// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example volumetric
    \meta tags {DataVisualization, QCustom3DVolume, Custom Item}
    \title Volumetric Rendering
    \ingroup qtdatavisualization_examples
    \brief Rendering volumetric objects.
    \since QtDataVisualization 1.2

    \e {Volumetric Rendering} shows how to use QCustom3DVolume to display volumetric data.

    \image volumetric-example.png
           {Volumetric rendering of 3D terrain with water and land layers}

    \include examples-run.qdocinc

    \section1 Initializing Volume Item

    The QCustom3DVolume items are special custom items (see QCustom3DItem), which can be used
    to display volumetric data. The volume items are only supported with orthographic projection,
    so first make sure the graph is using it:

    \snippet volumetric/volumetric.cpp 0

    Create a volumetric item tied to the data ranges of the axes:

    \snippet volumetric/volumetric.cpp 1

    Indicate that the scaling of the volume should follow the changes in the data ranges by setting
    the QCustom3DItem::scalingAbsolute property to \c{false}. Next, define the internal contents of
    the volume:

    \snippet volumetric/volumetric.cpp 2

    Use eight bit indexed color for the texture, as it is compact and makes it easy to adjust the
    colors without needing to reset the whole texture. For the texture data, use the data created
    earlier based on height maps.
    Typically, the data for volume items comes pregenerated in the form of a stack of images, so
    the data generation details can be skipped. For more information about the actual data
    generation process, see the example code.

    Since eight bit indexed colors are used, a color table is needed to map the eight bit color
    indexes to actual colors. In a typical use case, you would get the color table from the source
    images instead of using one manually defined:

    \snippet volumetric/volumetric.cpp 3

    To have an option to show slice frames around the volume, initialize their properties.
    Initially, the frames will be hidden:

    \snippet volumetric/volumetric.cpp 4

    Finally, add the volume as a custom item to the graph to display it:

    \snippet volumetric/volumetric.cpp 5

    \section1 Slicing into the Volume

    Unless the volume is largely transparent, you can only see the surface of it, which is often
    not very helpful. One way to inspect the internal structure of the volume is to view the slices
    of the volume. QCustom3DVolume provides two ways to display the slices. The first is to show
    the selected slices in place of the volume. For example, to specify a slice perpendicular to
    the X-axis, use the following method:

    \snippet volumetric/volumetric.cpp 6

    To display the slice specified above, the QCustom3DVolume::drawSlices property must also be set:

    \snippet volumetric/volumetric.cpp 7

    The second way to view slices is to use QCustom3DVolume::renderSlice() method, which produces
    a QImage from the specified slice. This image can then be displayed on another widget, such
    as a QLabel:

    \snippet volumetric/volumetric.cpp 8

    \section1 Adjusting Volume Transparency

    Sometimes, viewing just the slices doesn't give you a good understanding of the volume's
    internal structure. QCustom3DVolume provides two properties that can be used to adjust the
    volume transparency:

    \snippet volumetric/volumetric.cpp 9
    \dots
    \snippet volumetric/volumetric.cpp 10

    The QCustom3DVolume::alphaMultiplier is a general multiplier that is applied to the alpha value
    of each voxel of the volume. It makes it possible to add uniform transparency to the already
    somewhat transparent portions of the volume to reveal internal opaque details. This multiplier
    doesn't affect colors that are fully opaque, unless the QCustom3DVolume::preserveOpacity
    property is set to \c{false}.

    An alternative way to adjust the transparency of the volume is to adjust the alpha values of the
    voxels directly. For eight bit indexed textures, this is done simply by modifying and resetting
    the color table:

    \snippet volumetric/volumetric.cpp 11

    \section1 High Definition vs. Low Definition Shader

    By default, the volumetric rendering uses a high definition shader. It accounts for each
    voxel of the volume with the correct weight when ray-tracing the volume contents, providing an
    accurate representation of even the finer details of the volume.
    However, this is computationally very expensive, so the frame rate suffers.
    If rendering speed is more important than pixel-perfect accuracy of the volume contents, take
    the much faster low definition shader into use by setting QCustom3DVolume::useHighDefShader
    property \c{false}. The low definition shader achieves the speed by making compromises on
    accuracy, so it doesn't guarantee that every voxel of the volume will be sampled. This can lead
    to flickering or other rendering artifacts on the finer details of the volume.

    \snippet volumetric/volumetric.cpp 12

    \section1 Example Contents
*/
