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

/*!

\title Anti-Aliasing Best Practices
\page quick3d-asset-conditioning-anti-aliasing

Qt Quick 3D has multiple ways to combat aliasing (the jagged edges) while
rendering 3D models. Each technique offers its own benefits and limitations.
Multiple techniques can be combined, but with additional performance cost.

\section1 Aliasing in General

Aliasing occurs when there is more \e information present in the original than
we can represent in the pixels on screen. Anti-aliasing techniques fall into
three categories:

\list

\li Techniques that find additional information for a single pixel and
represent them all at the same time.

\li Image effects that attempt to find where things look bad and sweep the
problems under the carpet.

\li Techniques employed by artists that attempt to workaround the limitations.

\endlist

Although anti-aliasing is a useful tool in rendering graphics, it could affect
performance of your application if not used wisely. The following sections
describe a few different anti-aliasing techniques to choose from. Understanding
which technique best targets your problems will help balance visual quality
with sufficient rendering speed.

\section2 Geometry Aliasing

By default, all geometry is rendered one on-screen pixel at a time. As you can
see on the left in the image below, this leaves harsh edges that may be easily
noticeable in high-contrast cases, most noticeably here with black-and-white.

\image AA-GeometryAliasing.png {Comparison of five antialiasing
       techniques on rotated squares, circles, and text}
\caption Effective techniques for reducing aliasing for geometry

The most correct fix for this is to use \l{multisample-aa}{Multisample
Anti-Aliasing}, as it gathers more geometric detail only as needed. Using
\l{temporal-aa}{Temporal Anti-Aliasing} or \l{progressive-aa}{Progressive
Anti-Aliasing} can also mitigate the issue in a correct manner.

Finally, in certain cases you can use a \l{silhouette-opacity-maps}{silhouette
opacity map} to smooth the edges of the geometry.

\target texture-aliasing
\section2 Texture Aliasing

When a texture is sub-sampled, fewer pixels than in the original are displayed,
resulting in undesirable artifacts based on which pixels are chosen. This
effect is worsened when the model is moving, as different pixels are chosen at
different times. In the image below, notice how the line between E3 and F3 is
missing, strongly present between G3 and H3, then gone for the next 5 columns,
and so on.

\image AA-TextureAliasing.png {Comparison of four antialiasing
       techniques on receding grid texture}
\caption Effective techniques for reducing aliasing for textures

The simplest (and arguably the best) fix for this problem is to use
\l{mipmaps}{mipmapping in the image texture} itself. Alternative fixes include
using either \l{temporal-aa}{Temporal AA} or \l{progressive-aa}{Progressive AA}
to gather more information from the texture.

Using \l{multisample-aa}{Multisample Anti-Aliasing} will not fix this problem.

\target reflection-aliasing
\section2 Reflection Aliasing

Similar to \l{texture-aliasing}{Texture Aliasing}, a material reflecting the environment will sub-sample the image.
In some cases, as seen on the left in the image below, it becomes obvious when fine details are being
skipped.

\image AA-ReflectionAliasing.png
       {Comparison of four techniques for reducing reflection aliasing}
\caption Effective techniques for reducing aliasing for reflections

The most correct solution in this case is using \l{temporal-aa}{Temporal AA} or
\l{progressive-aa}{Progressive AA} to find the extra information.

A simple alternative solution that may be acceptable is to make the material less glossy, more
rough. In this case lower-resolution mipmaps of the environment are automatically used, blending
the sharp details together.

\section1 Anti-Aliasing Techniques in Qt Quick 3D

\note Check out the \l{Qt Quick 3D - Antialiasing Example}{Antialiasing
Example} and the \l{Qt Quick 3D - Scene Effects Example}{Scene Effects Example}
to exercise some of these features live. Keep in mind however that modern
windowing systems are often configured to perform \l{High DPI}{High DPI
scaling} with a high resolution screen connected. This means that the content
of any window shown on screen is rendered at a higher resolution and is then
scaled down by the system compositor or some other component of the platform.
That is in effect a form of \l{supersample-aa}{Supersample Anti-aliasing}.
Enabling antialiasing techniques in Qt Quick 3D may then show smaller, or
sometimes hard-to-see improvements, because aliasing is already eliminated to a
degree by the windowing system's automatic scaling. However, when deploying the
same application on another system, it could well be that that particular
system uses a platform or a screen where there is no such scaling, and so
aliasing and jagged edges are more visible out of the box. Developers are
advised to consider the potential presence, or lack of high DPI scaling in
their target environments, and experiment with and tune antialiasing settings
with this in mind.

