This post illustrates the effects of some features of the PROPixx projector, when displaying content meant for LCD screens, for example a movie.
A summary of possible solutions:
- ignore the issue and have colors rendered too bright, too saturated and with shifted hues
- correct the colors staying at 8 bits per color and full resolution, but introducing quantization errors, noticeable especially in dark areas
- correct the colors and use the C48 high bit depth mode, with half horizontal resolution
Briefly, the source of the issue is that the PROPixx does not render colors like a typical computer LCD monitor. It has “linear luminance” (gamma = 1), whereas a typical computer screen has a gamma of about 2.2, and BT.709 (SDR) videos are meant to be decoded with a gamma of 2.4 (Rec. 709 - Wikipedia, ITU-R BT.1886 - Wikipedia). Furthermore, its its color primaries are slightly different (the actual spectral characteristics of red, green and blue), leading to a generally wider color gamut (it can display more saturated colors) than the usual color space of computer content.
The following screenshots aim to illustrate how this would affect a movie, and the different solutions. They were generated such that viewing them on your monitor should approximate how it would look on the PROPixx. However, this is not exact and some effect appear slightly stronger or milder here than in reality.
First, here is the original image (capture of a decoded movie frame). It should be very close to how it was intended to look, though as mentioned, your LCD probably has a gamma of 2.2 and it may have been intended to have a gamma of 2.4.
If displayed without further processing, it will appear very “washed out” on the PROPixx.
If we only correct the gamma, some strange colors appear, especially in the darker areas, due to the limited precision of 8 bits per color (quantization errors).
This makes “banding” appear in the dark gradients, e.g. in the clouds:
The colors would also appear even more saturated and “pinker” than can be reproduced here on a normal screen. After converting the color space to account for the PROPixx gamut, the colors are much closer to the original, but some banding remains (not illustrated here).
The final solution is to also use a high bit depth mode (C48) on the PROPixx to avoid quantization errors in the previous conversions. However, it reduces the horizontal resolution by half. This is not too noticeable at a normal viewing distance.
The banding is actually practically gone on the PROPixx in this mode, compared to how it appears here.
Finally, a close-up showing slight blurring from the reduced resolution:
I’ll later post further details on how to apply these corrections, either with PsychToolbox in Matlab, or more generally with the use of an OpenGL shader.