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Chris Weaver

OpenGL GLSL Additive environment mapping of transparent fragments

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Chris Weaver    122
Hi, I'm making my first forays into GLSL and have stumbled across a problem with either my code or expectations of GLSL. I'm simply trying to texture an object with a base texture then to simulate a glossy coating a reflected cube map is added. The base texture has an alpha component and OpenGL has GL_BLEND enabled and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) setup. I originally got this working this using combiners which gets the colour of the previous fragment and adds the environment map to it as follows: // The environment map is modulated with the alpha channel of texture0, then // added to the previous fragment color. glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE4_NV); glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_ADD); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_EXT, GL_PREVIOUS_EXT); glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_EXT, GL_SRC_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB_EXT, GL_ZERO); glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB_EXT, GL_ONE_MINUS_SRC_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_EXT, GL_TEXTURE); glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_EXT, GL_SRC_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE3_RGB_EXT, GL_TEXTURE3_ARB); glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND3_RGB_EXT, GL_SRC_ALPHA); But I can't seem replicate this in GLSL because it looks like the alpha component of the base texture is modulating the environement map's intensisty giving a very dark reflection. I'm guessing this is a result of the blending in later stages? My fragment program is as follows: varying vec4 col; // gl_Color from vertex shader varying vec3 norm; // world normal vector varying vec3 viewVec; // world view vector uniform sampler2D TexUnit0; uniform samplerCube CubeMap; void main(void) { vec4 color = col; // Modulate the vertex colour by the transparent texel colour vec4 basetexture = texture2D(TexUnit0, gl_TexCoord[0].st); color *= basetexture; // Normalize the object space normal and view vector vec3 normViewVec = normalize(viewVec); vec3 envNormal = normalize(norm); // Reflect view vector around the normal vec3 reflVec = reflect(normViewVec, envNormal); // Get environment map contribution for this point vec4 envMapDiffuse = textureCube(CubeMap, -normalize(reflVec).xyz); // Add the environment map to the transparent colour color.rgb += envMapDiffuse.rgb; color = clamp(color, 0.0, 1.0); gl_FragColor = color; } What I need is the environment map to be additive so that even if the base texture is nearly fully transparent (glass) and blended with other polyons, you still get a bright reflection/specular highlight. Is this possible without having to do multiple passes or anything? Help, please! Cheers, Chris.

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zedz    291
yes it should be possible (esp if you have done it with GL_COMBINE4_NV)
im not 100% sure what u want
perhaps write in the color + alpha buffer seperatly
eg
gl_FragColor.xyz = color (from textures etc)
gl_FragColor.w = visability

+ then with glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) gl_FragColor.w will control the opaciniss

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Chris Weaver    122
Hi zedz,

Basically, I want to get rid of the GL_COMBINE4_NV code and replace it with GLSL code as much as possible. To explain it simply, the fragment colour I set in gl_FragColor.rgb is modulated by the opaicity set in gl_FragColor.a, which is fine for the actual basetexture colour because its a property of the material.

However, I don't want the environment/specular reflection to be modulated by the gl_FragColor.a component because it should be ADDED to whatever the final fragement colour is after blending is done. So, adding it to gl_FragColor.rgb isn't working.

The GL_COMBINE4_NV code added the reflection to the PREVIOUS fragment colour but I can't find equivalent functionality in GLSL to make the reflection appear unchanged "on top of" of the final blended result.

I guess what I want is a bit like "seperate specualar" but per-pixel.

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