深度检查OpenGL（glsl）

Question

我想知道是否有一种方法可以知道是否显示了特定的像素（例如，我想获取场景后方的所有位置，这意味着未在近平面中显示的所有背景对象）。

Answer 1

It's probably faster and easier to do some simple calculations yourself CPU side. For example...

clipSpacePoint = projectionMatrix * viewMatrix * modelMatrix * vec4f(0, 0, 0, 1); //using the model's origin and transform matrix
//or
clipSpacePoint = projectionMatrix * viewMatrix * vec4f(x, y, z, 1); //using the model's position

clipSpacePoint.xyz /= clipSpacePoint.w; //possible division by zero

//point is visible if clipSpacePoint.w is positive and clipSpacePoint.xyz are between -1 and 1

Alternatively, and to better answer your question, you can use the occlusion query to check how many fragments have been rendered during a draw call. An example application might be checking if a bright light is visible and drawing a lens flare if it is. The occlusion query, like many OpenGL calls, run asynchronously so if you need the results right away you can stall the rendering pipeline. If you don't mind waiting a frame or two for the result it may run a bit faster.

• http://www.opengl.org/wiki/Query_Object#Occlusion_queries

This won't tell you where the pixels were drawn though. Two ways to find the pixel locations come to mind...

1. You could write pixel coordinates in your fragment shader to a texture, use the histopyramid /stream compaction method to move them to the start of the texture, and read them back from the GPU.

2. If you don't mind using recent GL features, ARB_atomic_counter can be used in the fragment shader to create a unique index which you could then use to write the fragment's coordinates to a buffer or texture with ARB_image_load_store. You'll probably also want to enable the early depth test for this too.

These are both far more complex than doing the point in box check or an occlusion query.