If I load an image for a sprite thats lets say 300 x 400, and I then resize the sprite to 75 x 100 on the layout, will I be saving any memory? Will i save any processing on the gpu, or cpu?
Nope. It'll still take up the same amount of VRAM no matter how big or small you stretch it.
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I'm pretty sure I recall David saying that all textures are scaled to a power of 2. So your 300x400 image would be scaled to 512x512 therefore using as much VRAM as a 512x512 texture does.