WO2015128603A1 - Procédé et appareil de traitement de signal pour vidéo à plage dynamique élevée - Google Patents

Procédé et appareil de traitement de signal pour vidéo à plage dynamique élevée Download PDF

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Publication number
WO2015128603A1
WO2015128603A1 PCT/GB2015/050334 GB2015050334W WO2015128603A1 WO 2015128603 A1 WO2015128603 A1 WO 2015128603A1 GB 2015050334 W GB2015050334 W GB 2015050334W WO 2015128603 A1 WO2015128603 A1 WO 2015128603A1
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Prior art keywords
signal
function
range
luminance
bit
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PCT/GB2015/050334
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Tim BORER
Andrew Cotton
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British Broadcasting Corp
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British Broadcasting Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/14—Picture signal circuitry for video frequency region
    • H04N5/20—Circuitry for controlling amplitude response
    • H04N5/202—Gamma control
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/50—Control of the SSIS exposure
    • H04N25/57—Control of the dynamic range
    • H04N25/571—Control of the dynamic range involving a non-linear response
    • H04N25/573—Control of the dynamic range involving a non-linear response the logarithmic type
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
    • H04N23/70—Circuitry for compensating brightness variation in the scene
    • H04N23/76—Circuitry for compensating brightness variation in the scene by influencing the image signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/50—Control of the SSIS exposure
    • H04N25/57—Control of the dynamic range
    • H04N25/571—Control of the dynamic range involving a non-linear response
    • H04N25/575—Control of the dynamic range involving a non-linear response with a response composed of multiple slopes
    • 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

  • This invention relates to processing a video signal from a source, involving converting between a Iuminance value and signal value.
  • OETF opto-electronic transfer function
  • L is Iuminance of the image 0 ⁇ L ⁇ 1
  • V is the corresponding electrical signal Note that although the Rec 709 characteristic is defined in terms of the power 0.45, overall, including the linear potion of the characteristic, the characteristic is closely approximated by a pure power law with exponent 0.5.
  • the proposed Dolby OETF quantised to 10 bits, has a remarkably constant Weber fraction of about 1 % (the difference in brightness represented by adjacent quantisation levels, as a fraction of the brightness, is known as the Weber fraction).
  • the Weber fraction increases only slightly to 2%.
  • the Dolby proposal extends the dynamic range to 10 7 :1 , from 0.001 cd/m 2 to 10 4 cd/m 2 .
  • This dynamic range is similar to the simultaneous dynamic range of the human visual system, which is about 10000:1 . That is, humans can simultaneously, in the same scene, see brightness variations of this range, for example between shadows and highlights.
  • Such dynamic range far exceeds the dynamic range of printed material (less than 100:1 ), of (now obsolete) CRT displays (less than 100:1 ) and, in practice, of modern fiat panel displays (which, despite claiming huge dynamic ranges, are limited by the low dynamic range signals supported on their interfaces).
  • the invention provides a method of processing a video signal from a source, comprising converting between a luminance value and signal value according to a conversion in which: a power law is used for a lower range of luminance values in which the signal value is a function that includes a power of the luminance value; and a log law is used for an upper range of luminance values in which the signal value is a function that includes a log of the luminance value.
  • the power law function may be substantially the same as the Rec 709 standard noted above.
  • any existing display system processing a signal so produced in accordance with the inverse function of Rec 709 will produce an acceptable result.
  • a level of backward compatibility to existing equipment is therefore maintained.
  • the dynamic range may be extended, when processed by equipment using the inverse of the log law function, but the signal can still be processed in this range by any existing display system using the inverse function of Rec 709 and produce an acceptable result. Again, a level of compatibility is achieved.
