I have new shader files here (direct link to zip).
I’d say this is a pretty experimental release. It contains a few things.
Mask support
The shaders now have basic mask support. Currently only aperture grille is supported, and only for monitors with RGB or BGR subpixel arrangements. There is no “mask strength” parameter yet. Masks fade out as the area gets brighter so that they do not reduce the total brightness at all. (@kokoko3k, I ended up using the cubic blending as a “safer” default option.)
New presets: crt-beans-rgb and crt-beans-svideo
The original shader is now used in two preset files: crt-beans-rgb.slangp and crt-beans-svideo.slangp. They are the same set of shaders, but the S-Video preset has the low pass filter tuned to simulate S-Video signal bandwidth. Different consoles may have output signals with different bandwidths, so I just sort of picked some values that seemed like a reasonable default.
New preset: crt-beans-gaussian
There is a new crt-beans-gaussian.slangp preset. This is similar to the previous RGB preset, but uses a gaussian shape for the CRT spot. I would guess that this is more accurate to real CRTs. The image is slightly blurrier than the RGB preset because the gaussian has fatter tails.
The major advantage of this is that the spot width (and thus the scanline width) can be pushed to 1.2. You can basically get fatter, more overlapping lines. This allows simulating smaller, lower quality CRTs. The normal RGB preset doesn’t work properly with widths above 1.0 (that’s just a mathematical consequence of the spot function it uses).
The disadvantage is that the gaussian preset is much slower than the RGB preset. Because the gaussian’s tails are fatter, each area affects more pixels around it, which means a lot more texture samples are necessary. The scanline pass itself takes about 2x the time of the original, and I’d guess the whole shader takes about 1.6x the time of the original (I don’t have the numbers in front of me, currently).
New preset: crt-beans-monitor (no low pass filter)
There is a new crt-beans-monitor.slangp preset. The goal of this preset is to allow inputs other than 240p/480i or 288p/576i (basically, other than 15khz television). Thus the “monitor” name—it can be used for content at vga/480p and beyond.
This is a little complicated to explain. The original shader can’t be used without the low pass filter. It solves the scanline integral numerically, which requires the low pass filter to resample the input. This new preset solves the scanline integral analytically, which, conversely, requires that there is no resampling of the input. The input is interpreted as a piecewise-constant function and the integral is solved in pieces.
When resampling, I need to pick a number of samples to output for each line (a new image width, basically). The number of samples required changes based on the number of lines there are (the input image height). Unfortunately there is no way to do this within the limits of the slangp preset format. This, and another subtle issue with the presentation of the low pass frequencies parameters, led me to remove the low pass filter as a way to accommodate higher input resolutions.
There are two downsides to this. First is that there is no low pass filter, so you lose that capability. The output is a bit sharper horizontally than it probably should be. Second is that it is slightly slower. The analytical solution generally requires fewer texture samples but significantly more math. The worst performance is for very elongated pixel aspect ratios. For example, a 640x240 input will lead to worse performance than a 640x480 input.
Final thoughts
I may be able to upload some sample images tomorrow (the Streets of Rage screenshots above are pretty representative of the RGB preset).
I don’t know if there is really any interest in the gaussian or monitor presets. They might end up being a bit more confusing. They were interesting to make, though!
