by acting in different ways on the colors of the lights projected when drawing the preexisting images to compensate for the defects of sensitive surfaces and/or of printing inks. The invention also provides to act on the black when four-color printing is used to print the composite image.

Description

METHOD FOR OBTAINING AND OBSERVING COLOR AND RELIEF IMAGES

The sensation of the relief comes from the fact that the left eye and the right eye of an observer see the objects from two distinct points. To recreate this sensation of relief, it is therefore necessary to present respectively to the left eye and the right eye of the observer, two images representing the objects as they are seen from two distinct points.

The distance between the two human eyes being, on average from 60 mm to 65 mm, the imaginary segment which connects the two images should normally be horizontal, perpendicular to the axis of shooting and of length equal to 60 mm to 65 mm approximately, but these conditions can be changed to produce special effects.

With the exception of the hologram process, the principle of which is entirely different, all existing processes use two images produced separately, for example by two cameras placed side by side so that the centers of their objects are placed at two separate points .

To allow the separate observation by the left eye and by the right eye of the two images, several methods and devices exist: a) the stereoscope, in which two slide images, in color or monochrome, are observed separately through two eyepieces adapted respectively to the left eye and the right eye of the observer. b) The projection on screen of relief images, slide photos or cinema films, for which two projectors can be used which form the two images on the same screen through two crossed polarizers, one on the left and the other on right. The observer looks at the screen through two polarizers, one placed in front of the left eye and the other in front of the right eye, oriented in such a way that each eye receives only the image intended for it. This process gives good results in color or monochrome, but can only be applied if there is projection, that is to say emission of light and requires special equipment (dual projectors, metal screen). c) Anaglyphs, the principle of which allows the printing of monochrome relief images (in black and white). For this, two printing colors are used, for example blue and red.

If you print on the same support the monochrome image "left" in red and white (red in the dark parts and white in the light parts) and the monochrome image "right" in blue and white, you get an image composite by the superposition of the two left and right images. If we observe this composite image through a red filter, only the right image printed in blue appears, because the red left image merges with the background. Through a blue filter, only the left image appears.

The present invention provides a means by which color anaglyphs can be obtained.

The process in accordance with the invention can be applied both to photographic reproduction and to printing, but in both cases the final document obtained may have relatively large defects which hinder the correct and comfortable observation of this document. .

In fact, for example in photography, the reproduction processes use three-color printing, that is to say that they use sensitive surfaces which respond separately to the primary red, green and blue lights. However, their sensitivity curves to these three lights generally overlap partially, which means that for certain colors confusion can occur. When it is about images in color but not in relief, this confusion is practically not important whereas it becomes primordial when it is about documents in relief because, then, it can be that a light that should only impress one of the layers impresses two. This has the effect of showing two colors which will each be seen by one eye where a single color intended for a single eye should have appeared, hence a lessening of the relief effect, eyestrain and a defect which gives the impression of a duplication of the image. The annoying overlap of the sensitivity curves occurs in the region of the neighboring spectrum of 590 nanometers, that is to say that the sensitive surface intended for reproduction reacts as if it were both red light and green light. If these wavelengths are not eliminated during the cyan and red filtering of the left and right images, the image on the left will be weakly reproduced as the image on the right and vice versa, which will ultimately show a duplication of certain parts of the image. image, annoying duplication that is called a "ghost". In addition, the dyes used for the graphic reproduction of images, especially in printing, are not perfect and they exhibit, in practice, completely undesirable color absorptions which are detrimental to the correct execution of color and relief. In particular, the cyan dye absorbs green light and even a little blue light instead of absorbing only red light. These defects are usually corrected using masks, the principle of which is to subtract magenta dye at the places where cyan dye appears in order to correct the absorption of green light and to subtract yellow dye at the same places to correct the light absorption. blue.