  • Fig, 1 is a pair of images showing a "contour" effect when quantisation is insufficient to render subtle shading changes
  • Fig, 2 is a graph showing an example "Knee" characteristic as used in some cameras to extend dynamic range
  • Fig. 3 is a graph showing a comparison of known opto electronic transfer functions in comparison to a function of the present invention
  • Fig. 4 shows a graph of Weber fractions comparing known opto electronic transfer functions to a function for the present invention
  • Fig, 5 shows a schematic system diagram for a known proposed opto electronic transfer function and the signals processed
  • Fig, 6 shows the arrangement of Figure 5 but with the improvement of the present embodiment of the invention showing a reduction in signals processed
  • Fig, 7 is a schematic diagram of a processing component of the embodiment of the invention.
  • Fig, 8 is a schematic diagram of a display device of the embodiment of the invention.
  • the Rec 709 gamma curve is often modified in cameras, though the use, of a "knee" to extend the dynamic range and prevent the signal saturating. This is adequate for live television but for higher quality production higher dynamic range is achieved through the use of numerous non-standard, quasi logarithmic, OETFs.
  • a BBC Research Department Report from 1 974 [Moore] provides both theoretical analysis and subjective experiments to estimate the number of bits required to quantise a gamma corrected video signal.
  • These results were in line with their theoretical analysis. They also tested real pictures, which they found required only 6 bits with or without dither. However only 3 images were used which may not have been representative of critical modern images. The report recommended using 7 bits but noted that visible contouring could still arise under some circumstances.
  • Rec 709 signals which is specified in ITU Recommendation 2020, does not increase the maximum scene brightness that can be supported either.
  • Camera manufacturers would like to be able to support higher dynamic range and, particularly, brighter scenes. Their camera sensors have a dynamic range significantly greater than 100:1 and, if they could provide this dynamic range through to the display, pictures from their camera would look better. But with a standard Rec 709 signal about 100:1 dynamic range is all that can be achieved. To try to circumvent this limit camera manufacturers commonly modify the standard Rec 709 transfer characteristic and add a "knee" to the characteristic.
  • a knee in the OETF transfer characteristic In order to extend contrast at the bright end, camera manufacturers introduce a knee in the OETF transfer characteristic.
  • the knee puts a break point in the OETF and compresses the contrast above the break point to fit into available signal range.
  • An example of a knee characteristic is shown in figure 2 (for a 10 bit signal). in this example the knee is at 100% reference white and the headroom available in the signal (levels 941 to 1019 in SMPTE standards 259 & 292) is used to increase the available exposure to 300% of reference white.
  • the break point is set at about 85% of reference white output level (corresponding to an input exposure of about 70% of peak white).
  • the dynamic range of the video signal may be increased by using an alternative OETF.
  • the gamma curve specified in Rec 709 was designed to produce an approximately uniform perception of video noise in an analogue signal. So it was designed to approximate the subjective lightness curve experience by the human visual system. But in quantising a video signal the objective is to avoid contouring, not to provide uniform perception of noise. So in quantising a video signal the important characteristic is the human visual system's ability to distinguish similar values of brightness. in quantising a video signal we wish to avoid contouring, and so it is the likelihood of detecting the difference between adjacent quantisation levels that is important (not lightness).
  • the just noticeable difference in brightness is governed by Weber's law mentioned above (modified to the De Vries-Rose law at low luminance as discussed below).
  • Weber's iaw is that detectable difference between brightnesses (or, more generally, other stimuli too) is proportional to the brightness. That is that just noticeable difference in brightness is a constant fraction of the brightness, known as the Weber fraction.
  • the Weber fraction of cone cells in the eye is between 2% and 3%, which means that subjects can reliably detect a change of between 2% and 3% in brightness.
  • the present embodiment provides benefits in terms of dynamic range using a logarithmic transfer characteristic that has not previously been adopted for video production and distribution.
  • the dynamic range of television signals has been limited to 100:1 , which may not require a logarithmic curve, and that a logarithmic curve would be appear to be incompatible with the installed television infrastructure, if a higher dynamic range for the end viewer is required in new television standards a conventional gamma curve, even extended to 12 bits, is no longer adequate.
  • OETF Any new OETF should, we appreciate, also intemperate with existing standards and infra-structure.