Below is an example rendering of the
\l{https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/Sponza}{Sponza}
scene with some antialiasing methods enabled. The screenshots were taken
without any system scaling applied to the window (no high DPI scaling), so the
effects of the various methods are more pronounced.

\table
\header
\li AA used
\li Result
\row
\li No AA
\li \image aa_disabled.jpg {Architectural scene with no antialiasing
           showing jagged edges}
\row
\li Supersample AA, high (1.5x)
\li \image aa_ssaa_high.jpg {Architectural scene with supersample
           antialiasing at 1.5x}
\row
\li Multisample AA, high (4x)
\li \image aa_msaa_high.jpg {Architectural scene with multisample
           antialiasing at 4x}
\row
\li FXAA
\li \image aa_fxaa.jpg
           {Architectural scene with fast approximate antialiasing}
\row
\li Temporal AA, default strength (0.3)
\li \image aa_temporal_default.jpg
           {Architectural scene with temporal antialiasing}
\endtable

\target multisample-aa
\section2 Multisample Anti-Aliasing

Multisample AA (MSAA) operates either on the color buffer of the \l View3D item
(this is the default), or, if a \l{View3D::renderMode}{renderMode} other than
\c Offscreen is used, on the entire Qt Quick window (\l QQuickWindow, \l QQuickView,
\l Window, \l ApplicationWindow).

The edges of geometry are super-sampled, resulting in smoother silhouettes.
This technique has no effect on the materials inside geometry, however.

\list

\li \b{Pros}: Good results on geometry silhouettes, where aliasing is often
most noticeable. Works with fast animation without an issue. Many recent GPUs
support 2x or 4x MSAA without any performance issue.

\li \b{Cons}: Can be expensive to use, especially on older mobile and embedded
hardware. Does not help with texture or reflection issues.

\endlist

When the View3D is using the default \l{View3D::renderMode}{renderMode} of \c
Offscreen, the View3D itself is in full control of multisample antialiasing.
Applications can configure this via the
\l{SceneEnvironment::antialiasingMode}{antialiasingMode} and
\l{SceneEnvironment::antialiasingQuality}{antialiasingQuality} properties of
the environment (\l SceneEnvironment or \l ExtendedSceneEnvironment) associated
with the \l View3D.

The following example requests the commonly used 4x MSAA, because
antialiasingQuality defaults to \c{SceneEnvironment.High}.

\qml
    View3D {
        environment: SceneEnvironment {
            antialiasingMode: SceneEnvironment.MSAA
        }
    }
\endqml

MSAA is not implemented by Qt itself, but is rather performed by the underlying
3D API. Hence performance and quality may vary between different hardware and
their 3D API implementations.

\target temporal-aa
\section2 Temporal Anti-Aliasing

Temporal AA operates on the color buffer of the \l View3D. The camera is
jiggled \e {very slightly} between frames, and the result of each new frame is
blended with the previous frame.

\list

\li \b{Pros}: Due to the jiggling camera it finds real details that were
otherwise lost. Low impact on performance.

\li \b{Cons}: Fast-moving objects cause one-frame ghosting.

\endlist

Temporal AA has no effect when combined with Multisample AA. It can however be
combined with Progressive AA.

To control temporal anti-aliasing, use the environment's
\l{SceneEnvironment::temporalAAEnabled}{temporalAAEnabled} and
\l{SceneEnvironment::temporalAAStrength}{temporalAAStrength} properties.

\qml
    View3D {
        environment: SceneEnvironment {
            temporalAAEnabled: true
        }
    }
\endqml

\target progressive-aa
\section2 Progressive Anti-Aliasing

Progressive AA operates on the color buffer of the \l View3D. When all the
content in the scene rendered by the View3D has stopped moving, the camera is
jiggled \e {very slightly} between frames, and the result of each new frame is
blended with the previous frames. The more frames you accumulate, better
looking the result.

\list

\li \b{Pros}: Provides detailed static images with no performance cost.

\li \b{Cons}: Does not take effect if any visual changes are occurring. 8x PAA
takes one eighth of a second to finish rendering (at 60fps), which may be
noticeable.

\endlist

\qml
    View3D {
        environment: SceneEnvironment {
            antialiasingMode: SceneEnvironment.ProgressiveAA
        }
    }
\endqml

Use \l{SceneEnvironment::antialiasingQuality}{antialiasingQuality} to control
how many frames are blended together (2, 4, or 8).

