• # Coverage Buffer as main Occlusion Culling technique

Graphics and GPU Programming

# Introduction

Recently I came across a awesome presentation from Crytek named Secrets of CryEGNINE 3 Graphics Technology authored by Nickolay Kasyan, Nicolas Schulz and Tiago Sousa. In this paper I've found a brief description of a technique called Coverage Buffer. You can find whole presentation HERE. This technology was presented as main occlusion culling method, actively used since Crysis 2. And, since there was no detailed paper about this technology, I decided to dig into this matter myself. [hr]

# Coverage Buffer - Occlusion Culling technique

## Overview

Main idea of method was clearly stated by Crytek presentation I mentioned before:
• Get depth buffer from previous frame
• Make reprojection to current frame
• Softwarely rasterize BBoxes of objects to check, whether they can be seen from camera perspective or not - and, based on this test, make a decision to draw or not to draw.
There's, of course, nothing revolutionary about this concept. There's another very similar method called Software Occlusion Culling. But there are few differences between these methods, and a crucial one is that in SOC we must separate objects into two different categories - occluders and occludee, which is not always can be done. Let's see some examples. If we have FPS game level, like Doom 3, we have corridors, which are perfect occluders, and objects - barrels, ammo, characters, which are, in turn, perfect occludees. In this case we have a clear approach - to test objects' BBoxes agains corridors. But what if we have, let's say, massive forest. Every tree can be both occluder - imagine large tree right in front of camera, which occludes all other world behind it, - and occludee - when some other tree occludes it. In case of forest we cannot use SOC in its pure form, it'd be counterproductive. So, summarizing cons and pros of Coverage Buffer: PROS:
• we need not to separate objects into occluders/occludee
• we can use already filled depth buffer from previous frame, we need not to rasterize large occluder's BBoxes
CONS:
• small artifacts, caused by 1 frame delay (even reprojection doesn't completely solve it)
• small overhead if there's no occlusion happened (that, I guess, is common for all OC methods I know, but still)
[hr]

## Choice

When I started to investigate this matter, it wasn't out of pure academic interest. On a existing and live project there was a particular problem, which needed to be solved. Large procedural forest caused giant lags because of overdraw issue (Dx9 alpha-test stage was disabled due to other issues, which are not discussed in this article, and in Dx11 alpha test kills Early-Z, which also causes massive overdraw). Here's a short summary of initial problem:
• We need to draw an island, full of different, procedural trees. (Engine used is Torque3D)
Engine by default offers nice batching system, which batches distant trees into... well, batches, but decisions about "draw/no draw" is taken based on frustum culling results only. Also, distant trees are rendering as billboards-imposters, which is also a nice optimization. But this approach is not so effective, when we deal with large forest with thousands of trees. In this case there's a lot of overdraw done: batches behind mountains, batches behind the wall, batches behind other trees and so on. All of these overdraws cause FPS to drop gravely: even if we look through wall in the direction towards the center of an island, drawing of invisible trees took about 20-30ms. As a result, players got a dramatic FPS drop by just looking towards the center of an isle. To solve this particular issue it's been decided to use Coverage Buffer. I cannot say that I did not have doubts about this decision, but Crytek recommendations overruled all my other suggestions. Besides, CB fits into this particular issue like a glove - why not try it?

