// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!

\title Qt Quick 3D Introduction with glTF Assets
\page quick3d-asset-intro

The \l{Qt Quick 3D - Introduction} example provides a quick introduction to
creating QML-based applications with Qt Quick 3D, but it does so using only
built-in primitives, such as spheres and cylinders. This page provides an
introduction using \l{https://en.wikipedia.org/wiki/GlTF#glTF_2.0}{glTF 2.0}
assets, using some of the models from the
\l{https://github.com/KhronosGroup/glTF-Sample-Models}{Khronos glTF Sample
Models repository}.

\section1 Our Skeleton Application

Let's start with the following application. This code snippet is runnable as-is
with the \c qml command-line tool. The result is a very green 3D view with
nothing else in it.

\qml
    import QtQuick
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.Color
                clearColor: "green"
            }

            PerspectiveCamera {
                id: camera
            }

            WasdController {
                controlledObject: camera
            }
        }
    }
\endqml

\image assetintro_empty.jpg

\section1 Importing an Asset

We are going to use two glTF 2.0 models from the Sample Models repository:
Sponza and Suzanne.

These models typically come with a number of texture maps and the mesh
(geometry) data stored in a separate binary file, in addition to the .gltf
file:

\image assetintro_sponza_dir.jpg

How do we get all this into our Qt Quick 3D scene?

There are a number of options:

\list

\li Generate QML components that can be instantiated in the scene. The
command-line tool to perform this conversion is the \l{Balsam Asset Import
Tool}{Balsam} tool. Besides generating a .qml file, that is effectively a
subscene, this also repacks the mesh (geometry) data into an optimized,
fast-to-load format, and copies the texture map image files as well.

\li Perform the same using \c balsamui, a GUI frontend for \l{Balsam Asset
Import Tool}{Balsam}.

\li If using \l{Qt Design Studio},
the asset import process is integrated into the visual design tools. Importing
can be triggered, for example, by dragging and dropping the .gltf file onto the
appropriate panel.

\li For glTF 2.0 assets in particular, there is a runtime option as well: the
\l RuntimeLoader type. This allows loading a .gltf file (and the associated
binary and texture data files) at runtime, without doing any pre-processing via
tools such as \l{Balsam Asset Import Tool}{Balsam}. This is very handy in
applications that wish to open and load user-provided assets. On the other
hand, this approach is significantly less efficient when it comes to
performance. Therefore, we will not be focusing on this approach in this
introduction. Check the \l{Qt Quick 3D - RuntimeLoader Example} for an example
of this approach.

\endlist

Both the \c balsam and \c balsamui applications are shipped with Qt, and should
be present in the directory with other similar executable tools, assuming Qt
Quick 3D is installed or built. In many cases, running \l{Balsam Asset Import
Tool}{balsam} from the command-line on the .gltf file is sufficient, without
having to specify any additional arguments. It is worth being aware however of
the many command-line, or interactive if using \c balsamui or Qt Design Studio,
options. For example, when working with \l{quick3d-lightmap}{baked lightmaps to
provide static global illumination}, it is likely that one will want to pass
\c{--generateLightmapUV} to get the additional lightmap UV channel generated at
asset import time, instead of performing this potentially consuming process at
run-time. Similarly, \c{--generateMeshLevelsOfDetail} is essential when it is
desirable to have simplified versions of the meshes generated in order to have
\l{Qt Quick 3D Level of Detail}{automatic LOD} enabled in the scene. Other options
allow generating missing data (e.g. \c{--generateNormals}) and performing various
optimizations.

In \c balsamui the command-line options are mapped to interactive elements:
\image assetintro_balsamui_options.jpg

\section2 Importing via balsam

Let's get started! Assuming that the
\l{https://github.com/KhronosGroup/glTF-Sample-Models} \c git repository is
checked out somewhere, we can simply run balsam from our example application
directory, by specifying an absolute path to the .gltf files:

\c{balsam c:\work\glTF-Sample-Models\2.0\Sponza\glTF\Sponza.gltf}

This gives us a \c{Sponza.qml}, a \c{.mesh} file under the \c meshes
subdirectory, and the texture maps copied under \c maps.

