OpenGL trim/inline contour of stencil

Question

I have created a shape in my stencil buffer (black in the picture below). Now I would like to render to the backbuffer. I would like one texture on the outer pixels (say 4 pixels) of my stencil (red), and an other texture on the remaining pixels (red).

I have read several solutions that involve scaling, but that will not work when there is no obvious center of the shape.

How do I acquire the desired effect?

Answer 1

The stencil buffer works great for doing operations on the specific fragments being overlaid onto them. However, it's not so great for doing operations that require looking at pixels other than the one corresponding to the fragment being rendered. In order to do outlining, you have to ask about the values of neighboring pixels, which stencil operations don't allow.

So, if it is possible to put the stencil data you want to test against in a non-stencil format image (ie: a color image, maybe with an integer texture format), that would make things much simpler. You can do the effect of stencil discarding by using discard directly in the fragment shader. Since you can fetch arbitrarily from the texture (as long as you're not trying to modify it), you can fetch neighboring pixels and test their values. You can use that to identify when a fragment is near a border.

However, if you're relying on specialized stencil operations to build the stencil data itself (like bitwise operations), then that's more complicated. You will have to employ stencil texturing operations, so you're going to have to render to an FBO texture that has a depth/stencil format. And you'll have to set it up to allow you to read from the stencil aspect of the texture. This is an OpenGL 4.3 feature.

This effectively converts it into an 8-bit unsigned integer texture. That allows you to play whatever games you need to. But if you want to use stencil tests to discard fragments, you will also need texture barrier functionality to allow you to read from an image that's attached to the current FBO. But you don't need to actually use the barrier, since you should mask off stencil writing. You just need GL 4.5 or the NV/ARB_texture_barrier extension to be available, which they widely are.

Either way this happens, the biggest difficulty is going to be varying the size of the border. It is easy to just test the neighboring 9 pixels to see if it is at a border. But the larger the border size, the larger the area of pixels each fragment has to test. At that point, I would suggest trying to look for a different solution, one that is based on some knowledge of what pattern is being written into the stencil buffer.

That is, if the rendering operation that lays down the stencil has some knowledge of the shape, then it could compute a distance to the edge of the shape in some way. This might require constructing the geometry in a way that it has distance information in it.