## Implementation

Let's proceed to technical details and code. Obtaining Depth Buffer. First task was to obtain depth buffer. In Dx11 it's no difficult task. In Dx9 it's also not so difficult, there's a certain hack (found in Aras Pranckevi?ius blog, it's a guy, who runs render in Unity3D). Here's link: http://aras-p.info/texts/D3D9GPUHacks.html It appears, that one CAN obtain depth buffer, but only with special format - INTZ. According to official NVidia and AMD papers, most of videocards since 2008 support this feature. For earlier cards there's RAWZ - another hacky format. Links to papers: http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2012/10/Advanced-DX9-Capabilities-for-ATI-Radeon-Cards_v2.pdf http://developer.download.nvidia.com/GPU_Programming_Guide/GPU_Programming_Guide_G80.pdf Usage code is trivial, but I'll put it here - just in case:  #define FOURCC_INTZ ((D3DFORMAT)(MAKEFOURCC('I','T','N','Z'))) // Determine if INTZ is supported HRESULT hr; hr = pd3d->CheckDeviceFormat(AdapterOrdinal, DeviceType, AdapterFormat, D3DUSAGE_DEPTHSTENCIL, D3DRTYPE_TEXTURE, FOURCC_INTZ); BOOL bINTZDepthStencilTexturesSupported = (hr == D3D_OK); // Create an INTZ depth stencil texture IDirect3DTexture9 *pINTZDST; pd3dDevice->CreateTexture(dwWidth, dwHeight, 1, D3DUSAGE_DEPTHSTENCIL, FOURCC_INTZ, D3DPOOL_DEFAULT, &pINTZDST, NULL); // Retrieve depth buffer surface from texture interface IDirect3DSurface9 *pINTZDSTSurface; pINTZDST->GetSurfaceLevel(0, &pINTZDSTSurface); // Bind depth buffer pd3dDevice->SetDepthStencilSurface(pINTZDSTSurface); // Bind depth buffer texture pd3dDevice->SetTexture(0, pINTZDST);  Next step is processing depth buffer, so we could use it. Processing depth buffer.
• downscale to low resolution (I picked 256x128)
• reprojection
These steps are trivial. Downscale is performed with operator max - we're taking the elowest distance to camera, so we wouldn't occlude any of actually visible objects. Reprojection is performed by applying inverted ViewProjection matrix of previous frame and applying ViewProjection matrix of current frame to results. Gaps are filled with maxValue to prevent artificial occlusion. Here's some useful parts of code for reprojection:  float3 reconstructPos(Texture2D depthTexture, float2 texCoord, float4x4 matrixProjectionInverted ) { float depth = 1-depthTexture.Sample( samplerDefault, texCoord ).r; float2 cspos = float2(texCoord.x * 2 - 1, (1-texCoord.y) * 2 - 1); float4 depthCoord = float4(cspos, depth, 1); depthCoord = mul (matrixProjectionInverted, depthCoord); return depthCoord.xyz / depthCoord.w; }  Projection performed trivially. Software rasterization This topic is well known and already implemented a lot of times. Best info which I could find was here: https://software.intel.com/en-us/blogs/2013/09/06/software-occlusion-culling-update-2 But, just to gather all eggs in one basket, I'll provide my code, which was originally implemented in plain c++, and later translated to SSE, after which it became approximately 3 times faster. My SSE is far from perfect, so, if you find any mistakes or places for optimization - please tell me =)  static const int sBBIndexList[36] = { // index for top 4, 8, 7, 4, 7, 3, // index for bottom 5, 1, 2, 5, 2, 6, // index for