Fast polygon scanning with active edge list #96

antonfirsov · 2020-10-26T21:38:57Z

Prerequisites

I have written a descriptive pull-request title
I have verified that there are no overlapping pull-requests open
I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
I have provided test coverage for my change (where applicable)

Summary

This PR is intended to narrow the performance gap we have compared to other frameworks by implementing the algorithm described in #90.

~~The PR is WIP, opened it to get early feedback. Remaining tasks:~~

Review the output of 40+ failing tests with image comparer tool. Check if any of them indicates a bug VS minor difference in output.
Deal with any bugs found
Implement NonZero intersection rule in PolygonScanner
Utilize PolygonScanner in DrawTextProcessor
Dedupe DrawTextProcessor / FillRegionProcessor code
Deal with text drawing bugs
Remove the logic enforcing TessellatedMultipolygon orientation, and make sure the algorithm renders more or less nice output with CCW outermost contours + CW holes (see Fast polygon scanning with active edge list #96 (comment))
Remove sandbox code from both product and test (eg. ShapeOptions.UsePolygonScanner)
Cleanup and fix Style Cop findings

Implementation

The pipeline works as following: IShape -> TesselatedMultipolygon -> ScanEdgeCollection (compact representation of scannable edges) -> PolygonScanner
y coordinates are rounded to the scanlines. This allows easy handling of corner cases. See NumericCornerCases.jpg
- Horizontal lines are not emitted to ScanEdgeCollection. A line counts as horizontal by definition, if the whole line lies on the same scanline after rounding it's y coordinates.
For more details about the algorithm, see comments, and the drawings pushed to the repo with documentation purposes.

Results

Fixes #90 fixes #61 fixes #18.
Resolves #5 without offsetting the whole polygon, instead the calculation of coefficients in ScanEdge is centered around zero.

According to my visual observation, the new algorithm has fewer robustness issues now. I suspect the remaining ones are caused by an unhandled corner case rather than by numeric issue:

Performance

The performance gains scale with the number of points in the polygon to draw. ( ~8.5x speedup filling Utah's and ~62x speedup filling Missisippi's polygons.)

Configuration:
BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19042
Intel Core i7-8650U CPU 1.90GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=5.0.100-rc.1.20452.10
  [Host]     : .NET Core 3.1.8 (CoreCLR 4.700.20.41105, CoreFX 4.700.20.41903), X64 RyuJIT
  DefaultJob : .NET Core 3.1.8 (CoreCLR 4.700.20.41105, CoreFX 4.700.20.41903), X64 RyuJIT


FillPolygonSmall (Utah): 

|                 Method |         Mean |      Error |     StdDev |  Ratio | RatioSD |
|----------------------- |-------------:|-----------:|-----------:|-------:|--------:|
|          SystemDrawing |    210.91 us |   4.130 us |   6.670 us |   4.72 |    0.19 |
|    ImageSharp_Original | 28,572.78 us | 350.107 us | 292.356 us | 629.88 |   18.67 |
| ImageSharp_ActiveEdges |  3,342.78 us |  63.592 us |  68.043 us |  74.06 |    2.59 |
|              SkiaSharp |     44.79 us |   0.825 us |   1.184 us |   1.00 |    0.00 |


FillPolygonMedium (Mississippi):

|                 Method |         Mean |       Error |      StdDev |    Ratio | RatioSD |
|----------------------- |-------------:|------------:|------------:|---------:|--------:|
|          SystemDrawing |     358.2 us |     7.14 us |    10.68 us |     2.27 |    0.06 |
|    ImageSharp_Original | 187,078.3 us | 3,699.54 us | 6,181.11 us | 1,186.12 |   37.40 |
| ImageSharp_ActiveEdges |   2,884.7 us |    56.27 us |   109.75 us |    18.84 |    0.67 |
|              SkiaSharp |     156.8 us |     2.91 us |     2.43 us |     1.00 |    0.00 |


DrawPolygonMediumThin:


|                 Method |         Mean |       Error |       StdDev |    Ratio | RatioSD |
|----------------------- |-------------:|------------:|-------------:|---------:|--------:|
|          SystemDrawing |     697.7 us |    13.94 us |     25.49 us |     6.08 |    0.27 |
|    ImageSharp_Original | 407,472.0 us | 7,985.96 us | 10,931.27 us | 3,541.83 |  117.29 |
| ImageSharp_ActiveEdges |  18,913.2 us |   327.80 us |    350.75 us |   165.31 |    3.56 |
|              SkiaSharp |     114.6 us |     2.11 us |      1.98 us |     1.00 |    0.00 |


