Qt Gamma Compensation Lut Download
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VLC looks the same as Premiere but now upload the file to a video hosting platform like YouTube. You'll see that it's just as washed out as in QuickTime. I too believed for a long time that Premiere displays colors accurately and that Apple is at fault here. But it's actually Adobe that needs to fix things. So, exporting without gamma compensation will not get you the desired results. That's at least my conclusion from various tests.
I'm relatively new to video editing (more still photo experience), but if I understand correctly are you saying that ALL mac computers will display any video slightly lighter than intended (in mid tone/ shadow areas) due to the machines using a different gamma than the "standard" 2.4 for video. Or do movies displayed on Mac's somehow compensate for the difference? Also, I've noticed that VLC renders my files exported from Premiere with the correct tone (as opposed to QT)...how does that happen?
Thanks, appreciate your input!
Alan.
The display standards for grading and evaluation of Rec.709 media has been in use for over a decade, for all broadcast/streaming/web use: sRGB color primaries, D65 white point, 100 nits max brightness, gamma 2.4 (semi-darkened room) with 2.2 allowed for "bright room" or web use.
But for some unknown reason, when Apple started with their Retina monitors, they made the settings used by their OS level color management utility ColorSync to use the specified Rec.709 camera transform gamma of 1.96, rather than the specified display gamma of 2.4.
So, let's say you make a file that looks good when displayed with gamma 1.96, which lifts the shadows a lot and mids a bit ... meaning you have to drop the shadows/mids in the grading ... and yea, it looks fine on a Mac in a ColorSync managed application.
As I've often noted, the many colorists I'm around all the time are mostly Mac folk. Naturally, right? And they're mad as all get out about this mess. Because there's no way to display the same image when using two widely separate gamma settings.
________________ or 1_____ is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems.[1] Gamma correction is, in the simplest cases, defined by the following power-law expression:
Gamma encoding of images is used to optimize the usage of bits when encoding an image, or bandwidth used to transport an image, by taking advantage of the non-linear manner in which humans perceive light and color.[1] The human perception of brightness (lightness), under common illumination conditions (neither pitch black nor blindingly bright), follows an approximate power function (which has no relation to the gamma function), with greater sensitivity to relative differences between darker tones than between lighter tones, consistent with the Stevens power law for brightness perception. If images are not gamma-encoded, they allocate too many bits or too much bandwidth to highlights that humans cannot differentiate, and too few bits or too little bandwidth to shadow values that humans are sensitive to and would require more bits/bandwidth to maintain the same visual quality.[2][1][3] Gamma encoding of floating-point images is not required (and may be counterproductive), because the floating-point format already provides a piecewise linear approximation of a logarithmic curve.[4]
Although gamma encoding was developed originally to compensate for the brightness characteristics of cathode ray tube (CRT) displays, that is not its main purpose or advantage in modern systems. In CRT displays, the light intensity varies nonlinearly with the electron-gun voltage. Altering the input signal by gamma compression can cancel this nonlinearity, such that the output picture has the intended luminance. However, the gamma characteristics of the display device do not play a factor in the gamma encoding of images and video. They need gamma encoding to maximize the visual quality of the signal, regardless of the gamma characteristics of the display device.[1][3] The similarity of CRT physics to the inverse of gamma encoding needed for video transmission was a combination of coincidence and engineering, which simplified the electronics in early television sets.[5]
Photographic film has a much greater ability to record fine differences in shade than can be reproduced on photographic paper. Similarly, most video screens are not capable of displaying the range of brightnesses (dynamic range) that can be captured by typical electronic cameras.[6]For this reason, considerable artistic effort is invested in choosing the reduced form in which the original image should be presented. The gamma correction, or contrast selection, is part of the photographic repertoire used to adjust the reproduced image.
Analogously, digital cameras record light using electronic sensors that usually respond linearly. In the process of rendering linear raw data to conventional RGB data (e.g. for storage into JPEG image format), color space transformations and rendering transformations will be performed. In particular, almost all standard RGB color spaces and file formats use a non-linear encoding (a gamma compression) of the intended intensities of the primary colors of the photographic reproduction. In addition, the intended reproduction is almost always nonlinearly related to the measured scene intensities, via a tone reproduction nonlinearity.
