问题
I was trying to recover some vertex data from vertex shader, but I haven't found any relevant information about this on the internet.
I'm using the vertex shader to calculate my vertex positions using the GPU, but I need to get the results for the logic of my application in Javascript. Is there a possible way to do this without calculating it in Javascript too?
回答1:
In WebGL2 you can use transform feedback (as Pauli suggests) and you can read back the data with getBufferSubData although ideally, if you're just going to use the data in another draw call you should not read it back as readbacks are slow.
Transform feedback simply means your vertex shader can write its output to a buffer.
In WebGL1 you could do it by rendering your vertices to a floating point texture attached to a framebuffer. You'd include a vertex id attribute with each vertex. You'd use that attribute to set gl_Position. You'd draw with gl.POINT. It would allow you to render to each individual pixel in the output texture effectively letting you get transform feedback. The difference being your result would end up in a texture instead of a buffer. You can kind of see a related example of that here
If you don't need the values back in JavaScript then you can just use the texture you just wrote to as input to future draw calls. If you do need the values back in JavaScript you'll have to first convert the values from floating point into a readable format (using a shader) and then read the values out using gl.readPixel
回答2:
Transform feedback is OpenGL way to return vertex processing results back to application code. But that is only available with webgl 2. Transform feedback also outputs primitives instead of vertices making it unlikely to be perfect match.
A newer alternative is image load store and shader storage buffer objects. But I think those are missing from webgl 2 too.
In short you either need to calculate same data in javascript or move your application logic to shaders. If you need transformed vertex data for coalition detection you could use bounding box testing and do vertex level transformation only when bounding box hits.
You could use multi level bounding boxes where you have one big box around whole object and then next bounding box level that splits object in to small parts like separate box for each disjoint part in body (for instance, split in knee and ankle in legs). That way javascript mainly only transform single bounding box/sphere for every object in every frame. Only transform second level boxes when objects are near. Then do per vertex transformation only when objects are very close to touch.
来源：https://stackoverflow.com/questions/40198663/is-it-possible-to-get-the-actual-value-of-a-vertex