If a fragment shader would write to its depth value, early depth testing is impossible OpenGL won't be able to figure out the depth value beforehand.ĭepth testing is disabled by default so to enable depth testing we need to enable it with the GL_DEPTH_TEST option:
A restriction on the fragment shader for early depth testing is that you shouldn't write to the fragment's depth value. Whenever it is clear a fragment isn't going to be visible (it is behind other objects) we can prematurely discard the fragment.įragment shaders are usually quite expensive so wherever we can avoid running them we should. Early depth testing allows the depth test to run before the fragment shader runs. Today most GPUs support a hardware feature called early depth testing. This z value is the value that is compared to the depth buffer's content. The gl_FragCoord variable also contains a z-component which contains the depth value of the fragment. The x and y components of gl_FragCoord represent the fragment's screen-space coordinates (with (0,0) being the bottom-left corner). The screen space coordinates relate directly to the viewport defined by OpenGL's glViewport function and can be accessed via GLSL's built-in gl_FragCoord variable in the fragment shader. If the depth test fails, the fragment is discarded.ĭepth testing is done in screen space after the fragment shader has run (and after the stencil test which we'll get to in the next chapter). OpenGL performs a depth test and if this test passes, the fragment is rendered and the depth buffer is updated with the new depth value. When depth testing is enabled, OpenGL tests the depth value of a fragment against the content of the depth buffer. In most systems you'll see a depth buffer with a precision of 24 bits.
#The game creators selecting opengl 32 bit
The depth buffer is automatically created by the windowing system and stores its depth values as 16, 24 or 32 bit floats. The depth-buffer is a buffer that, just like the color buffer (that stores all the fragment colors: the visual output), stores information per fragment and has the same width and height as the color buffer. In this chapter we're going to elaborate a bit more on those depth values the depth buffer (or z-buffer) stores and how it actually determines if a fragment is in front.
In the coordinate systems chapter we've rendered a 3D container and made use of a depth buffer to prevent triangles rendering in the front while they're supposed to be behind other triangles. Depth testing Advanced-OpenGL/Depth-testing
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