Output to CRT-based television receivers and monitors does not usually require further gamma correction. The standard video signals that are transmitted or stored in image files incorporate gamma compression matching the gamma expansion of the CRT (although it is not the exact inverse). For television signals, gamma values are fixed and defined by the analog video standards. CCIR System M and N, associated with NTSC color, use gamma 2.2; systems B/G, H, I, D/K, K1, L and M associated with PAL or SECAM color use gamma 2.8.[12][13]
In most computer display systems, images are encoded with a gamma of about 0.45 and decoded with the reciprocal gamma of 2.2. A notable exception, until the release of Mac OS X 10.6 (Snow Leopard) in September 2009, were Macintosh computers, which encoded with a gamma of 0.55 and decoded with a gamma of 1.8. In any case, binary data in still image files (such as JPEG) are explicitly encoded (that is, they carry gamma-encoded values, not linear intensities), as are motion picture files (such as MPEG). The system can optionally further manage both cases, through color management, if a better match to the output device gamma is required.
The sRGB color space standard used with most cameras, PCs, and printers does not use a simple power-law nonlinearity as above, but has a decoding gamma value near 2.2 over much of its range, as shown in the plot to the right/above. Below a compressed value of 0.04045 or a linear intensity of 0.00313, the curve is linear (encoded value proportional to intensity), so 2 = 1. The dashed black curve behind the red curve is a standard 3 = 2.2 power-law curve, for comparison.
Gamma correction in computers is used, for example, to display a gamma = 1.8 Apple picture correctly on a gamma = 2.2 PC monitor by changing the image gamma. Another usage is equalizing of the individual color-channel gammas to correct for monitor discrepancies.
Some picture formats allow an image's intended gamma (of transformations between encoded image samples and light output) to be stored as metadata, facilitating automatic gamma correction. The PNG specification includes the gAMA chunk for this purpose[14] and with formats such as JPEG and TIFF the Exif Gamma tag can be used. Some formats can specify the ICC profile which includes a transfer function.
These features have historically caused problems, especially on the web. For HTML and CSS colors and JPG or GIF images without attached color profile metadata, popular browsers passed numerical color values to the display without color management, resulting in substantially different appearance between devices; however those same browsers sent images with gamma explicitly set in metadata through color management, and also applied a default gamma to PNG images with metadata omitted. This made it impossible for PNG images to simultaneously match HTML or untagged JPG colors on every device.[15] This situation has since improved, as most major browsers now support the gamma setting (or lack of it).[16][17]
The following illustration shows the difference between a scale with linearly-increasing encoded luminance signal (linear gamma-compressed luma input) and a scale with linearly-increasing intensity scale (linear luminance output).
On most displays (those with gamma of about 2.2), one can observe that the linear-intensity scale has a large jump in perceived brightness between the intensity values 0.0 and 0.1, while the steps at the higher end of the scale are hardly perceptible. The gamma-encoded scale, which has a nonlinearly-increasing intensity, will show much more even steps in perceived brightness.
where 4 is the Greek letter gamma. For a CRT, the gamma that relates brightness to voltage is usually in the range 2.35 to 2.55; video look-up tables in computers usually adjust the system gamma to the range 1.8 to 2.2,[1] which is in the region that makes a uniform encoding difference give approximately uniform perceptual brightness difference, as illustrated in the diagram at the top of this section.
For simplicity, consider the example of a monochrome CRT. In this case, when a video signal of 0.5 (representing a mid-gray) is fed to the display, the intensity or brightness is about 0.22 (resulting in a mid-gray, about 22% the intensity of white). Pure black (0.0) and pure white (1.0) are the only shades that are unaffected by gamma.
To compensate for this effect, the inverse transfer function (gamma correction) is sometimes applied to the video signal so that the end-to-end response is linear. In other words, the transmitted signal is deliberately distorted so that, after it has been distorted again by the display device, the viewer sees the correct brightness. The inverse of the function above is 5376163bf9
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