Thank you for your testing!
I could add a gamma parameter. The reason that I haven’t is that the current settings are basically “correct” from a theoretical point of view.
I am using 2.4 as the CRT gamma, which is basically standard. This does result in a slight darkening of the image as compared to the input without a shader, but it should accurately capture the gamma of an average CRT. CRTs varied a lot and people changed their own settings, so if I added a gamma parameter it would probably be for this.
The output gamma is pretty standard, and it should be set to match your screen. Most screens will be a simple 2.2 gamma, but some are actually calibrated to sRGB (with the linear portion in the shadows). They look very similar except for the dark shadows. If this isn’t set to approximately match your screen, you can get weird effects. The tonality of the image will not be accurate, the dither may not work correctly, and the scanline shapes may even be altered.
Regarding the brightness of the shader, if you set the maximum spot size to 1.0 it should be as bright as the original image if it was corrected for the difference between a CRT and modern screen gamma. The scanlines themselves would not actually darken the image at a maximum spot size of 1.0. Any value below that will decrease the brightness proportionally. That’s pretty much the only parameter that currently affects the brightness as a whole. The glow settings can sort of redistribute brightness, but they should have a minor effect on the average brightness of the image. The current mask blending method maintains the brightness of the pre-mask image on average (although, again, it sort of redistributes it). The minimum spot size also will not brighten or dim the image.
The low pass filter in the rgb and svideo presets can also dim small, bright detail. If you turn it up to 6.0MHz or use the monitor preset (which has no low pass filter), you may notice a difference. But this is faithful to the way CRTs worked as well. See @cgwg’s old post here:
Video amplifier. This is somewhat connected with the previous step, since the problem can be treated as an issue of signal processing. Since this applies before the gamma ramp, it can cause darkening of high-contrast areas — this is why gamma charts traditionally use horizontal lines.
The low pass filter is designed to mimic the limited bandwidth of the video amplifier (and other components in the signal path).
One thing that I have been careful about in this shader is not affecting the tonality of the image. The highlights don’t get rolled off and the shadows don’t get crushed or brightened. For example, here’s the output from one of my tests. I generate full screen grayscale images at varying brightness levels (corrected for CRT gamma) and run them through the shader. The x axis is the input brightness and the y axis is the output brightness. The solid, light blue line represents no change in brightness.
In this case, I used a maximum spot size of 0.9, so the actual values are shifted down slightly, representing a slight overall darkening. However, the curve stays linear. Changing the gamma will make this nonlinear, and this would change the tonality of the image.
All this is to say that, basically, it is complicated. The brightness of the shader is always at the maximum level that it can be without affecting the tonality of the image, and the current options guarantee linearity. Adding a simple gamma adjustment to brighten the output will cause details to be lost in the bright areas compared to an actual CRT, as it does in other shaders with such a feature. I don’t mean to rule it out as a future option, but that’s why I haven’t included it so far.
