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Depth Value VS Z coordinate? I'm confused...

For Beginners OpenGL 3D

Started by babaliaris January 21, 2019 01:02 PM

2 comments, last by Koen 5 years, 2 months ago

babaliaris

136

Author

January 21, 2019 01:02 PM

Hello!

I'm currently learning how the depth testing works in OpenGL from these tutorials and the tutorial says that


By default the depth function GL_LESS is used that discards all the fragments that have a depth value higher than or equal to the current depth buffer's value.

If i guess that the depth value it the z coordinate that I pass through the vertex data, then the above statement should not be true. Fragments with small Z values should be discarded because the depth is towards the -Z axis not fragments with higher z value. Does the depth values are created somehow else by using the z coordinate of the fragment? So the depth value is a number from 0...N so lets say a fragment has a depth value of 5 and the one that is behind it has 10, the 5 will pass the test?


void life()
{
  while (!succeed())
    try_again();

  die_happily();
}

1024

170

January 21, 2019 01:58 PM

Depth values are between 0 and 1. The z coordinates get mapped to this range with a formula, so that 0 is closest and 1 is furthest away. When comparison happens with GL_LESS, a fragment that is in front (closer, has a lower depth value) will pass and be rendered, and a fragment that is in the back (further, has a higher depth value) will be discarded, so it won't get rendered. In short, fragments that are behind other fragments will be skipped.

Koen

713

January 21, 2019 02:00 PM

If you set up the projection matrix the "default way", it will transform the near plane to -1 (or 0 for Direct3D) and the far plane to 1 (for both apis). If you'd prefer the other way around, you can do that too, but you'll have to properly build your projection matrix for your convention.

edit: reversing might even be a good idea: see this blog post

Depth Value VS Z coordinate? I'm confused...

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Depth Value VS Z coordinate? I'm confused...

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