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c++qtopenglframebuffer

Draw a transparent framebuffer onto the default framebuffer

I'm facing this situation where I need to render the content of a framebuffer object onto the screen. The screen already has some contents onto it and I would like to draw the contents of my framebuffer onto this content.

I'm using Qt5 and QNanoPainter to implement this.

The rendering commands I've implemented essentially take a QOpenGLFramebufferObject and converts this into a QNanoImage (using this) and then calls QNanoPainter::drawImage.

This works ok but the problem is that when the content of the fbo is rendered onto the screen, the previously existing content of the screen becomes "pixelated". So for example, before I draw the FBO the screen looks like this

enter image description here

Then when I draw the FBO onto the default render target, I get this (red is the content of FBO) enter image description here

I assume this has something to do with blending and OpenGL, but I'm not sure how to solve this problem.

Solution

  • This happens when you over-draw a semi-transparent image over itself multiple times. The white pixels become whiter, the blue pixels become bluer, and, consequently, the anti-aliased edge disappears over a couple iterations.

    I therefore deduce that your 'transparent framebuffer' already contains the blue line and the black grid lines. The solution thus will be to clear the 'transparent framebuffer' before you proceed with drawing the red line into in.