GPU Gems 2

GPU Gems 2

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Chapter 19. Generic Refraction Simulation

Tiago Sousa
Crytek

Refraction, the bending of light as it passes from a medium with one index of refraction to another (for example, from air to water, from air to glass, and so on), is challenging to achieve efficiently in real-time computer graphics. There are many well-known techniques for simulating light reflection (such as planar reflection maps and cubic environment maps), and these techniques work well in most situations. However, there aren't as many widely known and effective techniques for simulating refraction. This chapter describes an approach to refraction based on perturbing the texture coordinates used in a texture lookup of an image of the nonrefractive objects in the scene. This technique is very efficient and works well in many cases. The method presented here is an expansion of the techniques used in Far Cry for rendering water, heat haze, and the sniper-scope lens, among other effects.

There are several ways to simulate refraction: some are based on precomputing an environment map and then using it at runtime; others are based on computing the environment map on the fly. Drawbacks of these techniques are high texture memory usage and the performance penalty, especially if there are many refractive surfaces in the scene requiring different environment maps.

Another problem with current water refraction simulation techniques is that they require two rendering passes: one to generate the refraction map with geometry above the water plane clipped, and then another pass to render the water surface. This approach can also have poor performance, especially in complex rendering situations.

This chapter discusses a simple technique to overcome these problems. We start by introducing the basic technique, which is based on using the current back buffer as a refraction map and then adding displacement to the texture coordinates to simulate the refractive look. This basic approach can lead to artifacts, however, so we then discuss how to mask out geometry from the refraction map. Finally, we demonstrate some general techniques for rendering realistic water and glass using this refraction simulation technique. Figure 19-1 shows a simple example of refraction in a scene.

19_refraction_01a.jpg

Figure 19-1 An Example of Refraction in a Scene

19.1 Basic Technique

The first step of the basic refraction technique is to render the scene geometry into a texture, skipping all refractive meshes. This texture can be used to determine which objects are visible behind the refractive objects that will be rendered in a subsequent pass. We denote this texture as S.

The second step is to render the refractive meshes, looking up values from the texture S with a perturbation applied to simulate the refractive look. The perturbation can be achieved using a normal map N, where the normal-map red and green (XY) components are used and scaled by some small value to add a displacement into the projected texture coordinates. This approach is straightforward to implement in a shader: (1) fetch the texture N, (2) use the XY components scaled by a small value (such as 0.05), and (3) add this displacement value into the projected texture coordinates for S. Listing 19-1 shows a shader that demonstrates this approach; Figure 19-2 illustrates the three steps.

19_refraction_02a.jpg

Figure 19-2 A Visualization of the Rendering Steps

Example 19-1. Shader for Basic Refraction Technique

 half4 main(float2 bumpUV : TEXCOORD0,    float4 screenPos : TEXCOORD1,    uniform sampler2D tex0,    uniform sampler2D tex1,    uniform float4 vScale) : COLOR    {    // fetch bump texture, unpack from [0..1] to [-1..1]    half4 bumpTex=2.0 * tex2D(tex0, bumpUV.xy) - 1.0;      // displace texture coordinates    half2 newUV = (screenPos.xy/screenPos.w) + bumpTex.xy * vScale.xy;      // fetch refraction map      return tex2D(tex1, newUV);  }

19.2 Refraction Mask

The basic technique presented in the previous section will work reasonably well in a variety of situations, but it can be prone to artifacts if a bump with a large enough scale is used for perturbation when a mesh is in front of a refractive mesh. Figure 19-3 illustrates the problem: the brown sphere is inaccurately rendered as in the refraction in the teapot, even though it is in front of the teapot, not behind it.

19_refraction_03a.jpg

Figure 19-3 Artifacts Caused by the Technique

These artifacts are visible because the texture S has all the scene geometry rendered into it, including objects in front of the refractive mesh, and we are indiscriminately applying perturbation on every pixel. This leads to refraction "leakage" between objects in the scene. A straightforward solution is to decrease the amount of perturbation applied, until artifacts are reduced to an acceptable visual quality level. However, it is difficult to find a scale for the perturbation that works well in all circumstances, and this solution also has the negative side effect of capping how bumpy your refractive surfaces can be.