left 5, 8, 4, 5, 4, 1, // index for right 2, 3, 7, 2, 7, 6, // index for back 6, 7, 8, 6, 8, 5, // index for front 1, 4, 3, 1, 3, 2, }; __m128 SSETransformCoords(__m128 *v, __m128 *m) { __m128 vResult = _mm_shuffle_ps(*v, *v, _MM_SHUFFLE(0,0,0,0)); vResult = _mm_mul_ps(vResult, m[0]); __m128 vTemp = _mm_shuffle_ps(*v, *v, _MM_SHUFFLE(1,1,1,1)); vTemp = _mm_mul_ps(vTemp, m[1]); vResult = _mm_add_ps(vResult, vTemp); vTemp = _mm_shuffle_ps(*v, *v, _MM_SHUFFLE(2,2,2,2)); vTemp = _mm_mul_ps(vTemp, m[2]); vResult = _mm_add_ps(vResult, vTemp); vResult = _mm_add_ps(vResult, m[3]); return vResult; } __forceinline __m128i Min(const __m128i &v0, const __m128i &v1) { __m128i tmp; tmp = _mm_min_epi32(v0, v1); return tmp; } __forceinline __m128i Max(const __m128i &v0, const __m128i &v1) { __m128i tmp; tmp = _mm_max_epi32(v0, v1); return tmp; } struct SSEVFloat4 { __m128 X; __m128 Y; __m128 Z; __m128 W; }; // get 4 triangles from vertices void SSEGather(SSEVFloat4 pOut[3], int triId, const __m128 xformedPos[]) { for(int i = 0; i < 3; i++) { int ind0 = sBBIndexList[triId*3 + i + 0]-1; int ind1 = sBBIndexList[triId*3 + i + 3]-1; int ind2 = sBBIndexList[triId*3 + i + 6]-1; int ind3 = sBBIndexList[triId*3 + i + 9]-1; __m128 v0 = xformedPos[ind0]; __m128 v1 = xformedPos[ind1]; __m128 v2 = xformedPos[ind2]; __m128 v3 = xformedPos[ind3]; _MM_TRANSPOSE4_PS(v0, v1, v2, v3); pOut.X = v0; pOut.Y = v1; pOut.Z = v2; pOut.W = v3; //now X contains X0 x1 x2 x3, Y - Y0 Y1 Y2 Y3 and so on... } } bool RasterizeTestBBoxSSE(Box3F box, __m128* matrix, float* buffer, Point4I res) { //TODO: performance LARGE_INTEGER frequency; // ticks per second LARGE_INTEGER t1, t2; // ticks double elapsedTime; // get ticks per second QueryPerformanceFrequency(&frequency); // start timer QueryPerformanceCounter(&t1); //verts and flags __m128 verticesSSE[8]; int flags[8]; static Point4F vertices[8]; static Point4F xformedPos[3]; static int flagsLoc[3]; // Set DAZ and FZ MXCSR bits to flush denormals to zero (i.e., make it faster) // Denormal are zero (DAZ) is bit 6 and Flush to zero (FZ) is bit 15. // so to enable the two to have to set bits 6 and 15 which 1000 0000 0100 0000 = 0x8040 _mm_setcsr( _mm_getcsr() | 0x8040 ); // init vertices Point3F center = box.getCenter(); Point3F extent = box.getExtents(); Point4F vCenter = Point4F(center.x, center.y, center.z, 1.0); Point4F vHalf = Point4F(extent.x*0.5, extent.y*0.5, extent.z*0.5, 1.0); Point4F vMin = vCenter - vHalf; Point4F vMax = vCenter + vHalf; // fill vertices vertices[0] = Point4F(vMin.x, vMin.y, vMin.z, 1); vertices[1] = Point4F(vMax.x, vMin.y, vMin.z, 1); vertices[2] = Point4F(vMax.x, vMax.y, vMin.z, 1); vertices[3] = Point4F(vMin.x, vMax.y, vMin.z, 1); vertices[4] = Point4F(vMin.x, vMin.y, vMax.z, 1); vertices[5] = Point4F(vMax.x, vMin.y, vMax.z, 1); vertices[6] = Point4F(vMax.x, vMax.y, vMax.z, 1); vertices[7] = Point4F(vMin.x, vMax.y, vMax.z, 1); // transforms for(int i = 0; i < 8; i++) { verticesSSE = _mm_loadu_ps(vertices); verticesSSE = SSETransformCoords(&verticesSSE, matrix); __m128 vertX = _mm_shuffle_ps(verticesSSE, verticesSSE, _MM_SHUFFLE(0,0,0,0)); // xxxx __m128 vertY = _mm_shuffle_ps(verticesSSE, verticesSSE, _MM_SHUFFLE(1,1,1,1)); // yyyy __m128 vertZ = _mm_shuffle_ps(verticesSSE, verticesSSE, _MM_SHUFFLE(2,2,2,2)); // zzzz __m128 vertW = _mm_shuffle_ps(verticesSSE, verticesSSE, _MM_SHUFFLE(3,3,3,3)); // wwww static const __m128 sign_mask = _mm_set1_ps(-0.f); // -0.f = 1 << 31 vertW = _mm_andnot_ps(sign_mask, vertW); // abs vertW = _mm_shuffle_ps(vertW, _mm_set1_ps(1.0f), _MM_SHUFFLE(0,0,0,0)); //w,w,1,1 vertW = _mm_shuffle_ps(vertW, vertW, _MM_SHUFFLE(3,0,0,0)); //w,w,w,1 // project verticesSSE = _mm_div_ps(verticesSSE, vertW); // now vertices are between -1 and 1 const __m128 sadd = _mm_setr_ps(res.x*0.5, res.y*0.5, 0, 0); const __m128 smult = _mm_setr_ps(res.x*0.5, res.y*(-0.5), 1, 1); verticesSSE = _mm_add_ps( sadd, _mm_mul_ps(verticesSSE,smult) ); } // Rasterize the AABB triangles 4 at a time for(int i = 0; i < 12; i += 4) { SSEVFloat4 xformedPos[3]; SSEGather(xformedPos, i, verticesSSE); // by 3 vertices // fxPtX[0] = X0 X1 X2 X3 of 1st vert in 4 triangles // fxPtX[1] = X0 X1 X2 X3 of 2nd vert in 4 triangles // and so on __m128i fxPtX[3], fxPtY[3]; for(int m = 0; m < 3; m++) { fxPtX[m] = _mm_cvtps_epi32(xformedPos[m].X); fxPtY[m] = _mm_cvtps_epi32(xformedPos[m].Y); } // Fab(x, y) = Ax + By + C = 0 // Fab(x, y) = (ya - yb)x + (xb - xa)y + (xa * yb - xb * ya) = 0 // Compute A = (ya - yb) for the 3 line segments that make up each triangle __m128i A0 = _mm_sub_epi32(fxPtY[1], fxPtY[2]); __m128i A1 = _mm_sub_epi32(fxPtY[2], fxPtY[0]); __m128i A2 = _mm_sub_epi32(fxPtY[0], fxPtY[1]); // Compute B = (xb - xa) for the 3 line segments that make up each triangle __m128i B0 = _mm_sub_epi32(fxPtX[2], fxPtX[1]); __m128i B1 = _mm_sub_epi32(fxPtX[0], fxPtX[2]); __m128i B2 = _mm_sub_epi32(fxPtX[1], fxPtX[0]); // Compute C = (xa * yb - xb * ya) for the 3 line segments that make up each triangle __m128i C0 = _mm_sub_epi32(_mm_mullo_epi32(fxPtX[1], fxPtY[2]), _mm_mullo_epi32(fxPtX[2], fxPtY[1])); __m128i C1 = _mm_sub_epi32(_mm_mullo_epi32(fxPtX[2], fxPtY[0]), _mm_mullo_epi32(fxPtX[0], fxPtY[2])); __m128i C2 = _mm_sub_epi32(_mm_mullo_epi32(fxPtX[0], fxPtY[1]), _mm_mullo_epi32(fxPtX[1], fxPtY[0])); // Compute triangle area __m128i triArea = _mm_mullo_epi32(B2, A1); triArea = _mm_sub_epi32(triArea, _mm_mullo_epi32(B1, A2)); __m128 oneOverTriArea = _mm_div_ps(_mm_set1_ps(1.0f), _mm_cvtepi32_ps(triArea)); __m128 Z[3]; Z[0] = xformedPos[0].W; Z[1] = _mm_mul_ps(_mm_sub_ps(xformedPos[1].W, Z[0]), oneOverTriArea); Z[2] = _mm_mul_ps(_mm_sub_ps(xformedPos[2].W, Z[0]), oneOverTriArea); // Use bounding box traversal strategy to determine which pixels to rasterize __m128i startX = _mm_and_si128(Max(Min(Min(fxPtX[0], fxPtX[1]), fxPtX[2]), _mm_set1_epi32(0)), _mm_set1_epi32(~1)); __m128i endX = Min(Max(Max(fxPtX[0], fxPtX[1]), fxPtX[2]), _mm_set1_epi32(res.x - 1)); __m128i startY = _mm_and_si128(Max(Min(Min(fxPtY[0], fxPtY[1]), fxPtY[2]), _mm_set1_epi32(0)), _mm_set1_epi32(~1)); __m128i endY = Min(Max(Max(fxPtY[0], fxPtY[1]), fxPtY[2]), _mm_set1_epi32(res.y - 1)); // Now we have 4 triangles set up. Rasterize them each individually. for(int lane=0; lane < 4; lane++) { // Skip triangle if area is zero if(triArea.m128i_i32[lane] <= 0) { continue; } // Extract this triangle's properties from the SIMD versions __m128 zz[3]; for(int vv = 0; vv < 3; vv++) { zz[vv] = _mm_set1_ps(Z[vv].m128_f32[lane]); } //drop culled