  • a new signal format should be able to be carried over existing video connections (including compression) and be displayed, in a broadly compatible fashion, on existing 8 and 10 bit displays.
  • Television will increasingly encompass multiple formats (e.g. SD, HD and UHD) and frame rates. So a new signal format should also, ideally, be compatible with processing such as spatial up and down conversion and frame rate ("standards”) conversion,
  • OETFs would be equally compatible with existing standards and infra-structure. Any reasonable 10 bit OETF may be carried over existing 10 bit interconnects.
  • Video compression is optimised for the characteristics of conventional video, so that new formats are likely to require greater bit rate and/or exhibit more artefacts.
  • Video processing is typically performed on non-linear signals so a new signal format may degrade the quality of such processing.
  • Potential display distortion include altered brightness, changed colours, reduced or excessive sharpness and, perhaps, contouring.
  • Rec 709 is a two part curve with a linear part near black and a power law (gamma curve) for the majority of the input range. It is designed so that the value and the gradient of both curves match at the transition between them.
  • Camera makers modify the OETF by adding a third section near white, by using a "knee", to increase dynamic range and avoid clipping the signal. Unfortunately the section added above the "knee” risks introducing contouring artefacts (assuming an inverse ETOF) by failing to take full account of the psychovisual aspects of vision.
  • the present embodiment adds a further (third) part to the Rec 709 curve, at higher input luminance, to extend the dynamic range without re-introducing the risk of contouring.
  • the new, upper portion, of the curve is a logarithmic function to allow for the non-linear response of the eye.
  • the output value and the gradient of both curves are designed to match at the transition between them. Because this new proposal has some similarities to the widely used knee characteristic in cameras we can be confident that it is broadly compatible with Rec 709.
  • the OETF defined in Rec 709 is:
  • L is luminance of the image 0 ⁇ L ⁇ 1
  • V is the corresponding electrical signal in Recommendation ITU-R BT.2020 (Parameter values for ultra-high definition television systems for production and international programme exchange) the same equation is specified as:
  • E voltage normalized by the reference white level and proportional to the implicit light intensity that would be detected with a reference camera colour channel R, G, B; E' is the resulting non-linear signal
  • the present embodiment adds a third, logarithmic, portion to the transfer function at higher values of luminance such that (equation A):
  • a 10 bit signal conforming to the proposed OETF provides a sixteen fold increasing in dynamic range compared to an 8 bit Rec 709 signal. The maximum luminance is increased by a factor of 4 as above. Increasing the signal depth from 8 to 10 bits reduces the difference between successive quantisation levels by a further factor of 4, thereby increasing the dynamic range a low luminance ("in the blacks").
  • Figure 3 shows a comparative plot of several OETFs.
  • the Rec 709 curve (partially hidden by the "knee” curve), clips at reference white. Clipping is avoided up to 400% of reference white by using the camera knee characteristic illustrated.
  • the proposed OETF offers the same extension in dynamic range but with a smoother curve. At the bottom end it is (by design) very similar to the Rec 709 curve. At the upper end it is similar to the knee curve. Based on industry experience of cameras with knee curves we may be confident that the proposed curve has a high degree of compatibility with Rec 709. The known "perceptual quantiser OETF is included for comparison.
  • the proposed OETF prevents the Weber fraction from reducing to unnecessarily low values and by doing so allows a higher dynamic range.
  • the Perceptual Quantiser has a more or less constant Weber fraction of 0.09 across the range plotted and so corresponds closely with a pure logarithmic transfer function over this range.
  • the Weber threshold of visibility is replaced by the De Vries-Rose law. This says the threshold of visibility is proportional to the square root of the brightness rather than to brightness.
  • the brightness corresponding to the transition between De Vries-Rose and Weber depends on conditions, such as the size, frequency and duration of the (visual) signal. Typically the transition brightness is between .04 and 25 cd/m 2 [Sezan]. This transition brightness approximately corresponds to the breakpoint between the gamma and logarithmic sections of the curve in the proposed OETF. Consequently higher Weber fractions in Rec 709 (and therefore in this proposal), at low values of luminance, do not result in visible contouring. Furthermore, if the peak brightness of a display using the proposed OETF is approximately a few hundred cd/m 2 , then the OETF approximately corresponds to the psychovisual sensitivity of the eye.