In practice, this method gives satisfactory results in the parts of the images which are sufficiently dense in magenta and in yellow so that there can be effectively subtraction. However, this method does not work in the parts of the images which must be printed exclusively in cyan since these parts do not have enough magenta dye and yellow dye so that we can subtract enough using the masks described above. This is, in particular, what happens in the parts of the composite image which are highly offset (very marked areas of relief) and which, in addition, have a significant dark on light contrast since they appear in almost cyan pure.

The present invention remedies all of these defects and makes it possible to obtain images in color and in relief, both photographic and printed, of excellent quality and practically devoid of ghosts and other imperfections. To this end, the subject of the invention is a method for obtaining and then observing color and stereoscopic relief images from a first and a second pre-existing image representing points of view obtained from two points. distinct, characterized in that one selects one of the three trichromatic components of the first pre-existing image and the two other trichromatic components of the second pre-existing image, that one combines these three trichromatic components and that they produced on the same support using either photographic or printing dyes to obtain a single composite image in color, then observing said composite image in binocular vision through two filters, the first of which is placed in front of that of the two eyes of the observer to which the point of view of the first pre-existing image is intended for a correct stereoscopic vision and essentially transmits the composition trichromatic health selected on the first pre-existing image while the second filter is placed in front of the other of the two eyes of the observer and essentially transmits the two trichromatic components selected on the second pre-existing image. According to other characteristics of the invention:

- after having selected the trichromatic components of the two pre-existing images, to obtain the single composite image in color, the brightness and / or the contrast of at least one of the three trichromatic components are modified while, for the observation of said single composite image, the transparency and the chromatic balance of the filters are adapted to the characteristics of the three trichromatic components, after any modification;

- from a light source, the two pre-existing images are successively projected onto the same sensitive photographic surface, the first through a filter transmitting only two of the three trichromatic components of said light and the second through a filter transmitting only the third of the three trichromatic components of said light, this sensitive surface having to undergo physicochemical treatments known per se to produce negatives, slides, cinema films, prints on paper and other photographic products representing the single color composite image; - the sensitive photographic surface having essentially three layers of products reacting respectively to blue, green and red lights, one chooses as filters to project the pre-existing images on said surface, on the one hand a filter transmitting two of these three lights and whose value of the transmittance is substantially zero for all the wavelengths to which the sensitive layer must react to the third of these lights and, on the other hand, a filter transmitting the third of these lights and whose value of the transmittance is substantially zero for all the wavelengths to which the two layers sensitive to the two other lights must react respectively;

- the sensitive surface being of the type used to obtain a positive from a positive, the first pre-existing image is successively projected on said sensitive surface through a cyan filter and the second pre-existing image through a red filter in filtering, moreover, the light for these two projections using two superimposed filters respectively magenta and yellow, both of low density;

- the sensitive surface being of the type used to obtain a positive from a negative, we also practice insolation of the sensitive surface by means of green and blue lights;

the sensitive surface being of the type used to obtain a positive from a positive, a mask consisting of a low density negative of the other pre-existing image is superimposed on at least one of the positive pre-existing images;

- the sensitive surface being of the type used to obtain a positive from a negative, a negative of the first of the two pre-existing images is successively projected on the sensitive surface through a cyan filter, a negative of the second pre image -existing through a red filter and a positive of at least one of these images through the filter previously used to project the negative of the other; - from the two pre-existing images, a photographic document is produced representing the single composite image in color, an individual photograph is taken for each of the three trichromatic components and for black in order to constitute a four-color selection based on this single document and then the four shots are used to print by any known method the single composite image in four colors;

- a low density magenta dye and a lower density yellow dye than the previous one are added to the document which bears the single composite image, either by uniformly covering the document with two successive layers of these dyes, or by covering it with a pink filter, either by immersing the document in a bath of mixed magenta and yellow dyes to present a uniformly pink color; - a snapshot corresponding to the black color is produced from a film or a frame free of at least some of its sparse areas in the parts of the composite image where the two pre-existing images do not coincide.

