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Post Process Materials

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Post Process Materials enable you to set up materials that can be used with your post process to create visual screen effects for damage, area type effects, or overall look for your game that can only be achieved via Post Process Materials.

In the following sections, you'll learn about setting up Post Process Materials, the available settings you can use, along with some examples of how to setup some of your own Post Process Materials using different buffers, blending between Post Process Materials, and more.

Post Processing Graph

The engine already features complex post processing based on a graph of post processing nodes. The Post Processing Materials can be additionally inserted in some specific position. See Frequently Asked Questions section for r.CompositionGraphDebug to get a dump of the full graph. The graph is actually not only doing post processing but also some parts of the lighting.

Most of the time the graph is automatically creating intermediate render targets. This means if you want to blend with the former color, you need to do the blending in the shader (using the input from PostProcessInput0).

Post Process Materials should be used sparingly and only when you really need them. Whenever possible, such as for color correction or adjustments, bloom, depth of field, and various other effects, you should use the settings inherent to the Post Process Volume, which have been optimized and are more efficient.

Using a Post Process Material

Through post process settings (usually defined with post process volumes or camera settings) it is possible to blend (so called) blendable assets. At the moment, only Materials and Material Instances are blendable assets. The engine provides a few post process materials but with this feature you can create your own custom post processing without any programmer help.

Simply assign one or more post process materials to a post process volume in the Blendables section. First press the + to add new slots, select a material in the Content Browser, and press the left arrow to assign it. The order here is not important and unused slots are simply ignored.

PostProcessSettings.png

Making a Simple Post Process Material

For an overview of creating a simple Post Process Material effect from scratch, see the Post Process Material Example .

FinalPostEffect.png

Critical Settings for Post Process Materials

A post processing material needs to specify the material domain post process:

DomainPostProcess.png

The material should only use Emissive Color to output the new color. Additionally, you can define where during the post processing this pass should be applied and, if there are multiple, in which order they should be processed (Priority):

PostProcessMaterialProps.png

Blendable Location

Description

Before Tonemapping

PostProcessInput0 provides the scene color with all lighting in HDR. Use this to fix issues with temporal antialiasing and GBuffer lookups e.g. depth, normals.

After Tonemapping

Preferred location for performance as the color is LDR and therefore requires less precision and bandwidth. This is after tone mapping and color grading.

Before Translucency

This is even earlier in the pipeline than 'Before Tonemapping' before translucency was combined with the scene color. Note that SeparateTranslucency is composited later than normal translucency.

Replacing the Tonemapper

PostProcessInput0 provides the HDR scene color, PostProcessInput1 has the SeparateTranslucency (Alpha is mask), PostprocessInput2 has the low resolution bloom input.

The typical postprocess input comes from the pass before. That color can be accessed through the SceneTexture material expression when using PostProcessInput0. Using SceneColor might not give you the right results.

Blending Between Different Material Instances

Using A Post Process Material

With postprocess volumes, it is easy to setup soft transitions between multiple postprocess materials. Here we use one volume that is marked unbound and one volume that has larger blend radius (e.g. 1000):

BlendingAVolume.png

BlendingAVolume1.png

Post Process Set to Unbound

Post Process Bound Volume

Each volume we specify a different material instance of the same material. The color is specified as a material parameter which allows different settings for the two material instances.

BlendMatInst1.png

BlendMatInst2.png

Material Instance RED

Material Instance GREEN

Depending on the camera position the setting of one volume is used and blended when within the Blend Radius' range:

Blend1.png

Blend2.png

Blend3.png

Unbound Post Process Volume Material Instance (RED) set to 0.75

Blend Radius of 1000

Post Process Volume Material Instance (GREEN) set to 0.75

With camera movement, you can perceive soft linear transition between the two effects setting.

The following shows a top down view of a level that has how two volumes. The large unbound volume has a red material instance and the small volume has a green material instance specified as blendable. The smaller volume has a higher priority. The material params are getting blended depending on the camera position. The fuzzy borders are defined by the Blend Radius property specified in the volumes and extend the volume shape.

With the correct setup all blending happens as expected.

Bad Setup

Good Setup

The difference between the both setup is the default value specified on the material (scalar or vector) parameters. The good setup has values that make the pass appear to not have any effect (e.g. multiply with white or lerp with 0).

In both setup we see this: When the camera is outside the influence of either volume the postprocess pass is not getting rendered (visualize by the grey grid). If we are fully inside either of the volumes we also see the correct blending.

The bad setup : When the camera enters the influence radius we see a hard transition but because is uses the wrongly specified default parameters.

The good setup : The transition of entering the camera the influence radius is well hidden and we see a smooth transition to the volume colors.

All material instances properties are blended, no matter if the property checkbox is checked or not (in that case it blends the property from the parent). This is different from the post processing settings where an unchecked property is not having any effect. This means if you blend a material instance, all properties are getting blended.