triangle int startXx = startX.m128i_i32[lane]; int endXx = endX.m128i_i32[lane]; int startYy = startY.m128i_i32[lane]; int endYy = endY.m128i_i32[lane]; __m128i aa0 = _mm_set1_epi32(A0.m128i_i32[lane]); __m128i aa1 = _mm_set1_epi32(A1.m128i_i32[lane]); __m128i aa2 = _mm_set1_epi32(A2.m128i_i32[lane]); __m128i bb0 = _mm_set1_epi32(B0.m128i_i32[lane]); __m128i bb1 = _mm_set1_epi32(B1.m128i_i32[lane]); __m128i bb2 = _mm_set1_epi32(B2.m128i_i32[lane]); __m128i cc0 = _mm_set1_epi32(C0.m128i_i32[lane]); __m128i cc1 = _mm_set1_epi32(C1.m128i_i32[lane]); __m128i cc2 = _mm_set1_epi32(C2.m128i_i32[lane]); __m128i aa0Inc = _mm_mul_epi32(aa0, _mm_setr_epi32(1,2,3,4)); __m128i aa1Inc = _mm_mul_epi32(aa1, _mm_setr_epi32(1,2,3,4)); __m128i aa2Inc = _mm_mul_epi32(aa2, _mm_setr_epi32(1,2,3,4)); __m128i alpha0 = _mm_add_epi32(_mm_mul_epi32(aa0, _mm_set1_epi32(startXx)), _mm_mul_epi32(bb0, _mm_set1_epi32(startYy))); alpha0 = _mm_add_epi32(cc0, alpha0); __m128i beta0 = _mm_add_epi32(_mm_mul_epi32(aa1, _mm_set1_epi32(startXx)), _mm_mul_epi32(bb1, _mm_set1_epi32(startYy))); beta0 = _mm_add_epi32(cc1, beta0); __m128i gama0 = _mm_add_epi32(_mm_mul_epi32(aa2, _mm_set1_epi32(startXx)), _mm_mul_epi32(bb2, _mm_set1_epi32(startYy))); gama0 = _mm_add_epi32(cc2, gama0); int rowIdx = (startYy * res.x + startXx); __m128 zx = _mm_mul_ps(_mm_cvtepi32_ps(aa1), zz[1]); zx = _mm_add_ps(zx, _mm_mul_ps(_mm_cvtepi32_ps(aa2), zz[2])); zx = _mm_mul_ps(zx, _mm_setr_ps(1.f, 2.f, 3.f, 4.f)); // Texels traverse for(int r = startYy; r < endYy; r++, rowIdx += res.x, alpha0 = _mm_add_epi32(alpha0, bb0), beta0 = _mm_add_epi32(beta0, bb1), gama0 = _mm_add_epi32(gama0, bb2)) { // Compute barycentric coordinates // Z0 as an origin int index = rowIdx; __m128i alpha = alpha0; __m128i beta = beta0; __m128i gama = gama0; //Compute barycentric-interpolated depth __m128 depth = zz[0]; depth = _mm_add_ps(depth, _mm_mul_ps(_mm_cvtepi32_ps(beta), zz[1])); depth = _mm_add_ps(depth, _mm_mul_ps(_mm_cvtepi32_ps(gama), zz[2])); __m128i anyOut = _mm_setzero_si128(); __m128i mask; __m128 previousDepth; __m128 depthMask; __m128i finalMask; for(int c = startXx; c < endXx; c+=4, index+=4, alpha = _mm_add_epi32(alpha, aa0Inc), beta = _mm_add_epi32(beta, aa1Inc), gama = _mm_add_epi32(gama, aa2Inc), depth = _mm_add_ps(depth, zx)) { mask = _mm_or_si128(_mm_or_si128(alpha, beta), gama); previousDepth = _mm_loadu_ps(&(buffer[index])); //calculate current depth //(log(depth) - -6.907755375) * 0.048254941; __m128 curdepth = _mm_mul_ps(_mm_sub_ps(log_ps(depth),_mm_set1_ps(-6.907755375)),_mm_set1_ps(0.048254941)); curdepth = _mm_sub_ps(curdepth, _mm_set1_ps(0.05)); depthMask = _mm_cmplt_ps(curdepth, previousDepth); finalMask = _mm_andnot_si128(mask, _mm_castps_si128(depthMask)); anyOut = _mm_or_si128(anyOut, finalMask); }//for each column if(!_mm_testz_si128(anyOut, _mm_set1_epi32(0x80000000))) { // stop timer QueryPerformanceCounter(&t2); // compute and print the elapsed time in millisec elapsedTime = (t2.QuadPart - t1.QuadPart) * 1000.0 / frequency.QuadPart; RasterizationStats::RasterizeSSETimeSpent += elapsedTime; return true; //early exit } }// for each row }// for each triangle }// for each set of SIMD# triangles return false; }  Now we have Coverage Buffer technique up and running.

