EP0832533A1 - Circuit de traitement de signaux video - Google Patents

Circuit de traitement de signaux video

Info

Publication number
EP0832533A1
EP0832533A1 EP96914645A EP96914645A EP0832533A1 EP 0832533 A1 EP0832533 A1 EP 0832533A1 EP 96914645 A EP96914645 A EP 96914645A EP 96914645 A EP96914645 A EP 96914645A EP 0832533 A1 EP0832533 A1 EP 0832533A1
Authority
EP
European Patent Office
Prior art keywords
voltage
current
analog signal
clipping
circuit according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP96914645A
Other languages
German (de)
English (en)
Inventor
Patrick E. D'luzansky
Victor F. Fleury
James R. Garrett
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Polaroid Corp
Original Assignee
Polaroid Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Polaroid Corp filed Critical Polaroid Corp
Publication of EP0832533A1 publication Critical patent/EP0832533A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
    • H04N23/80—Camera processing pipelines; Components thereof
    • H04N23/82—Camera processing pipelines; Components thereof for controlling camera response irrespective of the scene brightness, e.g. gamma correction

Definitions

  • the present invention relates generally to video signal processing circuits, and, more particularly, the invention relates to video signal processing circuits for applying a specified gamma curve transformation function to a video signal in an electronic camera.
  • Video signal processing circuits are analog circuits which process video output signals representative of an image from a photosensitive semiconductor device such as a charge coupled device ("CCD"). The video signal processing circuit then applies a transformation function to the video output signal such that a downstream analog-to-digital conversion is optimized to produce better tone reproduction while limiting contouring seen within an output of the image.
  • CCD charge coupled device
  • the invention has application, by way of example, in an electronic still camera.
  • a camera captures an image with a CCD and then converts the analog representation of the image to digital image data representing the image where it may then be processed, stored on an electromagnetic media, or sent to a printing device.
  • a problem with such a camera is that a human eye does not readily perceive tone changes on extreme ends of a tone scale but is extremely adept at perceiving changes in a midrange of the tone scale. In other words, the human eye does not notice changes in black tones or white tones as readily as it perceives changes among the many intervening gray tones.
  • a linear digitization performed on the analog representation wastes digital image data in the black and white tones and then does not have enough digital image data available to represent the midtones which creates noticeable tone jumps in the image called contouring.
  • One commercially available camera uses the analog-to-digital converter itself to transform the image signal.
  • a tap on the analog-to-digital converter which is designed to lower noise is driven with a triangular voltage signal from zero volts to a maximum value and back.
  • the signal creates a piece-wise linear approximation of a Gamma function.
  • a problem with this method is that its approximation is rough having only a limited number of linear steps available to generate the curve.
  • the aforementioned and other objects are achieved by the invention which provides a video signal processing circuit for use in an electronic still camera.
  • the video signal processing circuit applies a gamma curve to an analog signal representative of an image to correct the analog signal and to enhance an ability of the electronic still camera to digitize the analog signal.
  • the latter is accomplished by readjusting tone scale mapping such that large changes in tone scale are mapped across a larger digital range while relatively flat changes in tone scale are compressed and mapped to a relatively small digital range.
  • the circuit comprises a cascaded resistance structure, clipping means, buffer means, adder means and current-to- voltage tr.anslation means.
  • the cascaded resistance structure comprises a resistive tree fed by a reference voltage.
  • the cascaded resistance structure has a plurality of terminals with a resistive element disposed between individual pairs of the terminals such that a resistive value of each resistive elements defines a cutoff voltage at each of the plurality of terminals.
  • each clipping means is in electrical communication with a current source.
  • the clipping means then draws a current from the current source which is proportional to a voltage of the analog signal. If the voltage of the analog signal becomes equal to the cutoff voltage, the current then becomes substantially constant.
  • buffer means is associated with each of clipping means to convert the voltage to a current which changes proportionally to the voltage.
  • buffer means is a transistor which is in saturation such that the currenc into the collector of the transistor is the current and also amplified to boost a magnitude of the current.
  • the adder means combines the currents from each of the buffer means and forms a single total current.
  • the current-to-voltage converter means then converts the total current to an output voltage which varies according to said analog signal along the gamma curve. Folowing the fluctuations in the total current, the output voltage varies according to the analog signal but follows the gamma curve which is defined by said cutoff voltages.