The Material Expression "SceneTexture"

You can add the SceneTexture material expression into your material and choose which texture you want to reference in the expression properties:

SceneTextureProps.png

The node has an optional input and multiple outputs:

SceneTextureExpression.png

The UV input allows you to specify where you want to make a texture lookup (only used for the Color output). The color output is a 4 channel output (actual channel assignment depends on the scene texture id). Size is a 2 component vector with the width and height of the texture. The inverse of that (1/width, 1/height) is available in the InvSize output. It is handy when referencing neighbor samples like in the example below:

DepthNextTo.png

The material expression computes the depth difference of the current pixel to a neighbor pixel (e.g. In = 0,1 to return the delta to the pixel below).

Using GBuffer Properties

A GBuffer consists of multiple textures that store material (e.g. subsurface/specular color, roughness, ...) and object attributes (e.g. normal, depth) without lighting to compute shading (how light interacts with a material). In a deferred renderer, we first render the GBuffer and then compute all lighting (deferred) with the GBuffer attributes. If UE4 uses the deferred shading path (e.g. DirectX 11 or high end OpenGL), we can get access to those buffers during post processing.

Anti-Aliasing generally makes this a bit more difficult as a GBuffer pixel/texel is no longer 1:1 associated with an output pixel (see section below).

CustomDepth

This separate feature enables masking of certain objects by rendering them into another depth buffer (called custom depth buffer). This adds extra draw calls but no more materials. The rendering is fairly cheap as we only output depth. The feature can be activated on the mesh (e.g. Static Mesh Properties / Render Custom Depth):

CustomDepth.png

In this scene, we activated the feature on two objects but without a post processing pass visualizing the content, this feature remains invisible:

scene.png

Here we can see how a visualization of the Custom Depth looks like:

sceneCustomDepth.png

This is the material we used to visualize it:

CustomDepthMat.png

CustomDepth Stencil

Custom Depth Stencil is an extension of Custom Depth where you can use a stencil, or cutout, of your rendered object to then do visually interesting things like the example below that enables you to visualize occlude objects, draw object outlines, or only be visible from certain viewing angles. There is a lot of potential with what you can do by having access to the stencil of an Actor in your scene. Using the following settings to enable and assign a stencil value.

CustomStencilSettings.png

In this scene, Custom Depth is enabled on three objects and a Custom Depth Stencil Value is set for each, but without any post processing pass to visualize the content, this feature remains invisible.

CustomDepthStencilScene.png

Once you've setup your Post Process Material you will be able to visualize how the Custom Depth Stencil looks, The occluded objects render with a randomly assigned color based on the Custom Depth Stencil Value used.

CustomDepthStencilVisualization.png

This is a setup of the Material we used to visualize it:

Click image for full size.

This is by no means, the only way to use Custom Depth Stencil and in this particular Material setup the stencil is being divided so that it uses values between 1 and 255, a mask is being used for any value that is between these values, a random color is being created for these values as well so as the Custom Depth Stencil Value changes so does the color, and finally the mask that is created is being used in a way that only colors the stencil if the object is occluded.

Temporal Anti-Aliasing or Why the GBuffer Jitters

Temporal anti-aliasing is a unique UE4 feature that greatly improves image quality with a very moderate performance cost.

By default, a post process material is inserted in the end of the post processing graph (after tone mapper). This means you get the final LDR color after tone mapping, color grading and after the temporal anti-aliasing was applied. This is the best spot for many simple post processing effects - for performance and ease of use.

Here you can see how we used the custom depth input to visualize a silhouette around specific objects:

sceneAfterTonemapper.png

Notice that the former image has no anti-aliasing on the silhouette but in motion you also would see the silhouette jittering around 1 pixel. This is because temporal anti-aliasing moves the rendering of the whole scene by a sub pixel each frame. Multiple frames together are getting combined to produce the final anti-aliased image. We can however move the material to an earlier spot in the post processing graph to fix this issue.

This is the result:

sceneBeforeTonemapper.png

We get a stable and anti-aliased image. In motion we might see some artifacts with the temporal anti-aliasing. This feature is using the depth buffer to replace old images. With the border rendered inside the object it works ok but outside the object we would also need to adjust the depth buffer (not done yet, costs extra performance) but ideally this should not be.

UV and ScreenPosition

With post process materials you can look up into screen aligned buffers, but you need to know the right UVs. The ScreenPosition material expression with the mapping option set to ViewportUV outputs the UV with 0,0 at the left top of the viewport and 1,1 at the bottom right. In contrast, using the SceneTextureUV mapping in this material expression might give you different results. This is because the actual texture (more correctly it is a render target) is potentially larger than the viewport. It can be larger in editor because we share this texture for multiple viewports and the largest extent is used for all viewports. Even in game it can be larger in some cases (e.g. SceneCaptureActors might have a smaller viewport, Matinee black borders, Splitscreen, VR, ...). The SceneTextureUV option gives you the UV for this larger texture. If you only need a relative offset (e.g. pixel sized edge detection) you need to scale with the right size. The SceneTexture material expression has outputs for the size and the inverse of the size (efficient and useful for pixel offsets). To test all that, you can use the console variable r.ViewPortTest which allows you to test various viewport configurations.