- we select a trichoromatic component of the first pre-existing image and the other two trichromatic components of the second pre-existing image, we take an individual snapshot for each of these three components to form a tri-chrome selection from the two images pre-existing and these three shots are then used to print by any known method the single composite image in three colors;

- the densities of the photos of the selection corresponding to magenta and yellow are increased at each point in proportion to the density of a negative of the photo of the selection corresponding to cyan and proportional to the respective percentages of green and blue lights absorbed by the cyan printing ink to be used for printing; - the shots of the selection being constituted by transparent films, on the one hand. superimpose a negative of low density of the film corresponding to the color cyan on the positive film corresponding to the color magenta, then one makes a copy in positive of the whole of these two superimposed films and, on the other hand, one superimposes a negative of very low density of the film corresponding to the color cyan to the positive film corresponding to the color yellow, then one makes a positive copy of all of these two superimposed films and, finally, one uses these two copies to the places magenta and yellow films to form the selection from which to print.

The invention will be better understood from the detailed description below made with reference to the accompanying drawing. Of course, the description and the drawing are given only by way of an indicative and nonlimiting example.

The single figure of the drawing illustrates an example of sensitivity curves of a sensitive surface and the transmittance value of filters used in accordance with the present invention to obtain the single composite image from the pre-existing images. According to the theory of three-color, the color of any light, pure or compound, can be reproduced by mixing in suitable proportions three lights called "primary": blue, green and red. It is the additive color synthesis.

A given light therefore has, in the sense of the three-color theory, three components, of generally unequal intensities, which are the three blue, green and red lights whose mixture reproduces the light considered.

Likewise, a colored image can always be considered as the additive superposition of the lights of three component images, blue, green and red, called here "trichromatic components of the image".

According to the invention, one selects, from a spectrum sharing, one of the three trichromatic components of the pre-existing image on the left and the two other trichromatic components of the pre-existing image of right and l '' we combine these three trichromatic components on a led support to achieve a unique composite image in color. Thus, in this single composite image, one of the trichromatic components represents only one of the two points of view, while the other two trichromatic components represent only the other point of view. By now observing this unique composite image with two different color filters in front of the eyes and corresponding to the sharing of the spectrum retained, we have both a binocular vision restoring the sensation of relief and the perception of colors.

According to the present invention, one therefore starts from two pre-existing images (for example two slides) in color forming a stereoscopic pair (image on the left and image on the right).

It should be noted that the two preliminary images playing symmetrical roles, one can indifferently name one of the two preliminary images "left" and the other "right" or vice versa. In what follows, we could therefore without inconvenience replace "left" by "right" and "right" by "left".

From the point of view of colors, three partitions of the spectrum are theoretically possible:

- 1st sharing possible: blue and green components on the left and red component on the right;

- 2nd sharing possible: blue and red components on the left and green component on the right;

- 3rd possible sharing: green and red components on the left and blue component on the right. In the following description, we adopted the first partition with which the trichromatic components of the composite image are the blue and green components of the left image and the red component of the right image.

But the other shares are also valid and in what follows we can replace "red" by "green", "cyan" by "magenta" and vice versa, or even replace "red" by "blue", "cyan" by "yellow" and vice versa. In order for the rendering of the relief to be entirely good, it is important that the left and right images (received by the left eye and by the right eye respectively) are not too different in terms of brightness and contrast . These characteristics of the two preliminary images depend on the manner in which the composite image is produced and on the characteristics of the filters through which it is observed. A possible imbalance can come from a chromatic imbalance of the composite image between its red component on the one hand and its green and blue components on the other hand or from too great a difference in transparency between the red and cyan filters through which this composite image is observed.

To correct such an imbalance, the chromatic balance of the composite image can be modified by weakening some of its components, either when taking photos, or during subsequent operations, by playing on the diaphragm apertures of the or objectives used or on exposure times or by the use of neutral or colored filters.