\note This qml file is not runnable on its own. It is a \e component, that
should be instantiated within a 3D scene associated with a \l View3D.

Our project structure is very simple here, as the asset qml files live right
next to our main .qml scene. This allows us to simply instantiate the Sponza
type using the \l{QML Documents}{standard QML component system}. (at run-time
this will then look for Sponza.qml in the filesystem)

Just adding the model (subscene) is pointless however, since by default the
materials feature the full \l{quick3d-pbr}{PBR lighting calculations}, so
nothing is shown from our scene without a light such as \l DirectionalLight, \l
PointLight, or \l SpotLight, or having \l{Using Image-Based
Lighting}{image-based lighting} enabled via
\l{SceneEnvironment::lightProbe}{the environment}.

For now, we choose to add a DirectionalLight with the default settings. (meaning
the color is \c white, and the light emits in the direction of the Z axis)

\qml
    import QtQuick
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.Color
                clearColor: "green"
            }

            PerspectiveCamera {
                id: camera
            }

            DirectionalLight {
            }

            Sponza {
            }

            WasdController {
                controlledObject: camera
            }
        }
    }
\endqml

Running this with the \c qml tool will load and run, but the scene is all empty
by default since the Sponza model is behind the camera. The scale is also not
ideal, e.g. moving around with WASD keys and the mouse (enabled by the
\l WasdController) does not feel right.

To remedy this, we scale the Sponza model (subscene) by \c 100 along the X, Y,
and Z axis. In addition, the camera's starting Y position is bumped to 100.

\qml
    import QtQuick
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.Color
                clearColor: "green"
            }

            PerspectiveCamera {
                id: camera
                y: 100
            }

            DirectionalLight {
            }

            Sponza {
                scale: Qt.vector3d(100, 100, 100)
            }

            WasdController {
                controlledObject: camera
            }
        }
    }
\endqml

Running this gives us:

\image assetintro_sponza_first.jpg

With the mouse and the WASDRF keys we can move around:

\image assetintro_sponza_second.jpg

\image assetintro_sponza_out.jpg

\note We mentioned \c{subscene} a number of times above as an alternative to
"model". Why is this? While not obvious with the Sponza asset, which in its
glTF form is a single model with 103 submeshes, mapping to a single \l Model
object with 103 elements in its \l{Model::materials}{materials list}, an asset
can contain any number of \l{Model}{models}, each with multiple submeshes and
associated materials. These Models can form parent-child relationships and can
be combined with additional \l{Node}{nodes} to perform transforms such as
translate, rotate, or scale. It is therefore more appropriate to look at the
imported asset as a complete subscene, an arbitrary tree of \l{Node}{nodes},
even if the rendered result is visually perceived as a single model. Open the
generated Sponza.qml, or any other QML file generated from such assets, in a
plain text editor to get an impression of the structure (which naturally always
depends on how the source asset, in this case the glTF file, was designed).

\section2 Importing via balsamui

For our second model, let's use the graphical user interface of \c balsam instead.

Running \c balsamui opens the tool:

\image assetintro_balsamui_startup.jpg

Let's import the
\l{https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/Suzanne}{Suzanne}
model. This is a simpler model with two texture maps.

\image assetintro_balsamui_open.jpg

As there is no need for any additional configuration options, we can just
Convert. The result is the same as running \c balsam: a Suzanne.qml and some
additional files generated in the specific output directory.

\image assetintro_balsamui_convert.jpg

From this point on, working with the generated assets is the same as in the
previous section.

