On Sun, Jun 20, 2010 at 6:48 AM, Chris Wood <pson...@gmail.com> wrote:
> George, > > I think your shader is the problem here (although I've not got the plugin > installed to be sure). You're setting the vertex colours to red with alpha > at 0.0, then in the fragment shader you're ADDING the vertex colour to the > texture colour. This totally messes up the colours, and of course if you > change the alpha value in the vertex shader to 1.0 you get 1.0 + texture > alpha, which means the whole quad is opaque. > I am totally aware that this is what I'm doing; I want to add color (with alpha at 0), to a texture that has large areas of alpha. So, I don't see why color add shouldn't work fine.... I guess I'm dubious about this "messing up" colors, but I am willing to suspend my lack of disbelief. It's just that in other paradigms alpha+alpha would be alpha. > The 'blobby' look is probably down to alpha premultiplication (or lack of). > Try something like: > > vec4 output = color + Color; // urgh at the naming there! ;D > output.rgb *= output.a; > return output; > > Maybe that will fix it. > This suggestion doesn't work for me, because it isn't legal in the shader (getting an "output is reserved" message...). Thanks for the suggestion though. I knew I would get some crap from someone about the "(color+Color)" naming ;-) -George -George Toledo > > On 20 Jun 2010, at 08:37, George Toledo wrote: > > > > > > > I hope I'm asking this question in the right place, and there are sort of > two separate issues at play - alpha calculation in GLSL shaders, and then > Over/Alpha Blend modes. > > > > Is there support for alpha in GLSL shaders in QC? > > > > In a vertex shader, I have the following line: > > > > Color = vec4(surfaceColor.rgb, 1.0); > > > > When I change it to: > > > > Color = vec4(surfaceColor.rgb, 0.0); > > > > I see an unexpected difference in the alpha content on the final textured > object in Over mode. While I do see it change from "quad" look to having > areas of obvious transparency, it looks clearly wrong. SO, I had the bright > idea to change the line... > > > > When I change it to : > > > > Color = vec4(surfaceColor.rgba); > > > > I don't see any errors, but I see no difference when I change > Opacity/Alpha value on the actual shader input for the surfaceColor. I would > think that would be the "fix". However, it doesn't work completely (the > color of the texture looks "blobby" in areas where alpha should be > fading....more smoothly?) > > > > The only workaround I've found is to use actual alpha blending on the > renderer, in this case, by using the Kineme AlphaBlend.plugin. > > > > Then, I see a clear difference between the two different alpha channel > values, and by setting alpha to 0 in the shader, things work as expected. > > > > This is a sample composition; I apologize, because it's sort of > non-standard, because one would need to install the AlphaBlend.plugin for > this to render as I would expect it should in QC normally (in Over mode). I > understand if no one is thrilled to look at this because of that, but it's > not a question about a bug being caused by a 3rd party plugin (in this case, > it's the only way to see what I expect to in Over with the shader at an > alpha of 0, or with the surfaceColor.rgba line). > > > > -George Toledo > > > > <Shaded and Textured Particles Alpha Blend.qtz> > _______________________________________________ > > Do not post admin requests to the list. They will be ignored. > > Quartzcomposer-dev mailing list (Quartzcomposer-dev@lists.apple.com > ) > > Help/Unsubscribe/Update your Subscription: > > > http://lists.apple.com/mailman/options/quartzcomposer-dev/psonice%40gmail.com > > > > This email sent to pson...@gmail.com > >
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