117 lines
4.1 KiB
Plaintext
117 lines
4.1 KiB
Plaintext
Title: Hand Optimized Simplex 2D
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Brief: Results of messing around with moving and hoisting stuff around.
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Date: 1688995095
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Tags: Programming, GLSL, OpenGL, Optimization
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CSS: /style.css
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Based on [webgl-noise repository](https://github.com/ashima/webgl-noise), which is based on [this paper](https://arxiv.org/pdf/1204.1461.pdf).
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Things tried:
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* Rearranging operations to reduce register pressure.
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* Calculating things as soon as possible.
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* Hand inlining.
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### Results ###
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For testing screen space *1024x1024* texture is generated, resulting in *1048576* fragment invocations,
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with 4 octave fractal brownian motion.
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Hardware: `Mobile Intel® GM45 Express Chipset`
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Driver: `DRI Mesa 21.2.6`
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Original:
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```
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Benchmark Iterations Min(ns) Max(ns) Variance Mean(ns)
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----------------------------------------------------------------
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full(0) 100 124848395 510494575 1473830605484805 129237053
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```
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Hand optimized:
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```
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Benchmark Iterations Min(ns) Max(ns) Variance Mean(ns)
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----------------------------------------------------------------
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full(0) 100 119354512 731397135 3705581696928414 125714847
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```
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Mean difference is `3ms 522µs 206ns (-2.7%)`, min difference is `5ms 493µs 883ns (-4.4%)`
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This suggests that given driver is suboptimal in its optimizing capabilities,
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and I imagine there might be GLSL compilers a lot worse than this.
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Some intermediate shader representation comes to mind as a mean for
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automatic GLSL source level, profile guided and other optimizations;
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as well as polyfilling to different extensions, profiles and APIs. But welp.
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### Source ###
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```glsl
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#version 120
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// Author : Ian McEwan, Ashima Arts.
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// Maintainer : stegu
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// Lastmod : 20110822 (ijm)
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// License : Copyright (C) 2011 Ashima Arts. All rights reserved.
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// Distributed under the MIT License. See LICENSE file.
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// https://github.com/ashima/webgl-noise
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// https://github.com/stegu/webgl-noise
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//
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#define MOD289(p_x) ((p_x) - floor((p_x) * (1.0 / 289.0)) * 289.0)
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#define PERMUTE(p_result, p_x) { vec3 _temp = (((p_x) * 34.0) + 10.0) * (p_x); p_result = MOD289(_temp); }
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float simplex_noise_2d(in vec2 v) {
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const vec4 C = vec4(0.211324865405187, // (3.0-sqrt(3.0))/6.0
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0.366025403784439, // 0.5*(sqrt(3.0)-1.0)
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-0.577350269189626, // -1.0 + 2.0 * C.x
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0.024390243902439); // 1.0 / 41.0
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// First corner
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vec2 i = floor(v + dot(v, C.yy));
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vec2 x0 = v - i + dot(i, C.xx);
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i = MOD289(i); // Avoid truncation effects in permutation
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// i1.x = step( x0.y, x0.x ); // x0.x > x0.y ? 1.0 : 0.0
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// i1.y = 1.0 - i1.x;
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vec2 i1 = (x0.x > x0.y) ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
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// Other corners
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// x0 = x0 - 0.0 + 0.0 * C.xx ;
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// x1 = x0 - i1 + 1.0 * C.xx ;
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// x2 = x0 - 1.0 + 2.0 * C.xx ;
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vec4 x12 = x0.xyxy + C.xxzz - vec4(i1.xy, 0.0, 0.0);
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// Permutations
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vec3 pp;
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vec3 p = i.y + vec3(0.0, i1.y, 1.0);
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PERMUTE(pp, p);
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pp += i.x + vec3(0.0, i1.x, 1.0);
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PERMUTE(p, pp);
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p = fract(p * C.www);
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vec3 m = max(0.5 - vec3(dot(x0, x0), dot(x12.xy, x12.xy), dot(x12.zw, x12.zw)), 0.0);
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// Gradients: 41 points uniformly over a line, mapped onto a diamond.
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// The ring size 17*17 = 289 is close to a multiple of 41 (41*7 = 287)
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vec3 x = 2.0 * p - 1.0;
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vec3 a0 = x - floor(x + 0.5);
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vec3 h = abs(x) - 0.5;
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m = m * m;
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m = m * m;
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// Normalise gradients implicitly by scaling m
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// Approximation of: m *= inversesqrt( a0*a0 + h*h );
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m *= 1.79284291400159 - 0.85373472095314 * (a0 * a0 + h * h);
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// Compute final noise value at P
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return 130.0 * dot(m, vec3(a0.x * x0.x + h.x * x0.y, a0.yz * x12.xz + h.yz * x12.yw));
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}
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```
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### Possibilities
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[NVidia's TEGRA guide](https://docs.nvidia.com/drive/drive_os_5.1.6.1L/nvvib_docs/DRIVE_OS_Linux_SDK_Development_Guide/baggage/tegra_gles2_performance.pdf) states that uniform access is often better than constants.
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On our hardware it only degrades performance, but there's possibility of other chipsets having similar to TEGRA's preferences.
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`C` constant is legible for this.
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