I am looking to create a glsl shader program that will give me a variable width outline around an arbitrary 2d texture as shown in the picture. Is this a reasonable job for the GPU? I've looked at edge-detection approaches but those would only reasonably provide a few pixels border. I want arbitrary width. Is this doable?
One approach would be to render the object to a texture with a bit larger scale and color it all the way your outline color should be. Then you render it to another texture as it should be displayed. In a third render pass you could then combine the 2 textures by choosing the outline texture, when the color-texture is empty and else the color texture. This could be an expensive process, but it obviously depends on the scope of your project if that impacts performance too much.