// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example screenspacereflections
    \ingroup quick3d-examples
    \title Qt Quick 3D - Screen Space Reflections Example
    \examplecategory {3D}
    \brief Demonstrates reflections in Qt Quick 3D.
    \image screenspacereflections-example.jpg
           {Reflective surface with colored spheres and floating objects}

    This example demonstrates how to create reflections using
    \e {Screen Space Reflections} (SSR) on a model. SSR is a post-processing effect
    that can enhance the scene by adding reflections to it. The idea behind SSR is
    that the reflections can be calculated in Screen Space after the objects
    have been rendered. For each fragment, a ray is emitted from the camera
    to this fragment and then it is reflected around the fragment's normal.
    After that, we follow the reflected ray and determine whether it will hit
    an object or not. If an object was hit, then the fragment will reflect
    this object. There are situations where SSR will fail. For example, when
    the reflected ray hits an object behind the camera. Since the reflections are
    calculated in Screen Space after the objects have been rendered, no information
    about the color of the objects behind the camera is available. Although SSR has
    some drawbacks, it adds more realism to the scene.

    This example implements SSR using \l{CustomMaterial}{Custom Materials} that can
    be used on a \l{Model}, which will make it reflect its surroundings.
    \snippet screenspacereflections/main.qml reflectingsurface

    The rest of the scene has some objects which are either static or rotate above
    the surface to show the reflections.
    \snippet screenspacereflections/main.qml scene

    \section1 Shader Code

    Before diving into the shader's code, let's check some parameters that can
    be used to control the reflections.

    \table
    \row
        \li \c depthBias
        \li This parameter is used to check if the difference between
            the depth of the ray and object is within a certain threshold.
    \row
        \li \c rayMaxDistance
        \li Controls how far is the ending point of the ray in View Space.
    \row
        \li \c marchSteps
        \li Controls how many steps are used for computation. Increasing the number of steps
            decreases the amount of fragments the ray moves in each iteration and increases
            the quality.
    \row
        \li \c refinementSteps
        \li After finding where the reflected ray hit the object, a refinement process is done
            to try and find the exact location of the hit. This parameter controls how many
            steps should be used. It can give better results when the \c marchSteps is small.
    \row
        \li \c specular
        \li A value between 0 and 1 to control how much reflectivity the model has.
    \row
        \li \c materialColor
        \li Gives color to the model. This color is mixed with the reflection color.
    \endtable

    The shader starts by getting a direction from the camera to the fragment, and
    then reflects it around the fragment's normal. The starting point and the ending
    point of the ray are calculated in View Space, and then these points are transformed
    into Screen Space. The benefit of marching the reflected ray in Screen Space is that
    it results in better quality. Moreover, the ray could cover a large distance in
    View Space but only a few fragments in Screen Space.

    A vector pointing from the starting fragment to the ending fragment is calculated
    and divided by the \c marchSteps.

    The \c rayMarch function is called afterward. It marches the ray every step in
    Screen Space, and then transforms it back to View Space. It also gets the object
    at this fragment using the \c DEPTH_TEXTURE of the scene. The difference between
    the ray's and object's depths is calculated and compared with the \c depthBias.
    If a hit is found, the \c refinementStep function is called.
    \snippet screenspacereflections/material_screenspacereflections.frag rayMarch

    The refinement step is the same as \c rayMarch, except it tries to find
    the exact position where the hit happens, so it advances the ray by half the
    step's distance every iteration.
    \snippet screenspacereflections/material_screenspacereflections.frag refinementStep

    The visibility of the reflection is set to the hit value, and after that some
    visibility checks are made. As mentioned before, SSR will fail if the reflected ray
    hits something behind the camera, so the visibility is faded out according to
    how much the reflected ray is heading toward the camera. The visibility is also
    faded out according to how far the hit object is from the starting point of
    the ray.
    \snippet screenspacereflections/material_screenspacereflections.frag visibilitycheck

    Finally, the reflection color is calculated from the \c SCREEN_TEXTURE and
    mixed with the material color.
    \snippet screenspacereflections/material_screenspacereflections.frag reflectioncolor

    \section1 Helper functions
    There are some helper functions used in the shader's code.
    The \c correctTextureCoordinates function fixes the texture's coordinate according
    to the used Graphics API. This has to be done in the case of D3D11 or Metal.
    For more information, see \l{CustomMaterial} documentation.
    \snippet screenspacereflections/material_screenspacereflections.frag correctTexture

    The \c rayFragOutOfBound function checks if the ray is outside the screen after
    marching. In this case, no reflection color is used because no information is
    available for anything outside the screen.
    \snippet screenspacereflections/material_screenspacereflections.frag rayoutofbound

    The \c viewPosFromScreen function gets the object's position in View Space
    by using the \c DEPTH_TEXTURE.
    \snippet screenspacereflections/material_screenspacereflections.frag viewposfromscreen

    The \c rayViewDepthFromScreen function gets the current position of the ray in
    View Space. This time the depth value is obtained by linearly interpolating the
    value between the starting point depth and ending point depth of the ray.
    \snippet screenspacereflections/material_screenspacereflections.frag rayviewposfromscreen

*/