DrawPolygonMediumThick:


|                 Method |       Mean |     Error |    StdDev |     Median |  Ratio | RatioSD |
|----------------------- |-----------:|----------:|----------:|-----------:|-------:|--------:|
|          SystemDrawing |   1.539 ms | 0.0304 ms | 0.0621 ms |   1.515 ms |   0.97 |    0.04 |
|    ImageSharp_Original | 169.697 ms | 3.3095 ms | 4.7463 ms | 168.241 ms | 104.43 |    2.09 |
| ImageSharp_ActiveEdges |  38.795 ms | 0.7716 ms | 1.0562 ms |  38.591 ms |  24.16 |    0.74 |
|              SkiaSharp |   1.625 ms | 0.0158 ms | 0.0132 ms |   1.623 ms |   1.00 |    0.00 |

antonfirsov · 2020-11-13T16:50:58Z

tests/ImageSharp.Drawing.Benchmarks/Drawing/DrawTextOutline.cs

+        // |        Method | TextIterations |         Mean |      Error |     StdDev |  Ratio | RatioSD |     Gen 0 |    Gen 1 | Gen 2 | Allocated |
+        // |-------------- |--------------- |-------------:|-----------:|-----------:|-------:|--------:|----------:|---------:|------:|----------:|
+        // | SystemDrawing |              1 |     55.03 us |   0.199 us |   0.186 us |   5.43 |    0.03 |         - |        - |     - |      40 B |
+        // |    ImageSharp |              1 |  2,161.92 us |   4.203 us |   3.510 us | 213.14 |    0.52 |  253.9063 |        - |     - |  804452 B |
+        // |     SkiaSharp |              1 |     10.14 us |   0.040 us |   0.031 us |   1.00 |    0.00 |    0.5341 |        - |     - |    1680 B |
+        // |               |                |              |            |            |        |         |           |          |       |           |
+        // | SystemDrawing |             20 |  1,450.12 us |   3.583 us |   3.176 us |  27.36 |    0.11 |         - |        - |     - |    3696 B |
+        // |    ImageSharp |             20 | 28,559.17 us | 244.615 us | 216.844 us | 538.85 |    3.98 | 2312.5000 | 781.2500 |     - | 9509056 B |
+        // |     SkiaSharp |             20 |     53.00 us |   0.166 us |   0.147 us |   1.00 |    0.00 |    1.6479 |        - |     - |    5336 B |


Unless I'm doing something wrong in this benchmark, we are still a lot behind when it comes to draw text.

Hmmm... Anything obvious when profiling?

Code is same as with FillRegion. I suppose GDI and Skia are utilizing some further optimizations for text rendering. Probably a global rasterized glyph cache? (This could be good candidate for LRU caching!)

tocsoft

only thing I can see is the fact we don't seem have any need at all for any for the OrientationHandling logic which leaves us with additional maintenance.

src/ImageSharp.Drawing/Shapes/Rasterization/PolygonScanner.cs

tocsoft · 2020-11-13T17:45:16Z

src/ImageSharp.Drawing/Shapes/TessellatedMultipolygon.cs

+        public static TessellatedMultipolygon Create(
+            IPath path,
+            MemoryAllocator memoryAllocator,
+            OrientationHandling orientationHandling = OrientationHandling.ForcePositiveOrientationOnSimplePolygons)


ForcePositiveOrientationOnSimplePolygons ends up not actually influencing the algorithm do we need the extra complexity of the other types seeing as none of our library code uses anything but this option

I just wanted to keep the flexibility to change this later if we want. Also: having the 3 options kinda documents what are we doing here and why.

JimBobSquarePants

It all looks good to me, just a few questions really. I'll leave @tocsoft to do final approval as he actually understands this stuff! 😝

src/ImageSharp.Drawing/Processing/Processors/Drawing/FillRegionProcessor{TPixel}.cs

src/ImageSharp.Drawing/Shapes/Helpers/TopologyUtilities.cs

src/ImageSharp.Drawing/Shapes/InternalPath.cs

src/ImageSharp.Drawing/Shapes/Rasterization/ActiveEdgeList.cs

src/ImageSharp.Drawing/Shapes/Rasterization/PolygonScanning.MD

src/ImageSharp.Drawing/Shapes/Rasterization/ScanEdgeCollection.Build.cs

src/ImageSharp.Drawing/Utilities/NumericUtilities.cs

src/ImageSharp.Drawing/Utilities/SortUtility.KeyValueSort.cs

JimBobSquarePants

Very happy!

