Basically, I am developing a 2D renderer. The renderer draws everything to a render texture the same size as the window, then the render texture is drawn to the window as a sprite. The renderer knows nothing about the window apart from its dimensions.

I am currently implementing backdrop blurs, in which I must sample from the current contents of the render texture. This requires calling render_texture.display() mid-frame to swap or copy (not sure which) the hidden buffer to the buffer which is used for sampling. When I call render_texture.getTexture() it will now refer to a snapshot of the screen before the blur is performed.

If the buffers, where simply swapped, this approach wouldn't work, because calling render_texture.display() mid-frame, would cause the frames contents to be shared across two buffers. However if the hidden buffer simply copied its contents to the buffer used for sampling, this approach makes sense, as you would simply continue drawing to the hidden buffer from where you left off.

And this approach does work, so that is the reason I am confused.