I am drawing a rectangle with a texture stretched over it. The texture is a 512x512x4 RGBA image. I draw it orthographic, and I “zoom in/out” by changing the size of the box passed to glOrtho. I’m drawing using software OpenGL.
When zoomed out so the entire texture consumes very few pixels on my monitor, drawing is fast.
When zoomed in so that a small portion of the texture fills the entire monitor, drawing is slow.
When zoomed in, but pan over, so that my monitor shows a peek of the texture – albeit a very zoomed in peek – drawing is fast.
Why is case 2 slow? Why is case 3 fast? What can I do to speed up case 2?
Thanks for your help!
PS: I am writing an app for the Panasonic Toughbook using the Atom processor. The slowdown appears on my fast desktop when hardware acceleration is disabled.