\qml
    import QtQuick
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.Color
                clearColor: "green"
            }

            PerspectiveCamera {
                id: camera
                y: 100
            }

            DirectionalLight {
            }

            Sponza {
                scale: Qt.vector3d(100, 100, 100)
            }

            Suzanne {
                y: 100
                scale: Qt.vector3d(50, 50, 50)
                eulerRotation.y: -90
            }

            WasdController {
                controlledObject: camera
            }
        }
    }
\endqml

Again, a scale is applied to the instantiated Suzanne node, and the Y position
is altered a bit so that the model does not end up in the floor of the Sponza
building.

\image assetintro_suzanne_first.jpg

All properties can be changed, bound to, and animated, just like with Qt Quick.
For example, let's apply a continuous rotation to our Suzanne model:

\qml
            Suzanne {
                y: 100
                scale: Qt.vector3d(50, 50, 50)
                NumberAnimation on eulerRotation.y {
                    from: 0
                    to: 360
                    duration: 3000
                    loops: Animation.Infinite
                }
            }
\endqml

\section1 Making it Look Better

\section2 More light

Now, our scene is a bit dark. Let's add another light. This time a \l
PointLight, and one that casts a shadow.

\qml
    import QtQuick
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.Color
                clearColor: "green"
            }

            PerspectiveCamera {
                id: camera
                y: 100
            }

            DirectionalLight {
            }

            Sponza {
                scale: Qt.vector3d(100, 100, 100)
            }

            PointLight {
                y: 200
                color: "#d9c62b"
                brightness: 5
                castsShadow: true
                shadowFactor: 75
            }

            Suzanne {
                y: 100
                scale: Qt.vector3d(50, 50, 50)
                NumberAnimation on eulerRotation.y {
                    from: 0
                    to: 360
                    duration: 3000
                    loops: Animation.Infinite
                }
            }

            WasdController {
                controlledObject: camera
            }
        }
    }
\endqml

Launching this scene and moving the camera around a bit reveals that this
is indeed starting to look better than before:

\image assetintro_suzanne_morelight.jpg

\section2 Light debugging

The \l PointLight is placed slightly above the Suzanne model. When designing
the scene using visual tools, such as Qt Design Studio, this is obvious, but
when developing without any design tools it may become handy to be able to
quickly visualize the location of \l{Light}{lights} and other \l{Node}{nodes}.

This we can do by adding a child node, a \l Model to the \l PointLight. The
position of the child node is relative to the parent, so the default \c{(0, 0,
0)} is effectively the position of the \l PointLight in this case. Enclosing
the light within some geometry (the built-in cube in this case) is not a
problem for the standard real-time lighting calculations since this system has
no concept of occlusion, meaning the light has no problems with traveling
through "walls". If we used \l{quick3d-lightmap}{pre-baked lightmaps}, where
lighting is calculated using raytracing, that would be a different story. In
that case we would need to make sure the cube is not blocking the light,
perhaps by moving our debug cube a bit above the light.

\qml
        PointLight {
            y: 200
            color: "#d9c62b"
            brightness: 5
            castsShadow: true
            shadowFactor: 75
            Model {
                source: "#Cube"
                scale: Qt.vector3d(0.01, 0.01, 0.01)
                materials: PrincipledMaterial {
                    lighting: PrincipledMaterial.NoLighting
                }
            }
        }
\endqml

Another trick we use here is turning off lighting for the material used with
the cube. It will just appear using the default base color (white), without
being affected by lighting. This is handy for objects used for debugging and
visualizing purposes.

The result, note the small, white cube appearing, visualizing the position of
the \l PointLight:

\image assetintro_suzanne_cube.jpg

\section2 Skybox and image-based lighting

Another obvious improvement is doing something about the background. That green
clear color is not quite ideal. How about some environment that also
contributes to lighting?

As we do not necessarily have suitable HDRI panorama image available, let's use
a procedurally generated high dynamic range sky image. This is easy to do with
the help of \l ProceduralSkyTextureData and \l{Texture}'s support for non-file
based, dynamically generated image data. Instead of specifying
\l{Texture::source}{source}, we rather use the
\l{Texture::textureData}{textureData} property.