A better solution instead is to make sure that we actually don't add perturbation into the wrong pixels. To do so, we lay down a mask in the alpha channel of the S texture for all refractive meshes and use it to ensure that the perturbed coordinates are used only if they end up being inside a refractive object's area on the screen. See Figure 19-4 for an example.

19_refraction_04.jpg

Figure 19-4 The Alpha Channel in the Frame Buffer

For this approach to work, we need to change the rendering a bit: first we make sure that the alpha channel of texture S is cleared to white, and then we render refractive meshes as black only into texture S's alpha channel. When rendering the refractive meshes, we make use of the extra information stored in S's alpha channel, to discriminate which pixels will be processed. This is done by checking if alpha is white; if so, we do not add perturbation in that case. Only if alpha is black do we add perturbation. Thus, if perturbation would have included a pixel outside of the refractive object, we instead just use the original pixel, giving whatever geometry was directly behind the refractive object. Although this result is inaccurate—refraction may actually lead to objects that aren't directly behind the object becoming visible—it works well in practice and is quite efficient. Listing 19-2 shows the shader that implements this approach, and Figure 19-5 shows the results.

19_refraction_05a.jpg

Figure 19-5 Artifacts Removed by Using the Refraction Mask

Example 19-2. Improved Shader That Uses the Refraction Mask to Avoid Including Pixels from Objects in Front of the Refractive Object

 half4 main(float2 bumpUV : TEXCOORD0,    float4 screenPos : TEXCOORD1    uniform sampler2D tex0,    uniform sampler2D tex1,    uniform float4 vScale) : COLOR    {    // fetch bump texture    half4 bumpTex=2.0 * tex2D(tex0, bumpUV.xy) - 1.0;      // compute projected texture coordinates    half2 vProj = (screenPos.xy/screenPos.w);      // fetch refraction map      half4 vRefrA = tex2D(tex1, vProj.xy + bumpTex.xy * vScale.xy);    half4 vRefrB = tex2D(tex1, vProj.xy);      return vRefrB * vRefrA.w + vRefrA * (1 - vRefrA.w);  }

It's simple in this way to remove the artifacts, making it possible to use this refraction simulation technique on every mesh type. Although almost not noticeable, some artifacts may be visible, because we're replacing occluder pixel colors with the background color.

19.3 Examples

Many interesting effects can be achieved with the presented technique. For example, the normal map used may be an animated texture, or a transformation may be applied to its coordinates. In this section, we cover a few practical examples.

19.3.1 Water Simulation

Simulating refractive water is one of the applications in which the presented technique is particularly effective, as current methods use an extra pass to generate the refractive map, by rendering the scene again and clipping geometry above the water plane. With this technique, we can render water in just one pass, because the only extra work is to render the water plane into texture S's alpha channel for the refraction mask.

For Far Cry, we used an animated bump texture, which worked out really well. In later experiments, however, we achieved even better results by using multiple bump-map layers for animating water waves, then blending between reflection and refraction through a per-pixel Fresnel term. Figure 19-6 shows the rendering steps for water.

19_refraction_06a.jpg

Figure 19-6 The Rendering Steps for Water

The water simulation rendering is done by rendering the scene as described in previous sections, with the water plane rendered into the back-buffer alpha for the refraction mask. Next we need to generate a water reflection map, by rendering the scene reflected through the water plane into the reflection map and clipping the geometry below the water surface. Finally, there's no need to do an extra rendering pass for the refraction map, because we can use texture S and its refraction mask on the alpha channel.

Once the main texture input has been generated, we must render the water itself. In this example, we use four bump layers. The texture coordinates of the bump layers are scaled in the vertex shader by an increasing value for each, so that we have a nice mix of low-frequency and high-frequency details on the water waves. A translation is also added to the texture coordinates to simulate the motion of waves in the water. Listings 19-3 and 19-4 show the implementation.

