Render to texture problem with alpha
When I render to texture, and then draw the same image, it seems to make everything darker. To get this image:
http://img24.imageshack.us/img24/8061/87993367.png
I'm rendering the upper-left square with color (1, 1, 1, .8) to a texture, then rendering that texture, plus the middle square (same color) to another texture, then finally that texture plus the lower-right square (same color) to the screen.
As you can see, each time I render to texture, everything gets a little darker.
My render-to-texture code looks like: (I'm using OpenGL ES on the iPhone)
// gen framebuffer
GLuint framebuffer;
glGenFramebuffersOES(1, &framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
// gen texture
GLuint texture;
glGenTextures(1, &texture);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glBindTexture(GL_TEXTURE_2D, 0);
glDisable(GL_TEXTURE_2D);
// hook it up
glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, texture, 0);
if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES))
return false;
// set up drawing
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glViewport(0, 0, Screen::Width, Screen::Height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, Screen::Width, 0, Screen::Height, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor4f(1, 1, 1, 1);
// do whatever drawing we'll do here
Draw();
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);
Is there anything that I'm doing wrong here? Do you need more code to figure it out? What might be going on here?
I'm only guessing:
Drawing the first texture gives you 204 (0.8 * 255) in the RGB and alpha channels. When you draw for the second time (with GL_BLEND enabled, I presume), you're getting the light gray 204 RGB with 80% alpha which gives you a medium gray.
Solution: use glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) and premultiply your colors.
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