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The Depth Bias Blending Material Expression

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Overview

The DepthBiasedBlend material expression provides the ability to do a blend between the source object being rendered and the contents of the destination buffer based on a bias value. The math behind the determination of the final color is as follows:

if (DestinationDepth < SourceDepth)
{
    FinalColor = DestinationColor;  // Do not draw the source pixel
}
else
if (SourceDepth < (DestinationDepth - ((1 - Bias) * BiasScale)))
{
    FinalColor = SourceColor;  // Draw the source pixel
}
else
{
    FinalColor = LinearInterpolation(DestinationColor, SourceColor,
                     ((DestinationDepth - Source Depth) / ((1 - Bias) * BiasScale)));
}

The most obvious usage of this type of blending is to eliminate the harsh edges that occur when sprite particles intersect geometry. We will use this situation to provide a demonstration for the DepthBiasBlend expression.

Example

This example uses a simple particle sprite emitter to demonstrate the benefits of this new material expression. Particle systems are covered at the following page: https://udn.epicgames.com/Three/ParticleTopics

The Source Image and Emitter

The emitter will burst a single particle every five seconds with a lifetime of 4.5 seconds. The emitter actor is positioned in the level such that the particles will rise through the `floor' of the level. The particles utilize the following texture:

Figure 1: The source texture used. (The alpha of the image is on the right.)

The Problem Example

The simple material that would be applied to the particle sprites would be the following:

Figure 2: The `standard' sprite material

The particle system looks good with this applied in the Cascade viewport. However, when the emitter is placed in the level, there is a problem. The particles generate hard edges whenever the penetrate geometry in the level. The results are as shown below:

Figure 3: The standard particle/geometry intersection - Shot 1

Figure 4: The standard particle/geometry intersection - Shot 2

Figure 5: The standard particle/geometry intersection - Shot 3

Note the harsh edges that show up when the sprite is penetrating the level geometry.

The New Method

The new material that can be applied to the particle sprites utilizes the DepthBiasBlend expression and is shown below:

Figure 6: The DepthBiasBlend-based material

The DepthBiasBlend is derived from the TextureSample expression, so a source texture is set simply by selecting a texture in the Generic Browser, right-clicking the expression, and selecting "Use Current Texture".

The Coord input is the standard Texture Coordinate option. For the case of particles, this should be left `empty' so that the first UV set is utilized.

The Bias input provides the source pixel bias - the value plugged into the equation above to allow for a `blending' transition to occur as objects penetrate. It can be any expression combination that results in a single floating point value. In this case, the alpha channel of the source image is used. This means that at each pixel, the source texture will have the alpha value sampled and will utilize it as the bias.

The BiasScale parameter of the DepthBiasBlend expression allows for scaling the bias to give finer control over the effect. In this case, the bias scale is set relatively high (200.0) to demonstrate the effect more clearly.

The BlendMode of the material must be translucent, even if the texture were opaque. (NOTE: This may change in the future, but is the case as of the authoring of this document.) This is to guarantee the object using the material is rendered at a time where the destination buffer has the values it will blend with. In the case of wanting to blend an opaque texture, set the BlendMode to Translucent and feed a Constant expression of 1.0 into the opacity channel.

[IMPORTANT NOTE: Currently, this expression will not show up in the thumbnail rendering or the preview window. This is due to the way clears occur at the moment. It is a known issue and is in the database.]

The new material is applied to the emitter with the following results:

Figure 7: The new particle/geometry intersection - Shot 1

Figure 8: The new particle/geometry intersection - Shot 2

Figure 8: The new particle/geometry intersection - Shot 3

The screen shot below shows a side-by-side comparison of the two versions of the emitter:

Figure 9: Side-by-side comparison

USAGE NOTES:

- The material expression for DepthBiasBlend is not cheap. It takes 15 additional instructions at the moment - be sure to take this into account when creating material which utilize it!

IMPORTANT UPDATE

The DepthBiasedBlend module should be used over the DepthBiasBlend module (which will be removed from the engine in the future). The advantage of the new expression is that it no longer derives from the TextureSample expression. The RGB and Alpha values are inputs for the expression. This removes the need to implement specialized Depth Blend expression for the various sample expressions (FlipBook, Movie, ParticleSubUV, etc.).

The Alpha input is 'passed through' to the Alpha output. When no alpha input expression is present, a value of 1.0f will be output. The bias value will be 1.0f for an empty input expression. This is useful for the case of opaque textures. The figure below shows an opaque material implemented with the new expression.:

Figure 10: Opaque DepthBiasedBlend Material

The translucent material example from above is implemented with the new expression below:

Figure 11: Translucent DepthBiasedBlend Material