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Alpha as user var for collision detection

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I''m trying to use the alpha value for collision detection for a small 2d game. Every object will be drawned additionally to the rgb color with an individual alpha value using vertex4f. Alphatesting is disabled. Before drawing the moving bullets, I''m looking for the alpha value at the bullet''s new position in order to check, if there''s already an object. If there''s one, e.g. I can deactivate the object known by the local alpha value. That''s the theory. In praxis, readpixels(...RGBA...)always returns 0.0 for alpha. But obviously there is an object drawned before at the position, as the rgb value differs from 0/0/0 (but strangely only every fourth frame update). Do I have to make some initialisation for using the alpha channel? I''m working with doublebuffer and activated RGBA with 32bit. Thanks in advance for any hints! Regards Oliver

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The only thing that comes to mind is make sure you have Destination Alpha enabled...(Alpha Buffer in the Pixel Format Descriptor)

PIXELFORMATDESCRIPTOR pfd = //pfd Tells Windows How We Want Things To Be
sizeof (PIXELFORMATDESCRIPTOR), // Size Of This Pixel Format Descriptor
1, // Version Number
PFD_DRAW_TO_WINDOW | // Format Must Support Window
PFD_SUPPORT_OPENGL | // Format Must Support OpenGL
PFD_DOUBLEBUFFER, // Must Support Double Buffering
PFD_TYPE_RGBA, // Request An RGBA Format
window->init.bitsPerPixel,// Select Our Color Depth
0, 0, 0, 0, 0, 0, // Color Bits Ignored
0, // Shift Bit Ignored
0, // No Accumulation Buffer
0, 0, 0, 0, // Accumulation Bits Ignored
24, // 24Bit Z-Buffer (Depth Buffer)
8, // 8Bit Stencil Buffer
0, // No Auxiliary Buffer
PFD_MAIN_PLANE, // Main Drawing Layer
0, // Reserved
0, 0, 0 // Layer Masks Ignored

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Thanks, I''ve tried this, but without any success.

Anyway, I''ve written out the set bits used by red, green, blue and alpha within a pixel (glGetIntegerv), and always got 8, 8, 8 and 0. Thus, alpha isn''t activated yet.
I''ve also set then accum-bits to 0, and got 16. My PixelFormatDescriptor is identical to yours.

Any further hints ?



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