\qml
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.SkyBox
            lightProbe: Texture {
                textureData: ProceduralSkyTextureData {
                }
            }
        }
\endqml

\note The example code prefers defining objects inline. This is not mandatory,
the SceneEnvironment or ProceduralSkyTextureData objects could have also been
defined elsewhere in the object tree, and then referenced by \c id.

As a result, we have both a skybox and improved lighting. (the former due to
the \l{SceneEnvironment::backgroundMode}{backgroundMode} being set to SkyBox
and \l{SceneEnvironment::lightProbe}{light probe} being set to a valid \l
Texture; the latter due to \l{SceneEnvironment::lightProbe}{light probe} being
set to a valid \l Texture)

\image assetintro_sponza_ibl.jpg

\image assetintro_sponza_ibl_2.jpg

\section1 Basic Performance Investigations

To get some basic insights into the resource and performance aspects of the
scene, it is a good idea to add a way to show an interactive \l DebugView item
early on in the development process. Here we choose to add a \l Button that
toggles the \l DebugView, both anchored in the top-right corner.

\qml
    import QtQuick
    import QtQuick.Controls
    import QtQuick3D
    import QtQuick3D.Helpers

    Item {
        width: 1280
        height: 720

        View3D {
            id: view3D
            anchors.fill: parent

            environment: SceneEnvironment {
                backgroundMode: SceneEnvironment.SkyBox
                lightProbe: Texture {
                    textureData: ProceduralSkyTextureData {
                    }
                }
            }

            PerspectiveCamera {
                id: camera
                y: 100
            }

            DirectionalLight {
            }

            Sponza {
                scale: Qt.vector3d(100, 100, 100)
            }

            PointLight {
                y: 200
                color: "#d9c62b"
                brightness: 5
                castsShadow: true
                shadowFactor: 75
                Model {
                    source: "#Cube"
                    scale: Qt.vector3d(0.01, 0.01, 0.01)
                    materials: PrincipledMaterial {
                        lighting: PrincipledMaterial.NoLighting
                    }
                }
            }

            Suzanne {
                y: 100
                scale: Qt.vector3d(50, 50, 50)
                NumberAnimation on eulerRotation.y {
                    from: 0
                    to: 360
                    duration: 3000
                    loops: Animation.Infinite
                }
            }

            WasdController {
                controlledObject: camera
            }
        }

        Button {
            anchors.right: parent.right
            text: "Toggle DebugView"
            onClicked: debugView.visible = !debugView.visible
            DebugView {
                id: debugView
                source: view3D
                visible: false
                anchors.top: parent.bottom
                anchors.right: parent.right
            }
        }
    }
\endqml

\image assetintro_suzanne_debugview.jpg

This panel shows live timings, allows examining the live list of texture maps
and meshes, and gives an insight into the render passes that need to be
performed before the final color buffer can be rendered.

Due to making the \l PointLight a shadow casting light, there are multiple
render passes involved:

\image assetintro_suzanne_debugview_2.jpg

In the \c Textures section we see the texture maps from the Suzanne and Sponza
assets (the latter has a lot of them), as well as the procedurally generated
sky texture.

\image assetintro_suzanne_debugview_3.jpg

The \c Models page presents no surprises:

\image assetintro_suzanne_debugview_4.jpg

On the \c Tools page there are some interactive controls to toggle
\l{DebugSettings::wireframeEnabled}{wireframe mode} and various
\l{DebugSettings::materialOverride}{material overrides}.

Here with wireframe mode enabled and forcing rendering to only use the
\l{PrincipledMaterial::baseColor}{base color} component of the materials:

\image assetintro_suzanne_override.jpg

This concludes our tour of the basics of building a \l{Qt Quick 3D} scene with imported assets.

*/
