Currently, I've got a GLSL shader which can outline individual sprites by finding the edges of the sprite texture, however I've found this to be insufficient in the case where I have two or more sprites that are layered on top of each other to form a single sprite, because each individual layer is being outlined, while what I want is for the whole thing to be outlined as if it were a single sprite. For example, imagine a character sprite with a separate hat sprite rendered on top, I don't want all the hat's edges to be outlined because the bottom outline will overlap the player head.
An idea I had for a solution is, for each sprite that is to be outlined, instead of rendering each sprite layer to the screen individually, I will create a frame buffer object to use as an offscreen texture where I will render each of the sprite layers. Then, I will render that offscreen texture to the screen instead and outline it.
Does this idea make sense, or is it the wrong approach? If I'm understanding correctly, I could potentially be creating a lot of frame buffer objects, as each outlined sprite would need its own surface, and I believe I would also have to create a VBO and IBO to go with each of them too, is my understanding correct here? Furthermore, outlines will likely be turning on and off throughout the game, so I am wonder if I should just have an ever growing list of FBOs, or should I create and delete them only as they are needed? Another scenario I just realized is the overall sprite size could change (imagine the previous example with that hat, you might swap a small hat for a very tall hat), so in this case I think I would have to delete the offscreen surface and recreate it using the new size.