We can also play on the transparencies of the filters used to observe the composite image. if, for example, the red image seen by the right eye is too little contrasted because it is too bright, we can weaken the red component of this right image for example by using a neutral gray filter when shooting. We can then possibly restore the color balance by using a red filter more transparent than the cyan filter to observe the composite image.

In some cases, these changes in brightness and contrast of the different components of the composite image can lead to an unfavorable change in color rendering. We can thus be led to adopt a compromise making it possible to render both the colors and the relief in an acceptable manner.

The three trichromatic components of the composite image are therefore, as said above, the blue and green components of the left image and the red component of the right image, but the brightness and the contrast of these components do not According to the invention, these are not necessarily those which naturally exist in the subject reproduced by the composite image in relief. We start from two separate pre-existing color images, produced simultaneously from two distinct points by two cameras or successively by the same camera that we move from one point to another, as is known to make images in relief.

Left and right pre-existing images can be obtained by optical means, or be produced artificially, for example by drawing and coloring them by known means, or even by making them using a computer. In this case, the left and right images can be images of a real subject, reproduced realistically or not, as well as images of a totally or partially imaginary subject.

When optical means are used to obtain the left and right images, these images represent two points of view of the subject insofar as the shooting is carried out from two distinct points. When artificially fabricating these images, it is necessary to reconstruct the two distinct points of view by using laws of perspective and possibly calculating the lighting according to these two points of view. It is not necessary to produce the left and right images in full colors and we can limit ourselves to producing the green and blue components of the left image and the red component of the right image.

Since we only use the blue and green components of the left image and red of the right image, we must make a selection of the trichromatic components, for example by filtering the left image with a cyan filter which eliminates the red component and provides the blue and green components and by filtering the image on the right with a red filter which eliminates the blue and green components and provides the red component, either at the shooting, or later. In what follows, it is assumed that the left and right images are produced in three-color, the filtration intervening after they have been obtained, but the results are the same if we filter when shooting. For the projection of slide photographs or cinema films in relief, it is of course possible to project on the same screen the pre-existing left and right images through filters respectively cyan and red, using two projectors, but it is easier to make a slide or a film reproducing the composite image, which then makes it possible to use a single projector. For this, we copy the left image filtered in cyan and the right image filtered in red, superimposed on the same film. To make photographs in color and in relief on paper or other similar support, the composite image can be produced from the color negatives of the pre-existing left and right images or from the positives if a process is used which draws the positives . The left image filtered in cyan and the right image filtered in red are successively projected on sensitive paper, then processing is continued normally.

To print images in color and in relief, it is possible to start from a composite image produced, for example, on a color slide. Printing can be done in three or four colors. we can also start from two separate color images left and right. A three-color selection is made using the blue and green components of the left image and the red component of the right image to produce the yellow, magenta and cyan images respectively. We copy the left image in black and white through a blue filter and we print in yellow the image obtained; then we do the same for the green component of the left image which we print in magenta; then we do the same for the red component of the right image which we print in cyan. The composite image is thus reproduced by subtractive synthesis. For example, for offset printing, the three selection shots are three screened types. As is known in the printing industry, corrective masks must be used to obtain good color rendering. Masks are necessary due to unwanted absorption of printing inks (especially cyan ink). These unwanted absorptions may have the consequence that certain elements of the right image are seen (weakly) by the left eye and that certain elements of the left image are seen (weakly also) by the right eye. To eliminate this drawback, additional masks may prove useful.

As we know, photographic products (film and paper) basically have three layers sensitive to blue, green and red light respectively. The sensitivity curves of these layers as a function of the wavelength of light are slightly different depending on the type of emulsion and it is advantageous to choose for the production of the single composite image, an emulsion whose sensitivity curve of the layer sensitive to red light covers a wavelength range entirely separate from that covered by the curve of the layer sensitive to green light because, then, such an emulsion cannot reproduce for the left image of colors that should only be present in the image on the right and vice versa.

But, as mentioned above, such an emulsion practically does not exist. According to the invention, a correlation is established between the sensitivity of the three layers of the sensitive surface and the transmittance of the filters used to select the colors in order to produce the composite image.

