FBO - Render to Texture and Blending

导读：
　　I am working on GeForce 6800 GT and using Cg for shaders.
　　I am using a multi-pass rendering (render-to-texture) using FBO. In first compuatation pass, I render to Texture1 and in the second pass I want to use the values computed in the first pass (as a texture in the fragment shader) and render into a second texture (Texture2).
　　I switch between the textures, by simply attaching the texture to the FBObefore rendering (computing). I am using blendingwhile rendering (see below in the code) as my computation needs it.
　　 Code:
　　//Enable Blending
　　glBlendFunc (GL_ONE, GL_ONE);
　　glEnable (GL_BLEND);
　　glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_RECTANGLE_EXT,Texture1, 0);
　　Computation1() //Render into Texture1
　　//Read back texture 1
　　glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
　　glReadPixels(0, 0, texSize, texSize,textureParameters.texFormat,GL_FLOAT,data);
　　glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_RECTANGLE_EXT,Texture2, 0);
　　Computation2() //Texture Lookup using Texture 1, and render into Texture2
　　//Disable Blending
　　glDisable (GL_BLEND);
　　Rendering works fine (in the first pass) and the results I read back in between the two computation passes show correct results (even when the blendingis on), which means that the Computation1() is correctly done.
　　When I lookup this texture (Texture1 - rendered in Computation1()), in a fragment shader in the Computation2(), however, I get all zero values if the blendingis turned on. If the blendingis disabled, the texture can be read properly (and correctly) in the Computation2().
　　This leads me to believe that, if blendingis enabled, texture readback can
　　be a problem. Is this true or am I missing something ?
　　Thank you!

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http://www.gpgpu.org/forums/viewtopic.php?p=10989&sid=059c0a431529b00258e21a01f925daac