README.

Author: terryxsun

README.md

@@ -7,6 +7,8 @@ Terry Sun; Arch Linux, Intel i5-4670, GTX 750
 
 This project contains a simplied graphics pipeline implemented in CUDA.
 
+![](renders/demo.gif)
+
 ## Pipeline Overview
 
 1. Vertex shader: applies a model-view-projection transformation to each
@@ -16,51 +18,129 @@ This project contains a simplied graphics pipeline implemented in CUDA.
    primitives (triangles). Parallelized across primitives.
 
 3. Geometry shader: after primitives are assembled, the geometry shader
-   generates more (or fewer) triangles for each existing triangle, up to a fixed
-   N for each triangle. Examples of this are (fixed-number) tesselation and
-   backface culling (both implemented).
+   performs additional primitive generation (or deletion), up to a fixed factor
+   (4) per original primitive.
+
+   1. Backface culling: triangles that face away from the camera are removed.
+      `thrust::remove_if` stream compaction is used to filter these out before
+      rasterization occurs.
 
-3. Rasterization: uses a scanline algorithm to determine which fragments are
+   2. Tessellation shading/smoothing: Each triangle is subdivided into 4 smaller
+      triangles, with interpolated normals.
+
+4. Rasterization: uses a scanline algorithm to determine which fragments are
    covered by a particular primitive, performs depth testing, and stores into a
    depth buffer. Uses an axis-aligned bounding box for optimization, barycentric
    coordinate checking to test coverage, and CUDA `atomicMin` to avoid race
    conditions when doing depth testing. Parallelized across primitives.
 
-4. Fragment shading: computes color of each pixel using Lambert (diffuse)
+5. Fragment shading: computes color of each pixel using Lambert (diffuse)
    shading. Interpolates normals within a triangle. Parallelized across
    fragments.
 
+6. Copy to screen/frame buffer.
+
 ## Features
 
-### Geometry shader + backface culling
+### Geometry shader
+
+**Backface culling**. Triangles which do not face the camera are removed before
+rasterization. Triangles are tested by computing the cross product between the
+edges (v0-v1, v0-v2); by convention, triangles which face away from the front of
+the model will be defined such that this cross product has a negative z
+component. ([source][bfc-wiki])
+
+  [bfc-wiki]: https://en.wikipedia.org/wiki/Back-face_culling)
+
+Back-facing faces are stream compacted away. This improves the execution
+warp coherency because all threads going through the scanline function are
+guaranteed to draw to at least one pixel on the screen.
+
+![](renders/backface.gif)
+
+(In this example the back direction is fixed relative to the model in order to
+demonstrate missing faces. In practice, backface culling would be invisible to
+the viewer.)
+
+*Tessellation geometry shading*. A second geometry shader divides each triangle
+into 4 smaller triangles (see left, below). Three new vertices are generated
+from each existing triangle. The vertex transformation must be applied again to
+each of these vertices, thus blurring the pipeline stages. (I considered moving
+the entire vertex shader to within geometry shader, but made the optimization of
+splitting the vertex shader out and transforming the original vertices once.)
+
+Below: the middle triangle of the 4 generated triangles is colored lightly to
+show the pattern of tessellation.
+
+![](renders/tri-subdiv.png)
+![](renders/suzanne-subdiv.png)
 
 ### Color interpolation
 
 For every point, its normal is interpolated from its relative distance from the
 3 vertices of its triangle. This is calculated using barycentric coordinates.
-This normal is then used to calculate a Lambert (diffuse) shading, which is
-smooth.
+This normal is then used to calculate a Lambert (diffuse) shading (plus a small
+amount of ambient lighting).
+
+Comparison of non-interpolated and interpolated normals:
+
+![](renders/suzanne-nosmooth.png)
+![](renders/suzanne-smooth.png)
+
+Animation of a light moving across the screen:
+
+![](renders/light.gif)
 
 ### Antialiasing
 
-TODO: image
+Four fragments are generated for every pixel, spaced evenly within the pixel.
+The parallelization for this process varies between stages. In some (fragment
+shader), a thread is launched for each fragment; however, in others (scanline),
+a single thread will handle four fragments in succession. Future work might be
+to do analysis on methods of parallelizing this multi-fragmented approach.
 
-Four fragments are generated for every pixel, spaced evenly within the pixel. In
-general this is parallelized 
+At the very end of the pipeline, as fragments are translated into colors for the
+frame buffer, the four pixels associated with a frame are averaged together.
 
 ### Scissor test
 
 Clipping optimization. Define a `glm::vec4(xmin, xmax, ymin, ymax)` window in
 which to render to the screen. When performing scanline rasterization algorithm,
 this test discards data outside of this window.
 
-## Internals
+![](renders/suzanne-clipped.png)
+
+## Performance
+
+### Breakdown by Stage
+
+![](data/percent-stacked.png)
+
+By far the longest stage is the rasterization/scanline function. Many of the
+other stages are completely inconsequential in comparison, as expected -- very
+little computational work is done in the primitive assembly (mostly memory
+access) or vertex shader step. It surprised me that the fragment shader is the
+second largest stage and by that margin.
 
-![](renders/suzanne-red.png)
+A future work possibility is to use a more optimized scanline function or use a
+different rasterization technique altogether.
+
+### Backface Culling
+
+Data in this case taken *with* triangle subdivision active.
+
+File    | Triangles Orig | Triangles after BFC | % Removed | Time Orig | Time AFter | % Speedup
+--------|----------------|---------------------|-----------|-----------|------------|----------
+Cow     | 23216 | 9352 | 59.7% | 13.21 | 12.94 | 2.02%
+--------|----------------|---------------------|-----------|-----------|------------|----------
+Suzanne | 3862 | 2576 | 33.3% | 22.2 | 20.83 | 6.17%
+
+
+## Internals
 
 ![](renders/suzanne-normals.png)
 
-# Bloopers
+## Bloopers
 
 ![](renders/cow-oops1.png)
 

data/aggregate.csv

@@ -0,0 +1,6 @@
+file, vertex shader, assemble primitives, geometry shader, scanline, fragment shader
+cow.txt, 0.09513088, 0.152462336, 0.2157648, 8.98044288, 3.499964032
+cow_nobfc.txt, 0.08912416, 0.142252992, 0.450094592, 9.017903488, 3.514585152
+suzanne.txt, 0.022475968, 0.03150624, 0.063839936, 16.55485335, 4.152908032
+suzanne_nobfc.txt, 0.02208192, 0.03105344, 0.081809152, 17.909788102, 4.178921536
+flower.txt, 0.01738368, 0.023185152, 0.035760704, 43.651649022, 3.331216896