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rhi: Improve isClipDepthZeroToOne() doc 

Quick 3D is now having a use case for calling this directly, so use this
opportunity for enhancing the docs to make it clear what this is about.

Change-Id: I19ec956ac25175dbfb754192abaad8d65a6eeb0e
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
bb10
Laszlo Agocs 2020-01-28 16:08:34 +01:00
parent 71cf207b54
commit 75ad13d2bc
1 changed files with 24 additions and 9 deletions
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33
src/gui/rhi/qrhi.cpp
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@ -5058,13 +5058,24 @@ bool QRhi::isYUpInNDC() const
}
/*!
\return \c true if the underlying graphics API uses depth 0 - 1 in clip
space.
\return \c true if the underlying graphics API uses depth range [0, 1] in
clip space.
In practice this is \c false for OpenGL only.
In practice this is \c false for OpenGL only, because OpenGL uses a
post-projection depth range of [-1, 1]. (not to be confused with the
NDC-to-window mapping controlled by glDepthRange(), which uses a range of
[0, 1], unless overridden by the QRhiViewport) In some OpenGL versions
glClipControl() could be used to change this, but the OpenGL backend of
QRhi does not use that function as it is not available in OpenGL ES or
OpenGL versions lower than 4.5.
\note clipSpaceCorrMatrix() includes the corresponding adjustment in its
returned matrix.
returned matrix. Therefore, many users of QRhi do not need to take any
further measures apart from pre-multiplying their projection matrices with
clipSpaceCorrMatrix(). However, some graphics techniques, such as, some
types of shadow mapping, involve working with and outputting depth values
in the shaders. These will need to query and take the value of this
function into account as appropriate.
*/
bool QRhi::isClipDepthZeroToOne() const
{
@ -5074,11 +5085,15 @@ bool QRhi::isClipDepthZeroToOne() const
/*!
\return a matrix that can be used to allow applications keep using
OpenGL-targeted vertex data and perspective projection matrices (such as,
the ones generated by QMatrix4x4::perspective()), regardless of the
backend. Once \c{this_matrix * mvp} is used instead of just \c mvp, vertex
data with Y up and viewports with depth range 0 - 1 can be used without
considering what backend and so graphics API is going to be used at run
time.
the ones generated by QMatrix4x4::perspective()), regardless of the active
QRhi backend.
In a typical renderer, once \c{this_matrix * mvp} is used instead of just
\c mvp, vertex data with Y up and viewports with depth range 0 - 1 can be
used without considering what backend (and so graphics API) is going to be
used at run time. This way branching based on isYUpInNDC() and
isClipDepthZeroToOne() can be avoided (although such logic may still become
required when implementing certain advanced graphics techniques).
See
\l{https://matthewwellings.com/blog/the-new-vulkan-coordinate-system/}{this