opengl stencil buffer not initialized with zero?

Question

I work under MS WindowsXP,my video card is itel GMA4500, my code:

glClearStencil(0);
glClear(GL_STENCIL_BUFFER_BIT);

unsigned char* data = new unsigned char[width*height];
glPixelStorei(GL_PACK_ALIGNMENT,1);
glReadPixels(0,0,width,height,GL_STENCIL_INDEX,GL_UNSIGNED_BYTE,data);

but when i checked the data buffer, i can see that the bytes are not all zero, so what's the problem?

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YES, I'm sure i have a stencil buffer, and after call glReadPixels, i checked glGetError,there's no error. i also tried memset to fill data buffer with zero, but the result didn't changed.

Answer 1

Do you actually have a stencil buffer? If not, glReadPixels will raise an error (check glGetError(…)) and leave the target buffer's contents unchanged.