  • a 10 bit signal using the proposed OETF provides a dynamic range of at least 1600:1 (10.6 stops), compared to 100:1 for Rec 709. This is sufficient for consumer displays and for some video production. However film and modern electronic cameras can support dynamic ranges up to 14 stops. A higher dynamic range signal is needed to support high quality video and movie production.
  • the 10 bit signal for the monitor could be derived by simply omitting the MSB and the LSB of the 12 bit signal.
  • the 12 bit signal would exceed the range of the 10 bit signal (precisely the reason for using the 12 bit signal). But in this case the over range signal would be easily seen because the signal would "wrap round" to black.
  • Such out of range signals would be corrected during grading (one of the purposes of grading) to produce the final 10 bit output.
  • the screen luminance, L is a pure power law, with a exponent (gamma) of 2.4, of the signal, V.
  • the overall gamma of the system from scene luminance to screen luminance, is 1 .2.
  • the OETF specified in Rec 709 closely approximates a pure power law with exponent 0.5 once allowance is made for the linear part of the curve near black. Combining this with the power law from Rec 1886 yields the overall system gamma of 1 .2. This overall system non-linearity is intentional and designed to compensate for viewing conditions.
  • an electro-optical transfer function corresponding to the proposal herein should also provide an overall system gamma of 1 .2. This implies the following EOTF (Equation B):
  • Figures 5 and 6 demonstrate the reduction in processing needed with an embodiment of the invention in comparison to a proposed transfer function that departs from the Rec 709 standard.
  • Figure 5 shows the process in which two signals need to be produced to remain compatible with an existing standard dynamic range screen as well as a true high dynamic range screen.
  • a raw image is captured at capture step 10 and may be previewed at preview step 12 and passed to an imaging screen for editing at step 14.
  • a separate colour grading 16 may be performed on the HDR and SDR images and separate output files 18 produced in a format such as MXF which are then separately provided to a play out server at step 20.
  • a HDR and SDR transmission encoder 22 then provides two signals, a first standard dynamic range signal and a second different signal from which the HDR signal may be derived using the SDR signal. This is then sent to a broadcast chain 24 such as a satellite at step where both signals must be broadcast. As can be seen, there is redundancy in the chain as two separate signals must be prepared and transmitted. Separate SD and HD receivers 26,28 are then able to receive the respective signals/
  • Figure 6 shows the contrast with the improvement of the present invention.
  • an image is captured at step 10 but at step 12 a new transfer function according to the present disclosure is applied prior to editing in a single editing screen at step 14 and colour grading at step 16.
  • a single MXF file is produced 18 and sent to a transmission server 20 prior to encoding 22 and satellite uplink at step 24.
  • the single signal is now receivable and can be displayed by both the SDR and HDR receivers in the manner described above. A significant saving is thus achieved.
  • Figures 7 and 8 respectively show an image capture device such as a camera, and a display device such as a TV screen embodying the invention,
  • the camera of Figure 7 includes a detector CCD or CMOS 30, processor 34 and memory 36 and an output 38.
  • the memory 36 holds the EOTF function as described above, particularly in Equation A, by which the processor processes the incoming luminance signal.
  • the display device of Figure 8 has an input 40 for receiving a signal, processor 44 and memory 42 and display 46.
  • the memory 42 holds the OEFT function such as in Equation B herein.
  • Figure 9 shows a further embodiment of the OETF that provides for a lower range of luminance values that the signal value is derived using a first function that includes a power of the luminance value, and for an upper range of luminance values the signal value is derived using a second function that includes a log of the luminance value.
  • the compressive function of this embodiment outputs the same value as is input (linear) until the signal level reaches a threshold, or breakpoint. Thereafter, for larger values of the input signal, the output is a logarithmic function that compresses the input.
