Display Gamma should be Higher than 2.2
I'm cross posting this because it pertains even more to digital displays. CRT's when correctly set up inherently have a gamma response which is about right. Digitals need to emulate that behavior to make their images correct, but we keep seeing all sorts of problems. The most glaring I think is the recurrent idea that 2.2 is the ideal display gamma. It actually should be higher to properly emulate what is seen on the CRT monitor of the telecine operator.
Originally posted by sspears
A Sony BVM is usually the display of choice during color correction with the film to video transfer. The Sony BVM24 HD monitor actually has a gamma of ~2.7. It also has SMPTE C phospor.
The roughly gamma 2.5 ideal CRT response is such that it slighly over degammas the 0.45 gamma of the source signal. The overall system gamma response is supposed to be about 1.1 to 1.2 to allow dark surround viewing. If you make the display response too linear (lower gamma) the scenes will have their midrange values too bright and the image will appear too "flat."
I quote some useful material from Poynton http://www.poynton.com/notes/colour_and_gamma/
The actual value of gamma for a particular CRT may range from about 2.3 to 2.6. Practitioners of computer graphics often claim numerical values of gamma quite different from 2.5. But the largest source of variation in the nonlinearity of a monitor is caused by careless setting of the Black Level (or Brightness ) control of your monitor. Make sure that this control is adjusted so that black elements in the picture are reproduced correctly before you devote any effort to determining or setting gamma . display transfer characteristic (gamma, ?),” EBU Technical Review 257 (Autumn 1993), 32–40..... "
"......Now, here’s a surprise. If a film system is designed and processed to produce exactly linear reproduction of intensity, reflection prints look fine. But projected transparencies – slides and movies – look flat, apparently lacking in contrast! The reason for this involves another aspect of human visual perception: the surround effect.
As explained in Adaptation, on page 85, human vision adapts to an extremely wide range of viewing conditions. One of the mechanisms involved in adaptation increases our sensitivity to small brightness variations when the area of interest is surrounded by bright elements. Intuitively, light from a bright surround can be thought of as spilling or scattering into all areas of our vision, including the area of interest, reducing its apparent contrast. Loosely speaking, the vision system compensates for this effect by “stretching” its contrast range to increase the visibility of dark elements in the presence of a bright surround. Conversely, when the region of interest is surrounded by relative darkness, the contrast range of the vision system decreases: Our ability to discern dark elements in the scene decreases. The effect is demonstrated in Figure 6.4 above, from DeMarsh and Giorgianni.
The surround effect has implications for the display of images in dark areas, such as projection of movies in a cinema, projection of 35 mm slides, or viewing of television in your living room. If an image is viewed in a dark or dim surround, and the intensity of the scene is reproduced with correct physical intensity, the image will appear lacking in contrast.LeRoy E. DeMarsh and Edward J. Giorgianni, “Color Science for Imaging Systems,” in Physics Today, September 1989, 44–52. "
"As explained in Gamma in film, on page 96, it is important for perceptual reasons to “stretch” the contrast ratio of a reproduced image when viewed in a dim surround. The dim surround condition is characteristic
of television viewing. In video, the “stretching” is accomplished at the camera by slightly undercompensating the actual power function of the CRT to obtain an end-to-end power function with an exponent of 1.1 or 1.2. This achieves pictures that are more subjectively pleasing than would be produced by a mathematically correct linear system.
Rec. 709 specifies a power function exponent of 0.45. The product of the 0.45 exponent at the camera and the 2.5 exponent at the display produces the desired end-to-end exponent of about 1.13. An exponent of 0.45 is a good match for both CRTs and for perception."
Figure 6.6 above illustrates the transfer function defined by the international Rec. 709 standard for high-definition television (HDTV). It is basically a power function with an exponent of 0.45. Theoretically a pure power function suffices for gamma correction; however, the slope of a pure power function is infinite at zero. In a practical system such as a television camera, in order to minimize noise in the dark regions of the picture it is necessary to limit the slope (gain) of the function near black. Rec. 709 specifies a slope of 4.5 below a tristimulus value of +0.018, and stretches the remainder of the curve to maintain function and tangent continuity at the breakpoint. In this equation the red tristimulus (linear light) component is denoted R, and the resulting gamma-corrected video signal is denoted with a prime symbol, R’709. The computation is identical for the other two components"
The difference between the SMPTE 240M and Rec. 709 transfer functions is negligible for real images. It is a shame that international agreement could not have been reached on the SMPTE 240M parameters that were widely implemented at the time the CCIR (now ITU-R) discussions were taking place. The Rec. 709 values are closely representative of current studio practice, and should be used for all but very unusual conditions.
An idealized monitor inverts the transform: Real monitors are not as exact as this equation suggests, and have no linear segment, but the precise definition is necessary for accurate intermediate processing in the linear-light domain. In a color system, an identical transfer function is applied to each of the three tristimulus (linearlight) RGB components. See Frequently Asked Questions about Color . Incidentally, the nonlinearity of a CRT is a function of the electrostatics of the cathode and the grid of an electron gun; it has nothing to do with the phosphor. Also, the nonlinearity is a power function [which has the form f( x ) = x a ], not an exponential function [which has the form f( x ) = a x ]. For more detail, read the Gamma chapter in Poynton’s book .
Does NTSC use a gamma of 2.2? Television is usually viewed in a dim environment. If an images’s correct physical intensity is reproduced in a dim surround, a subjective effect called simultaneous contrast causes the reproduced image to appear lacking in contrast, as demonstrated above. The effect can be overcome by applying an end-to-end power function whose exponent is about 1.1 or 1.2. Rather than having each receiver provide this correction, the assumed 2.5-power at the CRT is under-corrected at the camera by using an exponent of about 1 ? 2.2 instead of 1 ? 2.5. The assumption of a dim viewing environment is built into video coding.
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In other words, an ideal display gamma is actually higher than 2.2 but closer to 2.5 so the overall system has a positive gamma. This keeps getting messed up in digital implementations because designers see the 2.2 in the spec and forget that 2.2 isn't the actual display gamma but the inverse which if used makes the system (incorrectly) linear. A real CRT display's transfer characteristics should be emulated, not a 2.2 power function. It's close to a 2.5 power function, but most accurately given by the inverse of the rec 709 as below.
(Now let's watch this drop like a rock out of the bottom of the forum. This is the sort of stuff that Stacey and I mean when we say that some our best postings go ignored on the boards.)
Attachment: rec 709 for monitor.jpg
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