Advancing Ray Tracing

Advanced Topics for PowerVR Ray Tracing

Gareth Morgan Luke Peterson

Anti-aliasing with a ray tracer Similar to traditional rasterization - but a bit different

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A pixel is not a point sample, but an integral Aliasing occurs when scene features have higher spatial frequency than pixel res

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Pixel awareness with derivative functions dFdx(var)

dFdy(var)

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fwidth(var)

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Mip Mapping automatically uses derivatives

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But a ray is a point sample, by definition Similar to traditional rasterization - but a bit different

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A prism is a better idea

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Ray Differentials are the solution • Ray Differentials involve tracking values for ∆x and ∆y •

The original work was done by Homan Igehy, SigGraph ‘99

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Selective Derivative Tracking • This is not a released part of the spec yet – It may change • Ray attributes declared with the derivatives keyword contain values for ∆x and ∆y tracks, in addition to the main track layout(binding=0) raytype PrimaryRay { derivatives vec3 gl_Origin; derivatives vec3 gl_Direction;

};

• Set the tracks in a qualified variable setTracksIMG( out, mainVal, dxVal, dyVal ); val = splatTracksIMG( tracks );

• Samplers, dFdx, dFdy, and fwidth operate on qualified values vec4 color = texture( txt, uv); facebook.com/imgtec

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Emitting a ray with differentials void main () { derivatives vec3 outRayDirection; derivatives vec2 frameCoordDXDY; vec2 frameCoordOffsetDX = gl_FrameCoordIMG + ivec2(1, 0); vec2 frameCoordOffsetDY = gl_FrameCoordIMG + ivec2(0, 1);

setTracksIMG( frameCoordDXDY, vec2(gl_FrameCoordIMG)/RenderSize.xy - vec2(0.5), vec2(frameCoordOffsetDX)/RenderSize.xy - vec2(0.5), vec2(frameCoordOffsetDY)/RenderSize.xy - vec2(0.5)); setTracksIMG( outRayDirection, vec3( getMainTrackIMG(frameCoordDXDY).x, getMainTrackIMG(frameCoordDXDY).y * aspectRatio , (-1.0 / (2.0 * tanTheta)) ), vec3( getDxTrackIMG(frameCoordDXDY).x, getDxTrackIMG(frameCoordDXDY).y * aspectRatio , (-1.0 / (2.0 * tanTheta)) ), vec3( getDyTrackIMG(frameCoordDXDY).x, getDyTrackIMG(frameCoordDXDY).y * aspectRatio , (-1.0 / (2.0 * tanTheta)) )); primRay.gl_OriginIMG = splatTrackIMG(cameraPosition); primRay.gl_DirectionIMG = outRayDirection; emitRayIMG(primRay);

}

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Using Derivate Values • Must be propagated to other variables in the shader derivatives vec3 hitCoordDXDY; derivatives vec2 uv;

• Overloaded interpolation functions assign to qualified values baryCoordsDXDY = coordAtRayHitDXDYIMG( gl_in[0].gl_Position.xyz, gl_in[1].gl_Position.xyz, gl_in[2].gl_Position.xyz, inputRay.gl_Origin, inputRay.gl_Direction); uv = interpolateIMG(baryCoordsDXDY, VtxData[0].uv, VtxData[1].uv, VtxData[2].uv);

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Differential values can be propagated

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Secondary ray with differentials reflectionRay.gl_OriginIMG = hitCoordDXDY ; reflectionRay.gl_DirectionIMG = reflect(primRay.gl_DirectionIMG, hitCoordDXDY ); emitRayIMG(reflectionRay);

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Handling dynamic geometry • Build component groups glBuildComponentGroupIMG(0, group, numComponents, components);

• Merge component groups glMergeComponentGroupIMG(0, group, numGroups, groups);

• Component groups bound to scene arrays sceneArray = glCreateSceneArrayIMG(); glBindSceneArrayComponentGroupIMG(sceneArray, sceneIdx, group);

• Synchronization sync = glFenceSync(GL_SYNC_RTU_COMMANDS_COMPLETE_IMG, 0); glWaitSync(sync, 0, GL_TIMEOUT_IGNORED);

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Simple approach Single component group Frame 0 Build Component Group

Emit Rays

Frame 1 Fence Sync Frame 0

Wait Sync Frame 0

Build Component Group

Emit Rays

Component Group

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Frame 2 Fence Sync Frame 1

Wait Sync Frame 1

Build Component Group

Emit Rays

Fence Sync Frame 2

Use merging for better performance Divide scene based on update frequency Build Static Component Group

Build Dynamic Component Group 0

Merge Root Component Group 0

Frame 2

Frame 1

Frame 0 Fence Sync Frame 0

Render Scene 0

Build Dynamic Component Group 1

Merge Root Component Group 1

Scene 0

Render Scene 1

Fence Sync Frame 1

Wait Sync Frame 0

Build Dynamic Component Group 0

Scene 1

Scene Array Root Component Group 0 Root Component Group 1

Static Component Group facebook.com/imgtec

Dynamic Component Group 0

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Dynamic Component Group 1

Questions? Get in touch: • [email protected] @griffin1977 Please come and visit us at booth #1902 in the South Hall to see our demos and to collect your very own Vulkan™ Gnome t-shirt!

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