CODEWITHSUNDEEP
arraysopenglpixel
Silverlan
Silverlan

Reputation: 2911

OpenGL - glReadPixels returns incorrect values

I'm probably missing something obvious, but I can't find anything wrong with the code. It's for testing purposes, and all it's supposed to do is set the color of all pixels to white and read the pixel information. The result is an array full of 0s.

    unsigned int frameBuffer;
    glGenFramebuffers(1,&frameBuffer);
    unsigned int texture;
    unsigned int depthTexture;
    glGenTextures(1,&texture);
    glGenTextures(1,&depthTexture);

    glBindFramebuffer(GL_FRAMEBUFFER,frameBuffer);
    glBindTexture(GL_TEXTURE_2D,texture);

    int w = 256;
    int h = 256;

    glTexImage2D(
        GL_TEXTURE_2D,
        0,
        GL_RGB,
        w,h,
        0,GL_RGB,
        GL_UNSIGNED_BYTE,
        0
    );
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
    glFramebufferTexture2D(GL_FRAMEBUFFER,GL_COLOR_ATTACHMENT0,GL_TEXTURE_2D,texture,0);

    glBindTexture(GL_TEXTURE_2D,depthTexture);
    glTexImage2D(
        GL_TEXTURE_2D,
        0,
        GL_DEPTH_COMPONENT16,
        w,h,
        0,GL_DEPTH_COMPONENT,
        GL_FLOAT,
        0
    );
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
    glFramebufferTexture2D(GL_FRAMEBUFFER,GL_DEPTH_ATTACHMENT,GL_TEXTURE_2D,depthTexture,0);

    int status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
    if(status == GL_FRAMEBUFFER_COMPLETE)
    {
        unsigned char *pixels = new unsigned char[w *h *3];
        for(unsigned int i=0;i<(w *h *3);i++)
            pixels[i] = 255;
        glDrawPixels(w,h,GL_RGB,GL_UNSIGNED_BYTE,&pixels[0]);
        glReadPixels(0,0,w,h,GL_RGB,GL_UNSIGNED_BYTE,&pixels[0]);
        for(unsigned int i=0; i<(w *h *3); i+=3)
        {
            unsigned int r = pixels[i];
            unsigned int g = pixels[i + 1];
            unsigned int b = pixels[i + 2];
            std::cout<<i<<": "<<r<<","<<g<<","<<b<<std::endl;
        }
        delete[] pixels;
        int err = glGetError(); // No error reported
    }
    glBindTexture(0,GL_TEXTURE_2D);
    glBindFramebuffer(0,GL_FRAMEBUFFER);

No errors reported and the frame buffer is fine. What's going on here?

Upvotes: 1

Views: 2176

Answers (2)

Silverlan
Silverlan

Reputation: 2911

Someone found out the cause on a different forum.

The solution is to call

glReadBuffer(GL_COLOR_ATTACHMENT0);

before calling glReadPixels, and

glReadBuffer(GL_BACK);

afterwards, to reset it.

Upvotes: 3

Jon Simpkins
Jon Simpkins

Reputation: 152

Try making sure that the raster position is valid for glDrawPixels before the call:

glRasterPos(0,0);
glDrawPixels(w,h,GL_RGB,GL_UNSIGNED_BYTE, pixels);
glReadPixels(0,0,w,h,GL_RGB,GL_UNSIGNED_BYTE, pixels);

Upvotes: 1

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