Thus, referring to the drawing, we see the theoretical sensitivity curves 1 for the blue color, 2 for the green color and 3 for the red color of a given sensitive surface.

It can be seen that curve 2 corresponding to the sensitivity to the green color and curve 3 corresponding to the sensitivity to the red color overlap at around 590 nanometers and cancel each other around 600 nm for the curve 2 for sensitivity to green and around 580 nm for curve 3 of sensitivity to red. It can be seen that curve 2 corresponding to the sensitivity to the green color and curve 3 corresponding to the sensitivity to the red color overlap at around 590 nanometers and cancel each other around 600 nm for the curve 2 for sensitivity to green and around 580 nm for curve 3 of sensitivity to red.

To produce the composite image on a sensitive surface having these sensitivity characteristics, cyan filters are used respectively for the left image and red for the right image.

Here, we choose a cyan filter having the transmittance represented by curve I and a red filter having the transmittance represented by curve II.

It can be seen that curve I has a practically zero transmittance at 580 nm, that is to say at the point corresponding to the shortest of the wavelengths of the region of the spectrum covered by curve 3 while curve II s' cancels around 600 nm, that is to say at the point corresponding to the longest wavelength in the region of the spectrum covered by curve 2.

Thus, all wavelengths between 580 and 600 nm are completely eliminated, which neutralizes the central zone in which color confusion can occur.

For the effects of this process to be the best possible, it is essential that the transmittance curves are as steep as possible and practice shows that this result can currently only be obtained with so-called "dichroic" interference filters.

When the images show very contrasting subjects, the differences between the two images appear in pure red or pure cyan. The imperfection of the dyes, in particular cyan, will make appear in these shifts "ghosts" which will be all the more annoying as these shifts are greater, that is to say that the relief is more marked.

To eliminate these ghosts, other dyes are brought in their vicinity to compensate for the absorption of light by these ghosts: magenta to compensate for the absorption of green light, yellow to compensate for the absorption of blue light, cyan to compensate for the absorption of red light. When these ghosts come from the imperfections of the cyan dye, they are eliminated by making magenta and yellow appear. When they are red, cyan appears.

According to a first means, a general magenta and yellow filtering is carried out during the drawing under the cyan filter, giving the magenta filter a density of for example 10% and the yellow filter a density of only 5% in the case where a process that draws a positive from a positive.

When we proceed by drawing a positive from a negative, we practice a sunstroke of the paper or film using green and blue lights. This can be achieved by using the cyan filter which is used for the draw and by establishing an additional yellow filtering of approximately 50%. This exposure brings a variable amount of light according to the cases and which can, for example, be 10% of the light brought then during the drawing.

You can also practice this sunstroke under a green filter then under a blue filter (for half the time).

These compensating dyes can also be provided by means of masks. Thus, to carry out the drawing with a process which draws a positive from a positive, we can superimpose on the left image which we project with the cyan filter, a mask made up of a low density negative, by example of 10%, from the right image.

Similarly, we can superimpose on the right image that we project with the red filter, a small negative of the image on the left. In the case where one uses a process which draws a positive from a negative, one obtains the same result by carrying out a third exposure, before or after having projected the left image filtered in cyan and the image of right filtered in red, and this while projecting for a short time a positive of the right image filtered in cyan. We can possibly plan a fourth exposure for a short time of a positive of the left image filtered in red.

We have just described different variants of the process according to the invention when it is applied to photographic products. When it comes to impressions on. a support, it was indicated in the main patent that this printing can be carried out either in four colors from a photographic document carrying the single composite image or in three colors from two photographic documents each carrying one of the two pre-existing images.

In both cases, ghosts may appear in the offset places, that is to say of accentuated relief where the contrast is high, due to the defects of printing inks and especially cyan ink. When we print in four colors from a single photographic document, we know that we use masks which are either calculated directly by a computer when we use a device called "scanner", or obtained by superimposing a positive of low density of cyan film to negative of magenta film for printing the magenta positive from the selection in conventional photogravure.

