Add a page on 3D resolution scaling (#6331)

tutorials/3d/index.rst

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

tutorials/3d/resolution_scaling.rst

@@ -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.