GL_COLOR_LOGIC_OP Oddity.

OpenGL OpenGL: Advanced Coding
1

Hello,

I have been experiencing an odd result when rendering with GL_COLOR_LOGIC_OP enabled.

The problem is as follows:

  • First I render a fairly large terrain surface in 2d. Once the surface has been drawn, I want to be able to draw a few simple lines on top of the surface without having to re-draw the surface. Using overlay planes was not an option, so I am using the following bit of code to draw the lines:

    glDrawBuffer( GL_FRONT );
    glEnable( GL_COLOR_LOGIC_OP );
    glLogicOp( GL_COPY );

    // draw a few lines

    glDisable( GL_COLOR_LOGIC_OP );
    glDrawBuffer( GL_BACK );

  • This works as expected (the lines are visible on the surface ) except when I swap between the front and back frame buffers.

  • I expected the lines to be drawn into the front frame buffer, thus requiring an eventual re-draw of the surface to clear those lines from that buffer. However if I swap the buffers twice, the line will no longer be visible. I do not understand why the front buffer is not modified by the line drawing. Any ideas?

Dan

2

I think you want to use polygon offset - I’m doing the exact same thing you are, and it works for me:

First, draw your lines as you would normally.

Next:
glEnable(GL_POLYGON_OFFSET);
glPolygonOffset(1.0,1.0); // example values
// Draw your filled polys as normal.
glDisable(GL_POLYGON_OFFSET);

Some notes from my own testing: This works best when you have a lot of resolution in your Z-buffer - that is, make sure your near plane is as far away from the camera as possible and your far plane is as close to it as possible.

Jason/Maunikar

3

What is the point of using the GL_COPY logic op? It’s equivalent to not using logic op at all.

  • Matt
4

Good point Matt, there is actually no need to use GL_COPY, I just happened to use a bad example in the code snippet. I have the same result no matter what logic op is used. Originaly, I was using GL_XOR in order to clear the lines without re-drawing the main surface. This worked fine, however I was trying to do a similar thing but with the ability to specify specific colors for the lines. It was at this point that I noticed that any drawing I do with GL_COLOR_LOGIC_OP enabled will be erased after the buffer swaps. This enables me to do what I want, however I do not understand why.

Dan

5

>>

  • This works as expected (the lines are visible on the surface ) except when I swap between the front and back frame buffers.

  • I expected the lines to be drawn into the front frame buffer, thus requiring an eventual re-draw of the surface to clear those lines from that buffer. However if I swap the buffers twice, the line will no longer be visible. I do not understand why the front buffer is not modified by the line drawing. Any ideas?
    <<

Very simple, the swap in your currently selected pixelformat does not exchange the front and back buffers, but copies the back buffer to the front buffer.

Before swap:
Front: FF
Back : BB

After swap:
Front: BB
Back : BB (or undefined, depending on swap method if not PFD_SWAP_COPY)

Now your lines are gone.

[This message has been edited by Relic (edited 01-29-2001).]