Example 19-3. Fresnel Approximation Computation for Water Rendering

 half Fresnel(half NdotL, half fresnelBias, half fresnelPow)  {    half facing = (1.0 - NdotL);    return max(fresnelBias +               (1.0 - fresnelBias) * pow(facing, fresnelPow), 0.0);  }

Example 19-4. The Fragment Program for Refractive/Reflective Water

 half4 main(float3 Eye : TEXCOORD0,    float4 Wave0 : TEXCOORD1,    float2 Wave1 : TEXCOORD2,    float2 Wave2 : TEXCOORD3,    float2 Wave3 : TEXCOORD4,    float4 ScreenPos : TEXCOORD5,    uniform sampler2D tex0,    uniform sampler2D tex1,    uniform sampler2D tex2) : COLOR    {    half3 vEye = normalize(Eye);    // Get bump layers    half3 vBumpTexA = tex2D(tex0, Wave0.xy).xyz;    half3 vBumpTexB = tex2D(tex0, Wave1.xy).xyz;    half3 vBumpTexC = tex2D(tex0, Wave2.xy).xyz;    half3 vBumpTexD = tex2D(tex0, Wave3.xy).xyz;      // Average bump layers      half3 vBumpTex=normalize(2.0 * (vBumpTexA.xyz + vBumpTexB.xyz +                                    vBumpTexC.xyz + vBumpTexD.xyz) - 4.0);      // Apply individual bump scale for refraction and reflection    half3 vRefrBump = vBumpTex.xyz * half3(0.075, 0.075, 1.0);    half3 vReflBump = vBumpTex.xyz * half3(0.02, 0.02, 1.0);      // Compute projected coordinates      half2 vProj = (ScreenPos.xy/ScreenPos.w);    half4 vReflection = tex2D(tex2, vProj.xy + vReflBump.xy);    half4 vRefrA = tex2D(tex1, vProj.xy + vRefrBump.xy);    half4 vRefrB = tex2D(tex1, vProj.xy);      // Mask occluders from refraction map      half4 vRefraction = vRefrB * vRefrA.w + vRefrA * (1 - vRefrA.w);      // Compute Fresnel term    half NdotL = max(dot(vEye, vReflBump), 0);    half facing = (1.0 - NdotL);    half fresnel = Fresnel(NdotL, 0.2, 5.0);      // Use distance to lerp between refraction and deep water color      half fDistScale = saturate(10.0/Wave0.w);    half3 WaterDeepColor = (vRefraction.xyz * fDistScale +                            (1 - fDistScale) * half3(0, 0.15, 0.115));      // Lerp between water color and deep water color    half3 WaterColor = half3(0, 0.15, 0.115);    half3 waterColor = (WaterColor * facing +                        WaterDeepColor * (1.0 - facing));      half3 cReflect = fresnel * vReflection;      // final water = reflection_color * fresnel + water_color      return half4(cReflect + waterColor, 1);  }

19.3.2 Glass Simulation

Glass simulation is usually done using cube maps for both reflection and refraction, and then the results are blended using the Fresnel term. Some techniques go further and make refraction wavelength-dependent, to simulate chromatic aberration. In this example, we use only the texture S and a 2D reflection map to simulate the refraction and reflections on the glass. But this approach could be extended to include the previously mentioned techniques.

First we render the scene as usual to generate the S texture (with the refraction mask in alpha). The glass surface is then rendered by using environmental bump mapping blended with the refraction map using the per-pixel Fresnel term. The glass color comes from the diffuse texture modulated by the refraction. Figure 19-7 shows the three steps of this process.

19_refraction_07a.jpg

Figure 19-7 The Rendering Steps for Stained Glass

By changing the bump texture, diffuse texture, and environment map, it's simple to simulate the appearance of different types of glass without having to change the fragment program itself. Figure 19-8 shows two examples, and the shader appears in Listing 19-5.