tocsoft

Agreed everything's looking good.... still not 100% convinced we want the extra (unused) logic in TessellatedMultipolygon but I'm happy to approve anyway. But I agree there is some follow up clean up actions to remove some unused/wanted codepaths, pretty much all of internal path can probably be dropped along with the more generic point in polygon/intersection logic etc from the paths.

antonfirsov · 2020-11-16T16:31:19Z

@tocsoft I will remove the OrientationHandling stuff as part of this PR in the next few days.

antonfirsov · 2020-11-20T11:42:42Z

@tocsoft all done, I'm letting you have a final look and/or merge.

tocsoft

approved 👍 nice work

antonfirsov added 30 commits September 19, 2020 21:54

hack dev inner loop as usual

907df3e

_IntersectionExperiments

e20fa94

ScanTests + infrastructure

7765409

TessellatedMultiPolygon works

053a907

EdgeData skeleton

abbe94d

ScanEdgeCollection tests

5ee8e1c

SimplePolygon_AllEmitCases works

6de0e46

cleanup + more tests

4c493fd

use Sign2 to determine polygon orientation

a077f4e

SortHelper

37d6d02

QuicSort, SortHelper -> SortUtility

2deaeff

started implementing PolygonScanner

c4e5001

ScanEdge.GetX

cb38780

VertexCategoriesAndEmitRules.jpg

2b68483

Some docs about VertexCategory rules

6697127

Move types

dbb7bdd

ActiveEdgeList skeleton

7fc5f82

NumericCornerCases

254827b

better VertexCategory rules

5bb6933

robust rules for categorizing horizontal edges

1fb0578

RoundToPositiveInfinity

ef3992c

+1 corner case

3bd5e94

some progress with PolygonScanner

48ce7cc

some tests pass

99d7052

even more tests passing

ea57b49

FuzzyFloat

722e57f

distort lines according to rounding rules

a2296be

comment on SelfIntersecting01

b6859e9

remove TolerantComparer

b634dea

FillPolygon_Solid_Basic

d3d1fe7

antonfirsov requested a review from tocsoft November 13, 2020 16:49

antonfirsov commented Nov 13, 2020

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antonfirsov changed the title ~~[WIP] Fast polygon scanning with active edge list~~ Fast polygon scanning with active edge list Nov 13, 2020

This was referenced Nov 13, 2020

Improve blending performance, make sure API is not a blocker #102

Closed

IPath: Remove unused features to enable simplifying and optimizing InternalPath #103

Closed

tocsoft reviewed Nov 13, 2020

View reviewed changes

antonfirsov added 3 commits November 13, 2020 19:08

improvve PolygonScanning.MD and add benchmark results

7bbb5af

simplify PolygonScanner factory method

1eccfb8

use latest stable versions of Fonts and ImageSharp

6d1e7a1

JimBobSquarePants reviewed Nov 13, 2020

View reviewed changes

JimBobSquarePants previously approved these changes Nov 16, 2020

View reviewed changes

tocsoft previously approved these changes Nov 16, 2020

View reviewed changes

antonfirsov added 4 commits November 20, 2020 11:46

dedupe EdgeData creation code

47f5e8e

remove OrientationHandling customization

3ff3e59

remove OrientationHandling

c221b81

more sophisticated way to "argue" with StyleCop

4ac3523

antonfirsov dismissed stale reviews from tocsoft and JimBobSquarePants via 4ac3523 November 20, 2020 11:40

antonfirsov mentioned this pull request Nov 20, 2020

The performance of 'DrawLines' degrades the thinner the lines are #18

Closed

4 tasks

do not DebugDraw on CI

722ea9f

antonfirsov mentioned this pull request Nov 20, 2020

Remove Region and implement "ProcessInsideShape" within ImageSharp.Drawing #105

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tocsoft approved these changes Nov 20, 2020

View reviewed changes

tocsoft merged commit 3d737b5 into master Nov 20, 2020

antonfirsov mentioned this pull request Nov 20, 2020

Rasterizing artifacts when drawing large and complex polygons #106

Open

4 tasks

antonfirsov mentioned this pull request Nov 30, 2020

IndexOutOfRangeException when drawing line near top when Stroke > 1.5f #108

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antonfirsov mentioned this pull request Oct 22, 2021

Fix SkipEdgesBeforeMinY algorithm #171

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JimBobSquarePants deleted the af/polygon-lab branch November 18, 2021 11:45

JimBobSquarePants mentioned this pull request Sep 7, 2023

Drawing performance tweaks #290

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Fast polygon scanning with active edge list #96

Fast polygon scanning with active edge list #96

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Fast polygon scanning with active edge list #96

Fast polygon scanning with active edge list #96

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Prerequisites

Summary

Implementation

Results

Performance

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