## Results

Using C-Buffer for Occlusion Culling in our particular case reduced frame render time by 10-20 ms (and in some cases even more). But it also gave about 2ms overhead in "nothing culled" case. This method was useful in our case, but it doesn't mean, that it can be used in all other cases. Actually, it puzzles me, how Crytek used it in Crysis 2 - imho, CB-unfriendly game. Perhaps I took some of it concepts wrong? Well, maybe =) So, as it appears to me, main restriction for this method would be: Do not use it unless you want to cull something, that takes forever to render (like forest with overdraw, for instance). CPU rasterization is a costly matter, and it's not worth it, when its applied to a simple easy-to-render objects with gpu-cheap materials.

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## User Feedback

is there any advantage in using the zbuffer from the last frame over simple occlusion queries?

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is there any advantage in using the zbuffer from the last frame over simple occlusion queries?

Yes, occlusion queries gives serious delay - sometimes up to 3 or 4 frames, thus you cannot rely
on it completely, it can cause serious artificial cullings, which in turn leads to objects, appearing out of
a thin air.
Using Z-Buffer from previous frame combined with reprojection gives, basically, zero delay and completely reliable.

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why would an occlusion query have any other access latency than reading the zbuffer?

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why would an occlusion query have any other access latency than reading the zbuffer?

Because occlusion query submits the command buffer, executes it, and sends the results back to the CPU - this means that it has 2 CPU-GPU sync points.

Of course you can force the query to execute and get the results immediately, but in this case you will get a whole GPU stalled.

You can find more detailed discussion on this topic here

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to get the zbuffer, you also submit the command buffer, execute it and send the zbuffer back via a staging buffer to the CPU. Otherwise you'd stall for the GPU. (GPU and CPU run asynchronously, that's why occlusion queries have that delay).

Occlusion queries on the zbuffer approach of CryEngine have exactly the same latency and quality on PC.

The reason CryEngine uses a zbuffer approach is because on consoles you can access the zbuffer without the cumbersome staging latency and all tests can be done in some 'job' which effectively is free, as the job can be triggered by the GPU as soon as the zbuffer is available and is ready the moment the CPU needs the test results to process drawcalls.

(check the presentation paper, it states consoles have 1 frame latency, PC up to 4).

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This would benefit from a more detailed explanation of the software bounding box stage rather than just a code dump. I read the article and, without analysing the code, I'm left completely clueless abou that step. Maybe it's just me.