  • the invention provides methods in accord with the apparatus described above.
  • the aforementioned and other aspects of the invention are evident in the drawings and in the description that follows.
  • FIG. 1 shows a block diagram of an electronic still camera in accordance with the invention
  • Figure 2A shows a D-log H curve for the camera shown in Figure 1.
  • Figure 2B shows a Log V-Log H curve for the camera shown in Figure 1;
  • Figure 2C shows a graph of scene reflectivity versus print reflectivity for the camera of Figure 1 ;
  • Figure 3A shows a clipping circuit for use in the video signal processor in the camera according to Figure 1;
  • Figure 3B illustrates a circuit implementing the gamma curve in a video signal processor for an electronic still camera in accordance with Figure 1;
  • Figure 3C shows a voltage output curve out of the video signal processor for the electronic still camera of Figure 1.
  • Electronic imaging cameras for recording either motion or still images are in common usage today.
  • Such cameras generally include, as is shown in Figure 1, a two-dimensional photosensitive array which may comprise a high-resolution charge coupled device (“CCD”), charge injection device (“CID”), or other photosensitive sensors.
  • CCD charge coupled device
  • CID charge injection device
  • a CCD 16 is depicted in the preferred embodiment but this type of photosensitive array should be considered illustrative and not restrictive.
  • the CCD 16 receives light 12 representative of the image scene in a well-known manner by way of an objective lens and a shutter as shown collectively as optics 14.
  • the CCD 16 typically comprises a plurality of image sensing elements or pixels arranged in a two-dimensional array with each image sensing pixel converting image defining light reflected from a scene into a corresponding analog voltage value. Sampling is done sequentially for the three primary colors red, green, and blue (hereinafter referred to as "RGB"), and the image sensing elements are preferably arranged in a plurality of rows and columns.
  • RGB red, green, and blue
  • the resolution of the electro-optically sampled image comprises approximately 1656 image points, or pixels, per line horizontally and 600 lines vertically. Accordingly, each image has an aggregate 1656 x 600 pixels wherein each pixel is assigned one of the RGB colors.
  • VSP video signal processor
  • An analog-to-digital converter 20 then transforms row by row the analog voltage values into a plurality of digital electronic image data signals representing the recorded image in a RGB color coordinate system.
  • the digitized signal is then passed into a processor 22 where it can be stored in a storage device 24 such as an electromagnetic storage device, a hard disk for example, an electro-optical storage device 24, or it may simply be passed onto a computer which is connected to the camera via a cable.
  • a storage device 24 such as an electromagnetic storage device, a hard disk for example, an electro-optical storage device 24, or it may simply be passed onto a computer which is connected to the camera via a cable.
  • the signal 26 will be passed externally to the computer.
  • a negative is exposed to varying amounts of illuminant, H.
  • a resulting image has varying densities according to the exposure.
  • the exposure is the illuminant level multiplied by the exposure time
  • a log - log plot is normally used in photography as a convenient way to express information over a wide range of illumination and transmittances of the negative. Scene reflectance (and hence illuminant levels, H, onto the negative) varies over ranges of 1000 : 1 from highlights to shadows. Expressed in log base 10 this is compressed to a range of 3 : 1.
  • Illuminants on the CCD 16 generate charge Q, linearly. Charge is linearly converted to voltage, V, by the capacitance of an output amplifier:
  • FIG. 2 A A D-log H curve as described is shown in Figure 2 A. It can be seen from the figure that the first region 28 has a slope that is very flat and as exposure increases reflection density does not rise noticeably.
  • the slope increases dramatically. This is shown as the second region 30 where as exposure increases, density increases greatly. In the third region 32, the density again flattens.
  • the video signal processor 18 transforms an analog signal coming out of the CCD 16 representative of the image such that uniform steps in the A-D are concentrated in the second region 30 of the D-Log H curve. This transform is known as the gamma curve.
  • G is a tonescale mapping function of a "system.”
  • An analog to G in standard photography is the D-log H curve where a psychophysical characteristic of the system is defined by the equation
  • CIELab is a psychovisual model of uniform color space.
  • L* is a measure of lightness in the CIELab space. Equal increments of L* are perceived as uniform changes in lightness over a wide range of display illuminants and print reflectances. L* can be expressed as a function of display or print reflectance:
  • R Wh i te is assumed to be equal to one since the print is printed on white paper. Because CIELab is visually uniform space, equal increments in L* are perceived as equally different. As a result, a well-known criterion for the absence of visual contours is that the changes in L* between levels, ⁇ L*, should be less than a given value.