  • the second stage of the OETF applies a power law function as defined in Rec 709.
  • the OETF is a power law defined, for example, by ITU Rec 709 (because combining the linear function and power law gives a power law).
  • the OETF is a power of a logarithm.
  • the choice of compressive function is explained below.
  • the function is intended to have a domain [0:k] and compress this to a range [0:1 ].
  • the desired function is continuous and has a continuous first derivative. This provides a pair of simultaneous equations, below, from which the curve may be defined. - ⁇ + ⁇
  • the breakpoint determines the amount of compression that the function provides.
  • this function provides compression by a factor of, approximately, 5.02.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Picture Signal Circuits (AREA)

Abstract

L'invention concerne un procédé de traitement d'un signal vidéo provenant d'une source pour produire un signal de sortie, le procédé consistant à passer d'une valeur de luminance à une valeur de signal au moyen d'une conversion qui utilise une puissance de la valeur de luminance pour la plage inférieure des valeurs de luminance et qui utilise un logarithme de la valeur de luminance pour une plage supérieure de valeurs de luminance. Cette disposition rend le signal compatible avec les fonctions de transfert existantes en utilisant une loi en forme de puissance sur la partie inférieure de la plage, et maintient une certaine compatibilité avec les systèmes existants sur la partie supérieure de la plage. Un signal produit par le procédé peut par conséquent être utilisé à la fois avec des écrans de plage dynamique élevée ou de plage dynamique normale.
PCT/GB2015/050334 2014-02-27 2015-02-06 Procédé et appareil de traitement de signal pour vidéo à plage dynamique élevée Ceased WO2015128603A1 (fr)

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US10148871B2 (en) 2016-12-07 2018-12-04 Microsoft Technology Licensing, Llc Advanced raw conversion to produce high dynamic range, wide color gamut output
CN110415634A (zh) * 2018-04-27 2019-11-05 苹果公司 用于高动态范围显示器的标准和高动态范围显示系统和方法
US10616497B2 (en) 2015-12-23 2020-04-07 Huawei Technologies Co., Ltd. Method and apparatus for processing image signal conversion, and terminal device
CN111510698A (zh) * 2020-04-23 2020-08-07 惠州Tcl移动通信有限公司 图像处理方法、装置、存储介质及移动终端
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CN111866414A (zh) * 2020-07-15 2020-10-30 大连理工大学 高动态图像传感器像素结构及时序控制方法
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US10257394B2 (en) 2016-02-12 2019-04-09 Contrast, Inc. Combined HDR/LDR video streaming
JP7081835B2 (ja) 2016-08-09 2022-06-07 コントラスト， インコーポレイテッド 車両制御のためのリアルタイムｈｄｒビデオ
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Cited By (8)

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Publication number Priority date Publication date Assignee Title
US10783621B2 (en) 2015-12-15 2020-09-22 Huawei Technologies Co., Ltd. Method and apparatus for processing high dynamic range image, and terminal device
US10616497B2 (en) 2015-12-23 2020-04-07 Huawei Technologies Co., Ltd. Method and apparatus for processing image signal conversion, and terminal device
US10148871B2 (en) 2016-12-07 2018-12-04 Microsoft Technology Licensing, Llc Advanced raw conversion to produce high dynamic range, wide color gamut output
US11158032B2 (en) 2017-03-20 2021-10-26 Dolby Laboratories Licensing Corporation Perceptually preserving scene-referred contrasts and chromaticities
CN110415634A (zh) * 2018-04-27 2019-11-05 苹果公司 用于高动态范围显示器的标准和高动态范围显示系统和方法
CN110415634B (zh) * 2018-04-27 2023-01-17 苹果公司 用于高动态范围显示器的标准和高动态范围显示系统和方法
CN111510698A (zh) * 2020-04-23 2020-08-07 惠州Tcl移动通信有限公司 图像处理方法、装置、存储介质及移动终端
CN111866414A (zh) * 2020-07-15 2020-10-30 大连理工大学 高动态图像传感器像素结构及时序控制方法

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