Filtered texture lookups

The SceneTexture material expression has a checkbox to get [bilinear] filtered lookups. Using this option can results in a slower rendering so it should be only used if needed. Many of the screen space textures do not support filtering (e.g. GBuffer). Not exposing this property allows the engine to compress the data if needed (packing prevents filtering).

Replacing the Tonemapper

It is possible to override the in engine tonemapper with your own one by using the "Replacing the Tonemapper" blendable location. This is a work in progress feature meaning it will likely change and it does not have all the functionality yet.

ReplacingTheTonemapper.png

We started exposing some Post process setting parameters to the tonemapper but that part is likely to change quite a bit. The values are exposed as material parameters and need to have the exact name.

Vector parameter:

Engine.FilmWhitePoint

Scalar parameter:

Engine.FilmSaturation
Engine.FilmContrast

In order to get the parameters you need to create a material instance from the post process material.

You still can use your own parameters and get them blended like other post process material settings.

Known Issues

The following issues still need to be fixed:

  • Material expression SceneTexture

    • SeparateTranslucency does not work.

    • Certain lookups do not work in certain passes (e.g. some we might not fix because they would cost too much performance).

    • MaterialFunction might report an error but it still can be used in a Material with the Post Process domain.

  • Material

    • UV in the PostProcesMaterial might not be in 0-1 range (e.g. in editor when reducing viewport size), this aligns with the lookup but makes it hard to implement something like a vignette effect.

    • The asset thumbnail of a post process material does not look right.

    • Outputting alpha is not yet supported (should go through Opacity).

    • The preview material in the material editor does not look right.

    • There has been a case where a material change was not resulting in a change in the post process. Workaround: restart editor.

    • The Content Browser should allow easy filtering for post process materials.

  • Blending

    • When blending two post process volumes with a blend radius, it is possible to see a non-soft transition. This can be prevented by using an unbound volume with a material instance setting that represents the default.

FAQ

  • Can I have the "Lighting only mode" texture as input?

    No, we do not have this data available as an intermediate step. For this view mode, we generate it by ignoring the material colors. To make this a fast option, it would require restructuring a large part of the rendering code.

  • Why a SceneColor lookup shows banding but when using PostProcessInput0, I do not see that?

    When SceneColor is used we create a lower quality copy of the scene to allow a lookup into a texture that we currently write to (General case is rendering any mesh where this is not possible). In Post Processing, you should use PostProcessInput0.

  • How much memory do I pay for a post process?

    Memory cost depends on screen resolution. Before tone mapping we use HDR (8bytes per pixel), after we use LDR (4 bytes per pixel).

  • How can I lower the render cost for post processing?

    Measure on the target platform, keep the texture lookup count low, do less math operations and reduce dependent texture lookups, avoid randomized texture lookup (can be slower due to texture cache misses).

  • How many passes I can use?

    Each pass adds to the performance cost. Try to combine passes and activate passes only if needed. General game features e.g. noise could be added to the engine passes for better performance.

  • How much CPU performance do I pay per post process and for blending?

    Blending materials is very cheap. All material instances properties are getting blended and only one post process material pass with those settings gets rendered.

  • I need to use "Before Tonemapper" to get proper TemporalAA. When I use a color, it is tone mapped and therefore looks different. How can I prevent that?

    There is no easy solution. You would need to do an inverse tonemaping operation (expensive). The color also might appear different depending on eye adaptation. You could expose the EyeAdaptation level to the SceneTextures to compensate for that.

  • How can I get a full dump of the post processing graph?

    r.CompositionGraphDebug can log the graph into the console. Example:

    FRenderingCompositePassContext:Debug 'PostProcessing' ---------
    Node#1 'SceneColor'
        ePId_Output0 (2D 1136x768 PF_FloatRGBA RT) SceneColor Dep: 2
    Node#4 'Velocity'
        ePId_Output0 (2D 1136x768 PF_G16R16 RT) Velocity Dep: 1
    Node#2 'SceneDepthZ'
        ePId_Output0 (2D 1136x768 PF_DepthStencil) SceneDepthZ Dep: 1
    Node#5 'MotionBlurSetup0MotionBlurSetup1'
        ePId_Input0: Node#4 @ ePId_Output0 'Velocity'
        ePId_Input1: Node#1 @ ePId_Output0 'SceneColor'
        ePId_Input2: Node#2 @ ePId_Output0 'SceneDepthZ'
        ePId_Output0 (2D 568x384 PF_FloatRGBA RT) MotionBlurSetup0 Dep: 2
        ePId_Output1 (2D 568x384 PF_FloatRGBA RT) MotionBlurSetup1 Dep: 1
    Node#6 'QuarterResVelocity'
        ePId_Input0: Node#5 @ ePId_Output0 'MotionBlurSetup0MotionBlurSetup1'
        ePId_Input1:
        ePId_Output0 (2D 284x192 PF_FloatRGBA RT) QuarterResVelocity Dep: 1
    Node#7 'VelocityBlurX'
        ePId_Input0: Node#6 @ ePId_Output0 'QuarterResVelocity'
        ePId_Input1:
        ePId_Output0 (2D 284x192 PF_FloatRGBA RT) VelocityBlurX Dep: 1
    ...