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to get the zbuffer, you also submit the command buffer, execute it and send the zbuffer back via a staging buffer to the CPU. Otherwise you'd stall for the GPU. (GPU and CPU run asynchronously, that's why occlusion queries have that delay).

Occlusion queries on the zbuffer approach of CryEngine have exactly the same latency and quality on PC.

The reason CryEngine uses a zbuffer approach is because on consoles you can access the zbuffer without the cumbersome staging latency and all tests can be done in some 'job' which effectively is free, as the job can be triggered by the GPU as soon as the zbuffer is available and is ready the moment the CPU needs the test results to process drawcalls.

(check the presentation paper, it states consoles have 1 frame latency, PC up to 4).

That's not exactly correct.
In our case, we have our command buffer executed, and all we need to do is to fetch Z-Buffer to CPU. It will cause GPU stall, of course, but a small one. Staging latency is rather small in this case - check for yourself.

As for Occlusion Query, delayed one is unreliable - I think we can agree on that, and forcing one to implement immediately causes large GPU stall, much larger than fetching z-buffer, not to mention performing Occlusion Query for a complex scene with a lot of objects is a costly operation - it's basically another full scene pass, and if we can replace some simple objects with BBoxes, we cannot replace complex objects, for example - terrain, with BBox.

And, besides that, using Z-Buffer gives much better quality of culling than using not accurate BBoxes in Occlusion Queries. And it's much more flexible - we don't need to adjust BBoxes, as we should do for OQ:
http://http.developer.nvidia.com/GPUGems/gpugems_ch29.html
That is, in fact, a great article, that gives a lot of points about why you shouldn't use OQ as main scene occlusion technique.

I'm not saying, that C-Buffer is always better than Occlusion Queries, but in some cases it surely is.

Btw, I wonder if anyone actually uses Occlusion Queries in actual projects or engines,..

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This would benefit from a more detailed explanation of the software bounding box stage rather than just a code dump. I read the article and, without analysing the code, I'm left completely clueless abou that step. Maybe it's just me.

This step is quite trivial. You get depth buffer from previous frame, and then softwarely rasterize BBoxes of objects you want to test for occlusion,

bool RasterizeTestBBoxSSE(Box3F box, __m128* matrix, float* buffer, Point4I res)

This function accepts bounding box, depth buffer as plain float array and parameters of this buffer.
Then it represents BBox as set of triangles and rasterize it, testing with buffer values.

I honestly don't know what's more to say. If you want to know more about rasterization process, It's rather trivial - you can find a lot of other sources in internet, - and, more importantly, not the point of this particular article.

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for fairness, I guess I should mention, I'm the guy who implemented all the culling in Crysis 1+2+3 and Ryse. for validating: https://twitter.com/rapso

I also don't aim to sabotage your article, I think it's well written and it's nice to see someone else took the effort to implement this. But it would be a little bit misleading to put one solution over the other, both do essentially the same and both can lead to mostly similar results (quality and performance wise), but implementation wise, occlusion queries might be simpler.

That's not exactly correct.
In our case, we have our command buffer executed, and all we need to do is to fetch Z-Buffer to CPU. It will cause GPU stall, of course, but a small one. Staging latency is rather small in this case - check for yourself.

the more you render, the bigger the GPU stall will be, because the GPU works more asynchronously. in real game scenes that's more obvious than in some test, that's why this might not seem that obvious in your tests. That's why CryEngine has a bigger latency on PC. (you need up to 4 due to Crossfire/SLI).

As for Occlusion Query, delayed one is unreliable - I think we can agree on that

why would it be? It's used by more games than coverage buffering.

, and forcing one to implement immediately causes large GPU stall, much larger than fetching z-buffer

no, it will cost exactly the same. The cost you pay for is to wait until the GPU is done processing and then have a bubble in your GPU execution pipe until you kick off the next command buffer.

