Add a page on 3D resolution scaling

This covers why resolution scaling should be used in certain cases,
and how to use use the various 3D resolution scaling options in Godot.

This is a separate page from the 3D antialiasing documentation,
as resolution scaling can be performed independently from antialiasing.

Author: Calinou

tutorials/3d/img/resolution_scaling_bilinear_0.5.png

@@ -16,6 +16,7 @@
    baked_lightmaps
    environment_and_post_processing
    3d_antialiasing
+   resolution_scaling
    volumetric_fog
    high_dynamic_range
    using_gridmaps

tutorials/3d/img/resolution_scaling_bilinear_msaa_4x_0.5.png

@@ -0,0 +1,239 @@
+Resolution scaling
+==================
+
+Why use resolution scaling?
+---------------------------
+
+With the ever-increasing rendering complexity of modern games, rendering at
+native resolution isn't always viable anymore, especially on lower-end GPUs.
+
+Resolution scaling is one of the most direct ways to influence the GPU
+requirements of a scene. In scenes that are bottlenecked by the GPU (rather than
+by the CPU), decreasing the resolution scale can improve performance
+significantly. Resolution scaling is particularly important on mobile GPUs where
+performance and power budgets are limited.
+
+While resolution scaling is an important tool to have, remember that resolution
+scaling is not intended to be a replacement for decreasing graphics settings on
+lower-end hardware. Consider exposing both resolution scale and graphics
+settings in your in-game menus.
+
+.. note::
+
+    Resolution scaling is currently not available for 2D rendering, but it can be
+    simulated using the ``viewport`` stretch mode. See :ref:`doc_multiple_resolutions`
+    for more information.
+
+Resolution scaling options
+--------------------------
+
+In the advanced Project Settings' **Rendering > Scaling 3D** section, you cany
+find several options for 3D resolution scaling:
+
+Scaling mode
+^^^^^^^^^^^^
+
+- **Bilinear:** Standard bilinear filtering (default).
+- **FSR 1.0:** `AMD FidelityFX Super Resolution 1.0 <https://gpuopen.com/fidelityfx-superresolution/>`__.
+  Slower, but higher quality compared to bilinear scaling. On very slow GPUs,
+  the cost of FSR 1.0 may be too expensive to be worth using it over bilinear
+  scaling.
+
+Here are comparison images between native resolution, bilinear scaling with 50%
+resolution scale and FSR 1.0 scaling with 50% resolution scale:
+
+.. image:: img/resolution_scaling_bilinear_0.5.png
+
+.. image:: img/resolution_scaling_fsr1_0.5.png
+
+FSR 1.0 upscaling works best when coupled with another form of antialiasing.
+Temporal antialiasing (TAA) or multisample antialiasing (MSAA) should preferably
+be used in this case, as FXAA does not add temporal information and introduces
+more blurring to the image.
+
+Here's the same comparison, but with 4× MSAA enabled on all images:
+
+.. image:: img/resolution_scaling_bilinear_msaa_4x_0.5.png
+
+.. image:: img/resolution_scaling_fsr1_msaa_4x_0.5.png
+
+Notice how the edge upscaling of FSR 1.0 becomes much more convincing once 4×
+MSAA is enabled.
+
+Rendering scale
+^^^^^^^^^^^^^^^
+
+The **Rendering > Scaling 3D > Scale** setting adjusts the resolution scale.
+``1.0`` represents the full resolution scale, with the 3D rendering resolution
+matching the 2D rendering resolution. Resolution scales *below* ``1.0`` can be
+used to speed up rendering, at the cost of a blurrier final image and more aliasing.
+
+The rendering scale can be adjusted at run-time by changing the ``scaling_3d_scale``
+property on a :ref:`class_Viewport` node.
+
+Resolution scales *above* ``1.0`` can be used for supersample antialiasing
+(SSAA). This will provide antialiasing at a *very* high performance cost, and is
+**not recommended** for most use cases. See :ref:`doc_3d_antialiasing` for more
+information.
+
+The tables below list common screen resolutions, the resulting 3D rendering
+resolution and the number of megapixels that need to be rendered each frame
+depending on the rendering scale option. Rows are sorted from fastest to slowest
+in each table.
+
+.. note::
+
+    The resolution scale is defined on a **per-axis** basis. For example, this
+    means that halving the resolution scale factor will reduce the number of
+    rendered megapixels per frame by a factor of 4, not 2. Therefore, very low
+    or very high resolution scale factors can have a greater performance impact
+    than expected.
+
+**1920×1080 (Full HD)**
+
++--------------------------+-------------------------+-------------------------------+
+| Resolution scale factor  | 3D rendering resolution | Megapixels rendered per frame |
++==========================+=========================+===============================+
+| ``0.50``                 | 960×540                 | 0.52 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.67``                 | 1286×723                | 0.93 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.75``                 | 1440×810                | 1.17 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.85``                 | 1632×918                | 1.50 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``1.00`` **(native)**    | **1920×1080**           | **2.07 MPix**                 |
++--------------------------+-------------------------+-------------------------------+
+| ``1.33`` (supersampling) | 2553×1436               | 3.67 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``1.50`` (supersampling) | 2880×1620               | 4.67 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``2.00`` (supersampling) | 3840×2160               | 8.29 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+
+**2560×1440 (QHD)**
+
++--------------------------+-------------------------+-------------------------------+
+| Resolution scale factor  | 3D rendering resolution | Megapixels rendered per frame |
++==========================+=========================+===============================+