\target supersample-aa
\section2 Supersample Anti-Aliasing

Supersample AA operates on the color buffer of a \l View3D. This involves
creating a color buffer (texture) larger then its normal size, and then
downsampling it. This means an increased resource usage and at large sizes the
scaling operation can be costly.

\list

\li \b{Pros}: Provides full-scene anti-aliasing with no limitations on animation.

\li \b{Cons}: Can severely degrade performance when your scene is already
limited by the fill-rate of the graphics system, especially an older mobile and
embedded hardware.

\endlist

\qml
    View3D {
        environment: SceneEnvironment {
            antialiasingMode: SceneEnvironment.SSAA
        }
    }
\endqml

Use \l{SceneEnvironment::antialiasingQuality}{antialiasingQuality} to control
the size multiplier (1.2, 1.5 or 2.0).

\target mipmaps
\section2 Mipmaps

Mipmapping stores the texture along with its pre-calculated lower resolution
versions. Whenever the texture is being displayed at a smaller size, the
rendering system automatically uses these low-resolution images (which combine
many details into fewer pixels).

\list

\li \b{Pros}: Low performance impact. Greatly improves image quality for
textures.

\li \b{Cons}: Requires potentially costly generation of the mipmap chain, or,
with some image container formats, pre-generating the mipmap images in the
image asset itself. Uses 33% more graphics memory than the same image without
mipmaps.

\endlist

To have Qt generate mipmaps for a \l Texture and enable using the mipmap chain
when performing texture sampling in the graphics shaders, set the
\l{Texture::mipFilter}{mipFilter} and
\l{Texture::generateMipmaps}{generateMipmaps} properties.

\qml
    Texture {
        source: "image.png"
        mipFilter: Texture.Linear
        generateMipmaps: true
    }
\endqml

\target specular-aa
\section2 Specular Anti-Aliasing

Artifacts from the specular lighting contribution may be reduced by enabling
Specular Anti-aliasing. These artifacts typically show up as bright dots,
perhaps with a flickering appearance.

\table
\header
\li Specular AA disabled
\li Specular AA enabled
\row
\li \image specular_aa_off.jpg
           {Architectural scene showing specular highlights without
           antialiasing}
\li \image specular_aa_on.jpg
           {Architectural scene showing smoothed specular highlights with
           antialiasing}
\endtable

\qml
    View3D {
        environment: SceneEnvironment {
            specularAAEnabled: true
        }
    }
\endqml

\note Materials with a very smooth appearance may change their appearance as if
they had a \l{PrincipledMaterial::roughness}{more rough} surface when enabling
specular AA. This is a result of the underlying lighting calculations.

\target fx-aa
\section2 Fast Approximate Anti-Aliasing

\l ExtendedSceneEnvironment offers another method of anti-aliasing in form of a
post-processing effect. To enable FXAA, set
\l{ExtendedSceneEnvironment::fxaaEnabled}{fxaaEnabled} to true.

\qml
    import QtQuick3D.Helpers

    View3D {
        environment: ExtendedSceneEnvironment {
            fxaaEnabled: true
        }
    }
\endqml

\section1 Artist-Employed Cheats

\target silhouette-opacity-maps
\section2 Silhouette Opacity Maps

When your model has a consistent silhouette, you can apply an opacity map that makes the outer edge
of the geometry transparent. Using a gradient for the opacity will let the edge of the object
smoothly disappear. However, even if your opacity map transitions directly from fully-opaque
to fully-transparent over the space of one pixel, the result will still provide anti-aliased edges
as seen in the example above. This is because image maps, including opacity maps, use bilinear
interpolation.

\list
\li
  \b{Pros}: Can show softer transitions than normal AA. Can be applied per model instead of
    per-layer.
\li
  \b{Cons}: Cannot be used if the silhouette of the object will ever change. Multiple overlapping
    models that employ transparency consume fill rate performance, which is often at a premium.
\endlist

\target modifying-materials
\section2 Modifying Materials or Geometry

As demonstrated in the image for \l{reflection-aliasing}{Reflection Aliasing} above, sometimes the
simplest fix for problems is to change the artwork. If you are getting distracting specular glints
on the corner of your model, ask yourself: \e {Can I make the material softer? Can I modify the
geometry to smooth or change the reflection angle? Can I edit the environment map to reduce
sharp transitions?}

*/
