Chroma Key Shader
Posted: Sun Apr 28, 2024 10:47 am
Hi, i have an ogv video with a black background because ogv dont support alpha channel. How can i create a shader that remove the black color of my video.
The reason I'm asking is that while you can try to derive the alpha channel from the RGB channels, it's a lossy process.
If your background color is black and you want to have soft degrees of transparency, this means that the darker a pixel is, the more transparent it'll be, and the only 100% opaque pixel is a fully white one -- anything less than white will have some level of transparency, especially dark colors, even if in the original scene they'd be opaque. So a dark but fully opaque object will end up looking transparent in your chroma-keyed video.
If you didn't create the video or if you can't have it as a PNG image sequence (that is, if you really need to derive alpha from RGB in a shader), then you can simply plug the Y component of the video into the alpha channel.
Löve internally uses 3 textures for a video, each for the Y, Cb and Cr components of the video, and the Y texture has the luma (the brightness) of each pixel in the video. That's your alpha channel.
You'd create a pixel shader with this code:
Code: Select all
void effect() {
vec4 videoColor = VideoTexel(VaryingTexCoord.xy) * VaryingColor;
float alpha = Texel(love_VideoYChannel, VaryingTexCoord.xy).r;
love_PixelColor = vec4(videoColor.xyz, alpha);
}