WO2018055304A1

WO2018055304A1 - Digital method for broadcasting a film for projection in theatres

Info

Publication number: WO2018055304A1
Application number: PCT/FR2017/052552
Authority: WO
Inventors: Cédric LEJEUNE
Original assignee: Ymagis
Priority date: 2016-09-23
Filing date: 2017-09-22
Publication date: 2018-03-29
Also published as: FR3056872A1; FR3056870A1

Abstract

According to a method for image processing and projection, for broadcast in cinemas, a calibration step in which a process for rendering image sequences suitable for a predetermined projector model, an encapsulation step in which the image sequences are completed with metadata to form a digital copy, and a projection step in which the digital copy is projected by a projection system (4) onto a screen (44) are carried out. In the encapsulation step, a datum indicating that the calibration has been performed according to a process with high dynamics is integrated into the metadata, the projection system reading said data and selecting, as a result, a first light amplification factor for a standard process or a second light amplification factor for the process with high dynamics, the second factor being greater than the first factor.

Description

DIGITAL FILM BROADCASTING METHOD FOR PROJECTION

INDOORS

FIELD OF THE INVENTION

The invention relates to a digital image processing and projection method for broadcasting in cinemas. In particular, it relates to steps including image sequence calibration, encapsulation of said sequences with metadata, and film projection.

PRIOR ART

After having used the filmstrip for a long time since the shooting until the theatrical projection, the activity of the cinema has now moved entirely to digital format, which requires specific techniques for mastering the restitution of the images. in terms of color balance, contrast and brightness.

The projection systems convert a signal representing a so-called electrical level into a light intensity. In modern digital projectors, this is achieved for example by a system of piloted micro-mirrors that return a light source to the screen or elsewhere. The returned power is modulated by the amount of light returning to the screen.

The difference perceptible by the human eye between two levels of illumination is a function of the luminous intensity and does not evolve linearly according to this intensity. For example, at an illumination power of 100 cd / m ² , the perception of threshold is of the order 1%, lcd / m ² , while at a level of illumination of 0.01 cd / m ² , it is 10%, or 0.001 cd / m ² . Linear coding would require encoding at the lowest pitch, for example at 0.001 cd / m ² , which on a scale of up to 100 cd / m ^{2 would} require 10 ⁵ values, ie 17-bit coding. Also, it has been chosen to encode the digital signal representing the light power to be projected by applying an encoding conversion and to restore the light power at the level of the projector by an inverse function performing a decoding function. The most common encoding function is a power function with a l / γ factor, followed by a decoding function with a γ factor power function. This factor is greater than one in order to favor shades in low brightness values. We achieve a correct quality with 12 bits of encoding per color in digital cinema.

The preparation of the distribution of a film goes through a step of calibration, which consists of starting from the filmed sequences, to adjust the intensities of each of the three colors used for the projection in order to restore an image whose balance of colors and colors. intensity are in agreement with the artistic intention of the author.

After the calibration step, we obtain a master digital source, referred to as DSM for "Digital Source Master". All of the information needed for the projection is combined into an encapsulation step to provide a Digital Cinema Package (DCP) transport package based on a Material eXchange Format (MXF) file structure and an encryption system based on on private and public keys for projection servers. This package is provided to movie theaters where it is played by the projection system.

The Digital Cinema Initiative (DCI) recommendation, since standardized by the SMPTE, defines the characteristics of a DCI certified projector and the projection conditions. The projector must have a contrast ratio of 2000 to 1 (2000: 1) and the white point is defined indoors at a value of 48 candelas per square meter (cd / m ² ), ie 14 foot Lamberts (fL). The encoding curve is based on a power function of value 1 / 2.6 for encoding. The restitution follows a power curve of 2.6.

These projection conditions do not currently make it possible to restore images with a high dynamic, that is to say with a contrast greater than 2000 for 1.

Document US Pat. No. 9,288,499 B2 proposes a method for encoding and decoding the source master, which makes it possible to maintain high dynamics. However, it is necessary that the projection system is equipped with a decoding program adapted to this encoding.

OBJECTIVES OF THE INVENTION

The aim of the invention is to provide a digital method for broadcasting a film for theatrical projection in order to preserve a high image dynamic, without requiring any major modifications to the projection system defined by the current standards.

DESCRIPTION OF THE INVENTION

With these objectives in view, the subject of the invention is a digital film diffusion method according to which a calibration step is carried out in which an image sequence rendering processing adapted to a projector model is carried out. predetermined, an encapsulation step in which the image sequences are supplemented with metadata to form a digital copy and a projection step in which the digital copy is projected by a projection system on a screen, characterized in that, in the encapsulation step, the metadata includes a data indicating that the calibration has been performed according to a high dynamic process, the projection system reading said data and consequently selecting a first light amplification factor for a standard method or a second light amplification factor for the high dynamic process, the second factor being superior at the first factor. The method according to the invention operates on two stages of the diffusion of the film. In the calibration step, the color coding is done taking into account the level of brightness that will be used for the projection in order to respect the artistic intention of the director. Indeed the projection of a film made for a standard projection on a projection system with an amplification factor for high dynamics would give too bright colors and a too bright image. The specific calibration maintains a global luminance level comparable to the classic version but uses the extra brightness to emphasize the details and to make some parts of the image more saturated if necessary: fine details like the hair can be rendered more accurately , the effects of explosions or light rays can be more spectacular. This can be achieved by using the existing hardware, by modifying the projection system to implement the method and project with the required brightness level.