+| ``0.50``                 | 1280×720                | 0.92 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.67``                 | 1715×964                | 1.65 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.75``                 | 1920×1080               | 2.07 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.85``                 | 2176×1224               | 2.66 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``1.00`` **(native)**    | **2560×1440**           | **3.69 MPix**                 |
++--------------------------+-------------------------+-------------------------------+
+| ``1.33`` (supersampling) | 3404×1915               | 6.52 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``1.50`` (supersampling) | 3840×2160               | 8.29 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``2.00`` (supersampling) | 5120×2880               | 14.75 MPix                    |
++--------------------------+-------------------------+-------------------------------+
+
+**3840×2160 (Ultra HD "4K")**
+
++--------------------------+-------------------------+-------------------------------+
+| Resolution scale factor  | 3D rendering resolution | Megapixels rendered per frame |
++==========================+=========================+===============================+
+| ``0.50``                 | 1920×1080               | 2.07 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.67``                 | 2572×1447               | 3.72 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.75``                 | 2880×1620               | 4.67 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``0.85``                 | 3264×1836               | 5.99 MPix                     |
++--------------------------+-------------------------+-------------------------------+
+| ``1.00`` **(native)**    | **3840×2160**           | **8.29 MPix**                 |
++--------------------------+-------------------------+-------------------------------+
+| ``1.33`` (supersampling) | 5107×2872               | 14.67 MPix                    |
++--------------------------+-------------------------+-------------------------------+
+| ``1.50`` (supersampling) | 5760×3240               | 18.66 MPix                    |
++--------------------------+-------------------------+-------------------------------+
+| ``2.00`` (supersampling) | 7680×4320               | 33.18 MPix                    |
++--------------------------+-------------------------+-------------------------------+
+
+FSR Sharpness
+^^^^^^^^^^^^^
+
+When using the FSR 1.0 scaling mode, the sharpness can be controlled using the
+**Rendering > Scaling 3D > FSR Sharpness** advanced project setting.
+
+The intensity is inverted compared to most other sharpness sliders: *lower*
+values will result in a sharper final image, while *higher* values will *reduce*
+the impact of the sharpening filter. ``0.0`` is the sharpest, while ``2.0`` is
+the least sharp. The default value of ``0.2`` provides a balance between
+preserving the original image's sharpness and avoiding additional aliasing due
+to oversharpening.
+
+.. note::
+
+    If you wish to use sharpening when rendering at native resolution, Godot
+    currently doesn't allow using the sharpening component of FSR (RCAS)
+    independently from the upscaling component (EASU).
+
+    As a workaround, you can set the 3D rendering scale to ``0.99``, set the
+    scaling mode to **FSR 1.0** then adjust FSR sharpness as needed. This allows
+    using FSR 1.0 while rendering at a near-native resolution.
+
+Mipmap bias
+^^^^^^^^^^^
+
+Godot automatically uses a negative texture mipmap bias when the 3D resolution
+scale is set below ``1.0``. This allows for better preservation of texture
+detail at the cost of a grainy appearance on detailed textures.
+
+The texture LOD bias currently affects both 2D and 3D rendering in the same way.
+However, keep in mind it only has an effect on textures with mipmaps enabled.
+Textures used in 2D don't have mipmaps enabled by default, which means only 3D
+rendering is affected unless you enabled mipmaps on 2D textures in the Import
+dock.
+
+The formula used to determine the texture mipmap bias is: TODO
+
+To counteract the blurriness added by some antialiasing methods, Godot also adds
+a ``-0.25`` offset when FXAA is enabled, and a ``-0.5`` offset when TAA is
+enabled. If both are enabled at the same time, a ``-0.75`` offset is used. This
+mipmap bias offset is applied *before* the resolution scaling offset, so it does
+not change depending on resolution scale.
+
+The texture LOD bias can manually be changed by adjusting the **Rendering >
+Textures > Default Filters > Texture Mipmap Bias** advanced project setting. It
+can also be changed at run-time on :ref:`Viewports <class_Viewport>` by
+adjusting the ``texture_mipmap_bias`` property.
+
+.. warning::
+
+    Adjusting the mipmap LOD bias manually can be useful in certain scenarios,
+    but this should be done carefully to prevent the final image from looking
+    grainy in motion.
+
+    *Negative* mipmap LOD bias can also decrease performance due to
+    higher-resolution mips having to be sampled further away. Recommended values
+    for a manual offset are between ``-0.5`` and ``0.0``.
+
+    *Positive* mipmap LOD bias will make mipmapped textures appear blurrier than
+    intended. This may improve performance slightly, but is otherwise not
+    recommended as the loss in visual quality is usually not worth the
+    performance gain.
+
+The example below shows an extreme case, with a mipmap LOD bias of ``-1.0`` and
+anisotropic filtering disabled to make the difference more noticeable:
+
+.. image:: img/resolution_scaling_texture_mipmap_bias_comparison.png
+
+Troubleshooting
+---------------
+
+Performance does not increase much when decreasing resolution scale
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If performance doesn't increase much when decreasing resolution scale to a value
+like ``0.5``, it likely means the performance bottleneck is elsewhere in your
+scene. For example, your scene could have too many draw calls, causing a CPU
+bottleneck to occur. Likewise, you may have too many graphics effects enabled
+for your GPU to handle (such as SDFGI, SSAO or SSR).
+
+See the :ref:`doc_performance` tutorials for more information.