BTW, is the depth testing on the Particle System "none" or "read only"? I've always found it irritating that it can't be set for read/write depth testing (even though it's really antiquated anyway... don't know why I would expect depth testing).
-George Toledo On Sun, Jun 20, 2010 at 12:34 PM, George Toledo <gtole...@gmail.com> wrote: > That *really* shed light on why that works how it does, in a way that > totally makes sense to me. You're better than the Orange Book... > > -George Toledo > > On Sun, Jun 20, 2010 at 11:07 AM, Christopher Wright > <cwri...@kineme.net>wrote: > >> Thanks cwright! I get it now. I wasn't putting the gl_FragColor line in >>> the right place.... really obvious oversight. >>> >>> I'm attaching the "working as expected" shader. (I'm still scratching my >>> head a little bit about why, if color is alpha=0 already in the vertex >>> shader, and my source texture is composited over alpha, why this is >>> necessary.) >>> >> >> I think you're thinking in more complicated texturing terms than is >> strictly necessary -- the blend mode affects the entire object (the final >> compositing of whatever the fragment shader spits out), not the input >> texture or the input color. Alpha blend mode (unofficial) simply tells >> the GPU to do the premultiplication itself. This is a bit slower, but >> simplifies writing shaders. (And in fact, if you need to do the >> premultiplication in the fragment shader anyway, it's actually the same >> speed -- the premultiplication needs to happen somewhere, so that cost gets >> paid somewhere). >> >> To do alpha blending, you perform this mathematical operation (Alpha blend >> mode tells the GPU to use this equation): >> >> outputColor = destinationColor * (1. - sourceAlpha) + sourceColor * >> sourceAlpha; (you provide sourceColor and sourceAlpha directly by setting >> gl_FragColor or gl_FragData[0]) >> >> This cost 2 multiplies, and an add per pixel ["1. - sourceAlpha" is often >> "free" since it's usually just a binary inversion in 8 bit mode] (back in >> the day, this was a gigantic cost - I remember writing assembly blitters >> that did this sort of thing when I was a kid, before graphics hardware was >> around to make this irrelevant). >> >> So some people thought about the problem, and they realized that you could >> pre-calculate the "sourceColor * sourceAlpha" part, since that's constant. >> Thus, "premultiplication" was born. Your textures don't store sourceColor >> anymore, they store sourceColor * sourceAlpha. this, of course, has the >> negative side effect of losing color fidelity when alpha is less than 1 -- >> for example, when alpha is 0, color is 0 (you've lost all the color >> information). In practice, this typically isn't a problem, and it saves a >> multiply per pixel, which is a lot of work. >> >> The equation then becomes the following (Over blend mode uses this): >> outputColor = destinationColor * (1. - sourceApha) + sourceColor; >> >> So if you feed it a sourceColor that _isn't_ premultiplied, the results >> are wrong (they kinda blend, but they're kinda opaque too). >> >> Add blend mode uses the following: >> outputColor = destinationColor + sourceColor; this is even simpler (0 >> multiplies!), but disregards alpha entirely.) >> >> So, when you're setting colors in QC, typically it does the >> premultiplication for you, and never need to know/care. However, it cannot >> do that in the shader for you, since you're taking control of the GPU >> directly at that point. >> >> >> >> Too bad that the shader is blowing up your machine ( I wonder if the >>> classic vertexnoise glsl would do the same, as it's largely the same). >>> >> >> Not sure -- I commented out all the dead code, and it started working, so >> I have no idea. *shrugs* >> >> >> -- >> [ christopher wright ] >> cwri...@kineme.net >> http://kineme.net/ >> >> >
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