CIS 565 Project 6: WebGL Deferred Shader
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README.md

WebGL Deferred Shading

University of Pennsylvania, CIS 565: GPU Programming and Architecture, Project 6

Terry Sun; Arch Linux, Intel i7-4600, integrated graphics

A tile-based deferred shader.

Check it out: live online! There are a lot of available sliders and options for you to play with.

Showing 300 lights at a tile size of 75x75, rendering in about 80ms.

Tile

This tile-based shader technique "inverts" the deferred shader step: rather than drawing to the entire scene once per each light, the scene is drawn in individual tiles, and each tile loops over the lights that intersect it.

(Figure from Tiled Shading)

Inspiration for texture layout was taken from the image above, which a few modifications: the global light list was stored as two separate textures with corresponding indices (one containing position and radius; the other containing light colors). The "tile light light indices" texture, which stored the list of lights for each tile, packed closely, was not an Nx1 texture but a 4092xM texture, expanding to allow larger lists of indices. (The light grid, however, never exceeded an Nx1 size.)

This technique will render 300 lights in ~66 ms and 400 lights in ~76 ms. (See below for more information.)

Toon Shading

In addition to Blinn-Phong lighting, this shader implements a non-photorealistic toon shader. This consists of two main effects:

  • Discretization of color values, which mimicks a hand-drawn or hand-colored shading style. This is a simple post-processing step (though not a separate deferred pipeline step) where the calculated Blinn-Phong light value is floored to the nearest color "edge".
  • Edge shading applies a bolding effect on the edges of geometry, calculated by sampling the depth texture at a fragment's neighbors. If the maximum depth difference is greater than some threshold, then consider the fragment to lie on an edge and shade it black. The edge can be made thicker by sampling pixels which lie farther away.

Watercolor

Actually a cool blooper: this is what happens if you discretize over the sum of colors in a tile rather than each individual light.

(Currently, supported on tile-based deferred shading only. On non-tile-based shading, this would have to be a post-processing step.)

Performance

Performance numbers were taken with stats.js, with the camera placed inside the scene and lights evenly distributed within the scene such that almost all of them are within view of the camera.

This results are similar to

The tile size actually doesn't have a particularly large effect at lower light counts. Here, the performance gain of partitioning fewer lights per tile (and decreasing the amount of time looping through lights on the GPU) is counteracted by the amount of work needed to compute the lights-per-tile partition texture on the CPU, leaving the time relatively consistent for many tile sizes between 40x40 and 100x100. The timing at about 30x30 is particularly inconsistent -- perhaps this is one place where the CPU/GPU tradeoff is close to even. At tile sizes smaller than 20x20, you might expect there to be few lights per tile, it takes quite a lot of time in order to generate the necessary textures (which are themselves much larger).

With larger numbers of lights in the scene, larger tile sizes also see a large hit to performance due to looping over lights in the fragment shader. With fewer lights, this takes much longer (not shown) to manifest as a problem.

At higher light counts, the majority of the time is spent on the CPU computing light textures. This is verified by using the Chome Profiling tool, which shows that a significantly higher percentage of time is spent in the texture computation function (~75% at 300 lights, ~50% at 30 lights) at more brightly lit scenes. With fewer lights in the scene, a much smaller percentage of time is spent on the CPU. I suspect that that the actual ms-per-frame spent on the GPU rises at a much slower rate per light than on the CPU, accounting for the large difference in relative time on each procesing unit.

Comparison take with 50x50-size tiles.

The main performance cost of toon shading is the additional texture accesses required to find edges, as each pixel must read four neighbors' depth values. The discretization of light values should add almost no cost, since it is implemented as a simple mathematical funciton.

References