  • a proper step level for L* must be chosen.
  • An example is quantizing the image in steps of equal print L*. With a properly exposed print the quantization steps are then less than 0.4L* apart. This step level is sensitive to an error in exposure such that the quantized steps can become visible. A one stop exposure error will produce differences between levels in excess of 1.5L*.
  • This step level can be modified to be robust in the presence of the exposure errors that occur.
  • the signal is quantized in equal steps for a wider range of world reflectances than can be printed. This does not produce as uniform steps as with the previous scheme; however, the presence of an exposure error does not produce the large steps seen with the previous scheme.
  • ⁇ is the optimal tonescale map. Therefore, for no visible artifacts or contouring to occur, ⁇ R pr i n t between quantization steps must be less than ⁇ G x ⁇ R scene .
  • Such a gamma curve is implemented in the video signal processor 18.
  • the video signal processor 18 contains circuitry to transform the analog signal representing the image from the CCD 16, referred to as the input voltage V BM , into a signal as previously described.
  • a subpart of the overall circuit is shown in Figure 3A.
  • the subpart depicts a clipping circuit which is an integral part of the overall circuit shown in Figure 3B.
  • the clipping circuit utilizes a comparator and a buffer.
  • a current source, i drives the comparator through a compensating resistor, R COMP. described in greater detail hereinafter.
  • the comparator utilizes two branched PNP transistors Ql and Q2 where Q2 is fed by a clipping voltage, V c , and Ql is fed by an input voltage, VI .
  • the clipping voltage determines the level to which the input voltage, VIN, is compared.
  • the input voltage is representative of an image captured by the CCD. If the voltage VIN is less th.an the clipping voltage, Vc, then current through Q3 will be determined by V IN R G2 - Once V I becomes equal to or greater than the clipping voltage, Vc, Ql goes into a cutoff mode and thus no longer effects changes to the current through Q3.
  • Tr.ansistor Q3 is an NPN transistor whose collector feeds off of an operational amplifier ("op amp"), Ai.
  • the compensating resistor, R COMP . is then used to match the VBE characteristics of the NPN transistor Q3 to characteristics of the PNP transistors Ql and Q2.
  • Q3 serves as a buffer, placing V I across the resistor R G2 - This ensures a current which is representative of the input voltage, VI .
  • This stage shown as K ⁇ in Figure 3A, is used to provide voltage-to-current conversion, as well as provide a current gain which is fixed by a ratio of R GI to R G2 when VIN ⁇ Vc- Voltage-to-current conversion is accomplished by converting a voltage at the base of Q3 into i c ⁇ VIN/R G 2- Once V IN > V c , I ⁇ - decreases as VI increases, i.e. ⁇ VI R sm where
  • the op .amp has as a non-inverting input the input voltage, VI N, increased by 0.7V through a diode, Di.
  • the voltage increase is introduced to level shift V IN equally with that of the inverting input.
  • the diode compensates for the voltage increase across the P-N junction of the base-emitter of Q3. Varying the base current to Q3 then varies current drawn from the op amp, Aj, causing the output voltage, VOUT, to vary proportionally.
  • a smoothing resistor, R sm is also shown which is tied to the input voltage, V IN .
  • the path established between VI N and the emitter of Q3 ensures that after the cutoff voltage, Vc, is reached by V ⁇ M .and the current drawn from the op amp due to Ki shifts to a constant value, there becomes a current contribution through R sm producing a smooth transition to the constant value.
  • the smoothing rounds the sharp corner produced when the current is suddenly clipped.
  • the current contributions from each gain block are determined by the clipping voltages and the gains of the gain blocks, thus determining a shape of the gamma curve.
  • the smoothing resistor, R sm then acts to provide smoother, sharper transitions on the Gamma curve.
  • the clipping circuit of Figure 3A is utilized extensively in the video signal processor as shown in Figure 3B.
  • the video signal processor has a tree structure where various taps are utilized along the tree. In describing the figure, exemplary voltage values are used, but these values must be altered for each application to shape the Gamma curve.
  • the tree has a reference voltage, V R , of one volt.
  • a voltage of 0.8V for the first clipping circuit C represents the clipping voltage Vc as previously described.
  • the output is a current which runs through the N-P-N transistor Q 3 of gain block -t . This output is then summed and current-to-voltage conversion is performed to provide a contribution to the final output VOUT-
  • a second clipping circuit is fed off of the tree below R 2 which like before has a
  • Vc 3 0.6 volts for the second clipping circuit C 3 .
  • VIN is then compared against V c in C 3 and is again summed to add its contributions to V out .
  • Figure 3C depicts a typical V out output which is then passed through an A/D, in this case sampled between 0 and 255, creating a more uniform transition between black and white where contouring has essentially been alleviated by spreading out the steps between 0 and 255 along a more gradual curve.