We do the same, but with a cyan positive of even lower density, for the yellow film.

But these masks are ineffective in areas of accentuated contrast because the cyan ghost contains little or no magenta and little or no yellow to subtract.

According to the invention, this defect can be remedied by proceeding in two stages: firstly adding the colors which should be removed and which do not exist and secondly removing them where necessary. for this, the composite document to be selected is uniformly covered with a low density magenta dye, for example 20% in the case where there are extreme contrasts, and with an even lower density yellow dye, by example of 5%.

This result can be obtained by immersing the composite document, whether it is a slide photograph or a print on paper, in a bath of pink ink, that is to say a mixture of magenta ink and yellow ink in small quantity, this pink ink fixing on the document. You can also cover the document with a transparent pink film before starting the selection. The density of this pink must be calculated according to the importance of the defect to be corrected. In practice, this density can reach 20% in the extreme case of a black on white contrast in the areas of large offset between the two images and which correspond to a very accentuated relief.

To make pink appear on the document, we can also use additional red filtering during the photographic production of the composite document. These operations reveal magenta and yellow everywhere which will then be subtracted in dense cyan places by photogravure masks or by the scanner.

With the process of four-color printing, black is used and it can be the source of ghosts because it can of course be eliminated neither for one eye nor for the other so that all the black dots are seen by both eyes. However, in practice, there is the presence of black in the offset parts of the images which should be practically in pure cyan or pure red. This can come, for example, from the fact that in these areas the scanner analyzes the "light" or "dark" character of the colors and that to make the effect of "dark" the scanner provides for the addition of weak black.

According to the invention, this defect is remedied by producing in the four-color selection a black film free of its weak parts in order to obtain a black which can be described as "skeletal" because it contains no weak black zone. density.

Better still, we can locally lower the screen of the black film in the offset places where impressions in cyan or practically pure red should appear.

Whether the printing is done using the four-color process or the three-color process (with which black is not used), the presence of ghosts can be remedied by modifying the colors very little by increasing the density of the magenta print of the selection in proportion to the density of the negative of the cyan plate and in proportion to the percentage of green light absorbed by the cyan printing ink which will be used. Similarly, the density of the yellow photograph of the selection is increased in proportion also to the density of the negative of the cyan photograph and in proportion to the percentage of blue light absorbed by this cyan ink. This can be achieved by using a special additional mask which is quite different from the usual photogravure masks.

In the case where the ghosts are cyan, this mask consists in superimposing a low density negative of the cyan film of the selection on the positive magenta film, in copying in positive all of these two superimposed films and replacing in the selection the film positive magenta by this positive copy. The same is then done for the yellow film by superimposing a very weak negative of the cyan film on the positive of this yellow film.

In the case where the ghosts are red, this mask consists in superimposing negatives of low density of the magenta and yellow films on the positive of the cyan film, in copying in positive all of these three superimposed films and replacing in the selection the film positive cyan by this positive copy.

The densities of the weak cyan negatives to be superimposed on the magenta and yellow positives must be calculated according to the density of the cyan film in the defect, or ghost, the most important of the image and the density of the magenta and yellow films in the 'immediate entourage of this ghost and the percentages of green and blue lights absorbed by printing inks. If for example in the most important ghost the cyan film has a density of 70% and if in the immediate vicinity of this ghost the magenta film has a density of 8% and the cyan film has a density of 10% and if, moreover, the cyan ink considered has the failure to absorb 20% of the green light, the cyan ghost has a density of: 70% - 10% = 60% and will absorb 12% of the green light. A density of 12% of magenta will therefore have to be nearby, but as there is already 8%, only 12% - 8% = 4% should be added, this being obtained by superimposing a cyan negative at 4% on the positive magenta. The figures given above by way of example are purely indicative since the densities depend on the characteristics of the cyan dye and correspond, in fact, to the respective percentages of green and blue light which it absorbs. Instead of using masks, we can give the desired density point by point to each film in the selection. To calculate the density of the films at each of these points, depending on the color of the image at the point considered and the characteristics of the dyes, you can use the computer of a scanner that is programmed accordingly .