According to a preferred characteristic, the second factor is greater than 1.5 times the first factor, preferably twice the first factor.

According to a particular arrangement, the projection system is set so that a white point corresponding to the maximum luminous power is greater than or equal to 100 cd / m ² . Depending on the distance from the projector to the screen and its power level, it may be necessary to complete the projection system with one or more projectors to achieve the required light output, as already known per se in the field of digital projection.

Preferably, the film is projected into a room whose surfaces are treated in black. This limits the reflections to the screen, which allows to obtain deeper blacks, and thus increase the perceived contrast. One is thus able to restore images with a strong contrast.

Preferably, the film is projected with a projector whose performance allows inter-image contrast of at least 6000: 1. Despite the increase in the maximum luminance level, black depths are respected. The Inter-image contrast is defined by the luminance level between two successive images, between a completely white image and a completely black image, with a colorimeter pointing towards the center.

Better results are still obtained when the film is projected with a projector whose performance allows an inter-image contrast of at least 7000: 1.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the invention, reference is made to the description which follows and to the appended drawings, among which:

FIG. 1 is a diagrammatic view of a method of diffusion of a film according to the invention

FIG. 2 is a schematic view of a projector according to the invention;

Figure 3 is a test chart for the theoretical calculation justifying the effect of the invention.

DETAILED DESCRIPTION

In an example of use of the method according to the invention, the workflow complies with the ACES recommendations (Academy Color Encoding System). Figure 1 shows a diagram of this flow recommended by these recommendations and which is used in the invention.

According to this scheme, the data from the camera can undergo an Input Input Transform (IDT) 1 to convert the colors into a normalized color space and provide them to an ACES 2 space. This conversion takes into account the behavior of the camera as well as, for example, lighting, whether natural or artificial, and lighting technology. The ACES 2 space allows for post-production of the film, including editing and large varieties of treatments.

The result is then processed in a stream 3 which comprises a calibration step 31 which may comprise part of the overall processing of the film sequence by LMT (Look Modification Transform) filters and / or a part of a localized treatment of zones. image, all under the control of a colorist. The result is taken in a rendering processing which includes a first general purpose rendering stage 311 RRT (Rendered Transform Reference) in order to produce a source digital master and a second specific rendering step ODT (Output Device Transform). the device that will perform the projection. During colorist work, the specific rendering step is adapted to a monitor or projector used by the colorist to judge the result. After an encapsulation step, not shown, in which we receive the output of the specific rendering step adapted to the projector for which the film is intended, we obtain the digital copy that will be delivered to the projection room to be projected. During the encapsulation step. according to the invention, a data item indicating that the calibration has been performed according to a high dynamic process is integrated. The specific rendering steps, whether for the monitor or for theatrical projection according to the method of the invention, are adapted to the process.

As shown in FIG. 2, the projection system 4 has an input 40 for receiving the digital copy DCP, a read block 41 for decrypting the data, extracting the metadata and the data from the images. The device comprises a light source 42 and a modulation unit 43 such as a micro-mirror system for forming the image to be projected on the screen 44 from the light source 42. The image data are transmitted to the micro-mirror system 43. A selection block 45 receives the metadata and controls the intensity of the light source 42 in accordance therewith.

The projection system 4 receives the DCP digital copy on the input, deciphers the copy, reads the metadata, and sends them to the selection block 45. The selection block 45 determines whether the data indicating that the calibration was performed according to a high dynamic process is present. If this metadata is missing, the light intensity is adjusted to a standard level. In the case of presence of the high dynamic metadata, the projection system 4 applies an adjustment of the light source 42 so that the pfd of the brightest point is multiplied by a factor greater than 2 compared to the standard setting. Alternatively, the power of the light source 42 is not adjustable and the adjustment of the light power level is performed in the micro-mirror system 43.

The specific rendering step has taken into account the projection power parameter such that an image portion to be rendered at a predetermined luminance level has been encoded with a value divided by said factor relative to the standard encoding, to be restored by the projector at the desired luminance level.

Thanks to the method according to the invention, the colourist has a larger color scale, making it possible to restore films with a high dynamic, this with means requiring little adaptation of the existing material.

The maximum light power level is intended to be used only on small parts of the image such as fringes of objects illuminated against light, light sources etc. To theoretically assess the depth of black in an image, one has to take into account the light that comes from the screen and that is reflected by the room, as well as the diffusion on the optical surfaces of the projection system. Depending on the quality of treatment of the room, it is considered that the reflectivity is between 1 and 6%.

It is proposed to evaluate at least theoretically the intra-image contrast, that is to say between the brightest white and the deepest black. The brighter white is set by the projection system 4. In the case of a standard projection, the brighter white is set for example at 48 cd / m ² . In the case of a projection according to the method of the invention, this value is multiplied by at least 1.5. We take for the calculation a value of 103 cd / m ² .

