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/****************************************************************************
**
** Copyright (C) 1993-2009 NVIDIA Corporation.
** Copyright (C) 2017 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of Qt 3D Studio.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
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** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
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** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
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****************************************************************************/
/*!
\title Disable Depth Test
\page best-practices-disable-depth-test.html
\ingroup qt3dstudio-best-practices
This page describes the
\c{Disable Depth Test} property of Layers edited in Qt 3D Studio.
\image DisableDepthTest.png
The Disable Depth Test property can be used to simultaneously override
depth testing and the order objects are rendered. The operation of this
property is not as obvious as many of the other properties in the system
so this page offers some insight into when, where, and why this property
might be used.
\section1 What Does It Do?
By default, the Disable Depth Test property is unchecked
(\c{false}). This is the normal mode of operation. In this mode
rendering is done in two passes. First, reading and writing from the
depth buffer is enabled and all opaque objects are rendered from front
to back (objects closer to camera are rendered first). In the second
pass, writing to the depth buffer is disabled and all transparent
(non-opaque) objects are rendered from back to front.
The first pass allows for optimal z-culling for normal opaque 3D objects
and the second pass allows for correct blending of transparent objects.
By checking the Disable Depth Test property - setting it to
\c{true} - a different single pass rendering mode is enabled for
the Layer. In this mode the depth buffer is not used at all, and all
objects are rendered in a single back to front pass.
\section1 Why?
Each setting for this property has pros and cons.
\section2 Disable Depth Test: \c{false}
\b{Pros:}
\list
\li
This mode ensures the best visual quality because the two pass
rendering technique enables the correct rendering of overlapping and
intersecting 3D objects as well as correct blending of transparent
objects.
\li
Performance can be higher in this mode if there are opaque objects
which occlude other objects. Skipping the rendering of pixels which
are hidden behind fully opaque objects is a common and powerful 3D
rendering optimization known as z-culling.
\endlist
\b{Cons:}
\list
\li
The two pass rendering technique can be more expensive in some
scenarios.
\endlist
\section2 Disable Depth Test: \c{true}
\b{Pros:}
\list
\li
This mode can increase performance if there are very few (or no)
objects benefiting from z culling. In cases with little overlap it can
be faster to render with the one pass technique.
\endlist
\b{Cons:}
\list
\li
In some scenarios, rendering errors may be introduced and/or
performance can suffer in this mode. When objects intersect or overlap
each other depth testing and the two pass rendering technique are
necessary to achieve the correct results. (For more details, see
\l{http://en.wikipedia.org/wiki/Painter\%27s\_algorithm}{\"Painters
Algorithm\" on Wikipedia}.
\endlist
\section1 Conclusion
The Disable Depth Test property on Layers is an advanced property that
can be used to increase performance under certain specific scenarios. It
can be activated per-Layer to enable a separate one pass rendering
technique that can achieve higher performance than the default rendering
technique by eliminating the overhead of doing expensive depth testing
in cases where 3D objects do not overlap or intersect.
This property should be enabled with some care. Though it is possible to
increase performance by enabling this property on some Layers it is also
possible to introduce undesirable rendering errors and even in some
cases adversely affect performance. Remember to measure thoroughly when
optimizing.
*/
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