***

Claims

RE V END I CATI ONS

1- Method for obtaining then observing images in color and in stereoscopic relief from a first and a second pre-existing image representing points of view obtained from two distinct points, characterized in that one select one of the three trichromatic components of the first pre-existing image and the other two trichromatic components of the second pre-existing image, combine these three trichromatic components and make them on the same support using dyes either photographic or printing to obtain a single composite image in color, then we observe said composite image in binocular vision through two filters, the first of which is placed in front of that of the two eyes of the observer to whom the point is intended view of the first pre-existing image for correct stereoscopic vision and essentially transmits the selected trichromatic component on the first pre-existing image while the second filter is placed in front of the other of the two eyes of the observer and essentially transmits the two trichromatic components selected on the second pre-existing image. 2- Method according to claim 1, characterized in that after having selected the trichromatic components of the two pre-existing images, to obtain the single composite image in color, the brightness and / or the contrast of one are modified at least of the three trichromatic components while, for the observation of said single composite image, the transparency and the chromatic balance of the filters are adapted to the characteristics of the three trichromatic components, after possible modification. CO t \ ï ro -. o σt o ai vt D- -t. XJ * - *. CX -h had had XJ

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7- A method according to claim 3, characterized in that the sensitive surface being of the type used to obtain a positive from a positive, a mask consisting of a negative is superimposed on at least one of the positive pre-existing images low density of the other pre-existing image.

8- A method according to claim 3, characterized in that the sensitive surface being of the type used to obtain a positive from a negative, a negative of the first of the two pre-existing images is successively projected through a cyan filter, a negative of the second pre-existing image through a red filter and a positive of at least one of these images through the filter previously used to project the negative of the other.

9- A method according to claim 1, characterized in that from the two pre-existing images a photographic document is produced representing the single composite image in color, that an individual photograph is taken for each of the three trichromatic components and for black in order to constitute a four-color selection from this single document and that the four shots are then used to print by any known method the single composite image in four-color.

10- Method according to claim 9, characterized in that one adds to the document which carries the single composite image a magenta dye of low density and a yellow dye of density lower than the previous one, either by uniformly covering the document by two successive layers of these dyes, either by covering it with a pink filter, or by immersing the document in a bath of magenta and yellow dyes mixed to present a uniformly pink color.

11- A method according to claim 9, characterized in that a stereotype corresponding to the black color is produced from a film or a screen free of at least some of its sparse areas in the parts of the composite image where the two preexisting images do not coincide. 12- Method according to claim 1, characterized in that one selects a trichromatic component of the first pre-existing image and the other two trichromatic components of the second pre-existing image, that one draws an individual snapshot for each of these three components in order to constitute a tri-chrome selection resulting from the two pre-existing images and which one then uses the three stereotypes to print by all known methods the single composite image in three colors.

13- Method according to claims 9 and 12, characterized in that the densities of the shots of the selection corresponding to magenta and yellow are increased at each point in proportion to the density of a negative of the shot of the selection corresponding to the cyan and in proportion to the respective percentages of green and blue lights absorbed by the cyan printing ink to be used for printing.

14- A method according to claim 13, characterized in that the stereotypes of the selection being constituted by transparent films, on the one hand a negative of low density of the film corresponding to the color cyan is superimposed on the positive film corresponding to the color magenta , then a positive copy of all of these two superimposed films is produced and, on the other hand, a very low density negative of the film corresponding to the cyan color is superimposed on the positive film corresponding to the yellow color, then a positive copy of all of these two superimposed films is made and, finally, these two copies are used in place of the magenta and yellow films to constitute the selection from which one proceeds to printing.

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