19_refraction_08a.jpg

Figure 19-8 Different Glass Types

Example 19-5. Shader for Refractive/Reflective Glass Simulation

 half4 main(float2 BaseUV : TEXCOORD0,    float4 ScreenPos : TEXCOORD1,    float3 Eye : TEXCOORD2,    uniform sampler2D tex0,    uniform sampler2D tex1,    uniform sampler2D tex2,    uniform sampler2D tex3) : COLOR    {    half3 vEye = normalize(Eye.xyz);      // Get bump and apply scale, then get diffuse    half4 vBumpTex = 2.0 * tex2D(tex1, BaseUV.xy) - 1.0;    half3 vBump = normalize(vBumpTex.xyz * half3(0.2, 0.2, 1.0));    half4 vDiffuse = tex2D(tex0, BaseUV.xy);      // Compute reflection vector      half LdotN = dot(vBump.xyz, vEye.xyz);    half3 vReflect = 2.0 * LdotN * vBump.xyz - vEye;      // Reflection vector coordinates used for environmental mapping    half4 vEnvMap = tex2D(tex3, (vReflect.xy + 1.0) * 0.5);      // Compute projected coordinates and add perturbation      half2 vProj = (ScreenPos.xy/ScreenPos.w);    half4 vRefrA = tex2D(tex2, vProj.xy + vBump.xy);    half4 vRefrB = tex2D(tex2, vProj.xy);      // Mask occluders from refraction map    half4 vFinal = vRefrB * vRefrA.w + vRefrA * (1 - vRefrA.w);      // Compute Fresnel term      half fresnel = Fresnel(LdotN, 0.4, 5.0);      // Lerp between 1 and diffuse for glass transparency    vDiffuse.xyz = saturate(0.1 + vDiffuse.xyz * 0.9);      // Final output blends reflection and refraction using Fresnel term    return vDiffuse * vFinal * (1 - fresnel) + vEnvMap * fresnel;  }

19.4 Conclusion

In this chapter we have presented a technique to simulate refraction. The technique, though not physically based, produces results with good visual quality and is extremely efficient. The examples presented are just a few of the diverse effects that can be achieved.

One limitation of this technique is that, when applied to different-colored refractive surfaces, it will yield incorrect results where the surfaces overlap. As long as the refractive surfaces have a similar color, the visuals will look correct. One possible solution would be to sort refractive meshes from back to front and update the refraction map every time a refractive mesh is rendered. Alternatively, a less accurate solution would be to sort the refractive meshes from back to front and use alpha blending while rendering them.

The technique is also applicable to a range of target hardware: even though it's not demonstrated in this chapter, the technique can easily be expanded so that it works on lower-end hardware using pixel shader versions 1.1 to 1.4.

19.5 References

Akenine-Möller, Tomas, and Eric Haines. 2002. "Planar Reflections." In Real-Time Rendering, 2nd ed., pp. 239–243. A K Peters.

Everitt, Cass. 2001."Projective Texture Mapping." NVIDIA Corporation. Available online at http://developer.nvidia.com/object/Projective_Texture_Mapping.html

Kilgard, Mark J. 2001."Chromatic Aberration." NVIDIA Corporation. Available online at http://developer.nvidia.com/object/chromatic_aberration.html

Oliveira, Gustavo. 2000. "Refractive Texture Mapping, Part One." On Gamasutra Web site. Available online at http://www.gamasutra.com/features/20001110/oliveira_01.htm

Vlachos, Alex, John Isidoro, and Chris Oat. 2002. "Rippling Reflective and Refractive Water." In ShaderX, edited by Wolfgang Engel. Wordware.

Wloka, Matthias. 2002. "Fresnel Reflection Technical Report." NVIDIA Corporation.

A special thanks to Martin Mittring and Carsten Wenzel for ideas and discussion about artifact minimization and to Márcio Martins for his model loader used in the chapter demos.

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Library of Congress Cataloging-in-Publication Data

GPU gems 2 : programming techniques for high-performance graphics and general-purpose
computation / edited by Matt Pharr ; Randima Fernando, series editor.
p. cm.
Includes bibliographical references and index.
ISBN 0-321-33559-7 (hardcover : alk. paper)
1. Computer graphics. 2. Real-time programming. I. Pharr, Matt. II. Fernando, Randima.

T385.G688 2005
006.66—dc22
2004030181

GeForce™ and NVIDIA Quadro® are trademarks or registered trademarks of NVIDIA Corporation.

Nalu, Timbury, and Clear Sailing images © 2004 NVIDIA Corporation.

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Copyright © 2005 by NVIDIA Corporation.

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Second printing, April 2005

Dedication

To everyone striving to make today's best computer graphics look primitive tomorrow