To evaluate the depth of the black, we consider firstly the inter-image contrast of the projection system 4, for example with a value of 6000 between the intensity of the brightest white and the deepest black and the intensity bright reflected by the room. This reflected light intensity is evaluated by considering the projection of a checkerboard-shaped pattern of black squares 50 and white 51 as shown in FIG. 3. The white of the white squares 51 is of a luminous intensity of a white "average" less than the maximum intensity, with two of the four edges having the maximum white intensity said specular white, with a surface portion of 1% relative to the average white. These projection conditions are compared with a standard projection in which the inter-image contrast is 2000, and the pattern is in the form of a checkerboard of black and white squares, without specular white,

When the average white is half of the specular white in light intensity, it is found that the intensity of the black is only 5% higher than the black intensity obtained under standard conditions, whatever the reflectivity of the room between 1 and 6%.

When the average white is one quarter of the specular white in light intensity, it is found that the intensity of the black is divided by 1.8 compared to the intensity of the black obtained in standard conditions, whatever the reflectivity of the room included between 1 and 6%, which represents a very important gain in depth of black.

A blind experiment was conducted with a sample of people in a cinema in Lille.

The recruitment of people was done exclusively via social networks. A call for participation message was sent to various communities who relayed to their members (medical students, audiovisual students, media and / or the arts, professionals from the Image Pole of the Hauts-de-France region, various non-profit associations profit; customer file loyal and movie subscribers; . . .). The degrees of expertise in image knowledge and technical analysis are very varied. In the same way, the ages vary from 14 years to 72 years with an average of 35.78 years. The presence of the participants being based on the voluntary basis, it was impossible to control the distribution of men and women. Despite this, we arrive at an almost balanced distribution with 36 men and 32 women in total. The 14-35 age group is overrepresented. Nevertheless, this state of affairs does not constitute a bias for the study, because it is also the age group that frequents cinema the most. We can therefore legitimately think that our sample is close to the distribution proper to moviegoers.

The same introductory form was sent to each participant as soon as he arrived before he filled out the informed consent form and the pre-vision questionnaire about his habits in terms of cinema, television and the media in general. People were not informed about the points that differed from one sequence to another. Then in small groups of 1 to 4, participants were led to watch.

Room extinct, in the conditions of a projection in session, the spectators saw:

tf a count;

> a sequence in version A; t> 1 minute of black;

> a second count;

> the same sequence in version B.

Everything was identical between version A and version B, except that one was in PS standard projection and the other was treated with the method according to the invention AE. In order to neutralize the effect of precedence, some saw AE then PS, and other PS and AE.

The projector was set to project an image with white at 103 cd / m ² for AE and with a white at 48 cd / m ² for PS. In practice, the same Projector was used, and to switch from one projection to the other, the projector settings were not changed, but the digital copy for the PS projection was processed by a specific rendering step taking into account that it was going to be projected with a system planned for an AE projection. The color rendition has been checked against the Digital Cinema Initiative (DCI) reference system.

After the viewing, the participant had to complete a questionnaire:

> what excerpt (from A or B) made him want to see the rest of the movie in its entirety,

> what excerpt was his favorite,

> what extract gave him the best immersion,

> what extract gave him the best commitment,

> finally, if one of them was more spectacular, more realistic, more naturalistic, or better restored.

For A, then for B, the participant had to indicate if the excerpt was animated, if he attracted his attention, if he made him live emotions. A last open question allowed to add comments in free verbalization.

The projected sequence was an excerpt from the movie "Brice de Nice 3", with mountain scenes showing dizzying peaks.

Thus, the projection with the method according to the invention is always preferred to the standard method, in particular because it is perceived more spectacular and giving a better feeling of immersion.

Claims

claims

1. A digital image processing and projection method for cinema-theater broadcasting according to which:

A calibration step in which an image sequence rendering process adapted to a predetermined projector model is carried out,

An encapsulation step in which the image sequences are supplemented with metadata to form a digital copy,

A step of providing a projection system (4), and

A projection step in which the digital copy is projected by the projection system (4) onto a screen (44),

characterized in that, in the encapsulation step, the metadata includes a data indicating that the calibration was performed using a high dynamic process,

the projection system (4) reading said data and accordingly selecting a first light amplification factor for a standard method or a second light amplification factor for the high dynamic process,

the second factor being superior to the first factor.

2. The method of claim 1, wherein the second factor is greater than 1.5 times the first factor, preferably 2 times the first factor.

3. Method according to one of the preceding claims, wherein adjusting the projection system (4) so that a white point corresponding to the maximum light power is at a value greater than or equal to 100 cd / m2.

4. Method according to one of the preceding claims, wherein the film is projected in a room whose surfaces are treated in black.

5. Method according to one of the preceding claims, wherein the film is projected with a projector whose performance allows an inter-image contrast of at least 6000: 1.

6. The method of claim 5, wherein the film is projected with a projector whose performance allows inter-image contrast of at least 7000: 1.