Abstract

L'invention se rapporte à un circuit de traitement de signaux vidéo destiné à être utilisé dans une caméra électronique. Le circuit de traitement de signaux vidéo transforme un signal analogique représentatif d'une image en fonction d'une courbe de correction gamma de sorte que le signal analogique soit allongé le long des demi-tons de l'image et contracté à chaque extrémité de l'échelle de luminances. Cette transformation améliore la qualité de l'image, et la simplicité de conception du circuit permet une fabrication à coûts réduits. Le circuit lui-même met en oeuvre une structure arborescente résistive qui crée des tensions de blocage dans un limiteur d'amplitude où les valeurs résistives forment la courbe de correction gamma.
EP96914645A 1995-05-18 1996-05-16 Circuit de traitement de signaux video Withdrawn EP0832533A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US44369895A 1995-05-18 1995-05-18
US443698 1995-05-18
PCT/US1996/006996 WO1996037073A1 (fr) 1995-05-18 1996-05-16 Circuit de traitement de signaux video

Publications (1)

Publication Number Publication Date
EP0832533A1 true EP0832533A1 (fr) 1998-04-01

Family

ID=23761849

Family Applications (1)

Application Number Title Priority Date Filing Date
EP96914645A Withdrawn EP0832533A1 (fr) 1995-05-18 1996-05-16 Circuit de traitement de signaux video

Country Status (2)

Country Link
EP (1) EP0832533A1 (fr)
WO (1) WO1996037073A1 (fr)

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4651227A (en) * 1982-08-20 1987-03-17 Olympus Optical Co., Ltd. Video signal recording apparatus with A/D conversion
JPS5945775A (ja) * 1982-09-09 1984-03-14 Sharp Corp ガンマ補正回路
JP2554955B2 (ja) * 1990-10-02 1996-11-20 池上通信機株式会社 非線形処理回路
EP0672324A4 (fr) * 1992-12-04 1995-12-06 Hughes Jvc Tech Corp Circuit de correction gamma pour projecteurs d'images.
JP3071590B2 (ja) * 1993-01-05 2000-07-31 日本電気株式会社 液晶ディスプレイ装置

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9637073A1 *

Also Published As

Publication number Publication date
WO1996037073A1 (fr) 1996-11-21

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