How expensive is OpenGL Texture Binding

Question

I'm currently working on a renderer for Tiled's .tmx maps. Among one of the features there is that layers and tilesets are independent, so that a layer can use any tile set on any tile. It's not so much of a problem, though, as my Image type is basically:

data Image = Image { imageTexture :: TextureObject
                   , imageMinTex2 :: TexCoord2
                   , imageMaxTex2 :: TexCoord2
                   } deriving (Show, Eq)

Drawing an image is therefore

1. Bind texture
2. Render Quad using min/max texture coords

A quite simple implementation. However, if every Tile is an Image, in my implementation, then this would mean that on every tile, a texture is bound. Often times it will be the same, and quite often just a few tiles have entirely different texture.

So how expensive is a texture binding in OpenGL? Do I have to sort the Tiles by texture or will the OpenGL drivers do that for me?

Answer 1

Guessing a bit here but from the little information you gave it sounds like you are thinking of something like the following

for each layer
    for each tile
        bind texture
        draw quad
    end
end

If that is the case this is the most inefficient way of drawing, and you will not get any good performance out of this because you end up with a draw call per tile rather than a draw call per tileset.

You should batch your draw calls, a simple implementation could be something like the following

// draw
for each layer
    for each tile
        add to render queue
    end
 end

 // flush
 sort render queue items by texture
 let buffers be index/vertex buffers on the GPU with a fixed size.

 for each item in render queue
    if item texture is not bound or buffers are full
        bind texture
        draw buffers
    end

    append item to buffers
 end