Question

I need to do some CPU operations on the framebuffer data previously drawn by openGL. Sometimes, the resolution at which I need to draw is higher than the texture resolution, therefore I have thought about picking a SIZE for the viewport and the target FBO, drawing, reading to a CPU bufffer, then moving the viewport somewhere else in the space and repeating. In my CPU memory I will have all the needed colordata. Unfortunately, for my purposes, I need to keep an overlap of 1 pixel between the vertical and horizontal borders of my tiles. Therefore, imagining a situation with four tiles with size SIZE x SIZE:

0 1
2 3

I need to have the last column of data of tile 0 holding the same data of the first column of data of tile 1, and the last row of data of tile 0 holding the same data of the first row of tile 2, for example. Hence, the total resolution I will draw at will be

SIZEW * ntilesHor -(ntilesHor-1) x SIZEH * ntilesVer -(ntilesVer-1)

For semplicity, SIZEW and SIZEH will be the same, and the same for ntilesVer and ntilesHor. My code now looks like

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
glViewport(0, 0, tilesize, tilesize);
glPolygonMode(GL_FRONT, GL_FILL);

for (int i=0; i < ntiles; ++i)
{
    for (int j=0; j < ntiles; ++j)
    {
        tileid = i * ntiles +j;

        int left = max(0, (j*tilesize)- j);
        int right = left + tilesize;
        int bottom = max(0, (i*tilesize)- i);
        int top = bottom + tilesize;

        glEnable(GL_DEPTH_TEST);
        glDepthFunc(GL_LESS);

        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(left, right, bottom, top, -1, 0);   

        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // Draw display list
        glCallList(DList);

        // Texture target of the fbo
        glReadBuffer(tex_render_target);

        // Read to CPU to preallocated buffer
        glReadPixels(0, 0, tilesize, tilesize, GL_BGRA, GL_UNSIGNED_BYTE, colorbuffers[tileid]);
    }
}

The code runs and in the various buffers “colorbuffers” I seem to have what looks like colordata, and also similar to what I should have given my draw; only, the overlap I need is not there, namely, last column of tile 0 and first column of tile 1 yield different values.
Any idea?

Answer 1

    int left = max(0, (j*tilesize)- j);
    int right = left + tilesize;
    int bottom = max(0, (i*tilesize)- i);
    int top = bottom + tilesize;

I’m not sure about those margins. If your intention is a pixel based mapping, as suggested by your viewport, with some constant overlap, then the -j and -i terms make no sense, as they’re nonuniform. I think you want some constant value there. Also you don’t need that max there. You want a 1 pixel overlap though, so your constant will be 0. Because then you have

right_j == left_(j+1)

and the same for bottom and top, which is exactly what you intend.