Most games are GPU limited, I recommend to do whatever it takes to not stall your GPU.

not to mention performing Occlusion Query for a complex scene with a lot of objects is a costly operation - it's basically another full scene pass, and if we can replace some simple objects with BBoxes, we cannot replace complex objects, for example - terrain, with BBox.

occlusion queries for previously visible objects are done in-place while you're drawing the scene. it's essentially free. for previously invisible objects, you're drawing just BBoxes, or lower LOD meshes if wanted.
for terrain you're testing individual chunks of a terrain, not the whole terrain as one object. One important thing is to test the water surface accurately, if it is invisible, you can skip all reflection rendering, thus this should get your special care.

And, besides that, using Z-Buffer gives much better quality of culling than using not accurate BBoxes in Occlusion Queries. And it's much more flexible - we don't need to adjust BBoxes, as we should do for OQ:
http://http.developer.nvidia.com/GPUGems/gpugems_ch29.html
That is, in fact, a great article, that gives a lot of points about why you shouldn't use OQ as main scene occlusion technique.

you have the choice of what accuracy you want. you can use whatever detailed objects you'd like, rects, AABBs, lod detail LODs,...

I'm not saying, that C-Buffer is always better than Occlusion Queries, but in some cases it surely is.

Btw, I wonder if anyone actually uses Occlusion Queries in actual projects or engines,..

e.g. Unreal engine and all games use it afaik. [edit: I mean, all games that use UE use it, pardon]

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for fairness, I guess I should mention, I'm the guy who implemented all the culling in

Well, I guess it's time for me to surrender =) If you're the guy, who invented the very method I struggled to implement, you're obviously much more experienced and knowledgeable in this area than myself =)

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Hmm, this reminds me of the Hierarchical Z-Buffer, though I have not seen the trick of grabbing the previous frame before.

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Hey Damon - looks like really nice work! Any chance you've thought about contributing to T3D? We're always looking for people with knowhow to add improvements like this which will benefit many users!

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Any chance you've thought about contributing to T3D?

I'm afraid it won't be an easy task, our build of T3D is very much different from current, official version. It won't be easy to port this thing to vanilla torque.
Besides, now it's a hot season for our project - release is coming, and there's a lot of work and optimization to be done yet. Maybe after release =)

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Makes sense. Best of luck with the release!

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Just to make it even more fun :)

I'm actually using the Occlusion Queries in our engine, and out of curiosity I also implemented the C-Buffer algorithm some time ago, but with a slight modification where I instead have a mipmap chain on the cpu side and project the corners of the bbox view rectangle into the mip where the corners exactly cover 2x2 pixels, like described in this article http://blog.selfshadow.com/publications/practical-visibility/ (but on the CPU instead of the GPU).

This gave me very fast tests and so I could perform culling at a finer level.

Alas this did not give me any kind of speed up compared to the Occlusion Query algorithm, as I'm almost always GPU bound in our game.

And with 1 frame delayed Occlusion Queries you are not stalling the GPU but the CPU, while the CPU waits for the result from the previous frame to be calculated by the GPU before it can be copied to main memory..

But maybe you can gain some performance by using the mipmap trick? Care has to be taken though when filling out the gaps in the reprojected buffer. I used a dilation/blur kind of filter instead, to avoid single pixels at far plane to dominate the mip chain.

Enjoy!

Henning

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But maybe you can gain some performance by using the mipmap trick? Care has to be taken though when filling out the gaps in the reprojected buffer. I used a dilation/blur kind offilter instead, to avoid single pixels at far plane to dominate the mip chain.

That's some great ideas, thx for sharing! It'll definitely come in handy! )

B.t.w, does anyone know, how exactly Umbra works? I've heard, that Witcher 3 actually use their OC solution

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Just wanted to drop that CryEngine is now open source, you can probably check out the whole source code of it (most likely the software rasterizer for occlusion culling is included, we decided on per-level base whether we use the zbuffer, the softwarerasterizer or both).

It should also include a nice little vector-math wrapper (VMath...) that made the culler work on SPU/PPU/XBox360/SSE/Neon (and as that came from my home-projects, it was working on ps2,gamecube,psp), hence should be a nice starting point for learning about occlusion culling and vectormath wrapper.

Maybe someone will even find a bug or optimization :)

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