Slow performance when drawing thick lines

[Maxim Rylov]

Hi,

I've tried to test Xamarin's C# wrapper for Skia called SkiaSharp. I guess that an issue that I've found in my tests might relate to Skia.

On Windows, SkiaSharp performs very slow in comparison to GDI+ when it comes to draw thick lines (stroke width greater than 1 pixel). To help you reproduce this issue, I've created a GitHub repository with a rather simple benchmark which compares the performance of SkiSharp and GDI+.

On my machine I got the following results:

Host Process Environment Information:
BenchmarkDotNet=v0.9.8.0
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i3 CPU M 380 2.53GHz, ProcessorCount=4
Frequency=2467784 ticks, Resolution=405.2219 ns, Timer=TSC
CLR=MS.NET 4.0.30319.42000, Arch=64-bit RELEASE [RyuJIT]
GC=Concurrent Workstation
JitModules=clrjit-v4.6.1532.0

Type=Benchmarks  Mode=Throughput  GarbageCollection=Concurrent Workstation  

Method	AntiAlias	Median	StdDev

Method	AntiAlias	Median	StdDev
Skia_Clear	False	626.0689 us	33.5835 us
GDI_Clear	False	1,883.3675 us	92.5697 us
Skia_DrawLines_SolidThick	False	34.3181 us	2.3956 us
GDI_DrawLines_SolidThick	False	69.2968 us	3.5125 us
Skia_DrawLines_SolidThin	False	33.6100 us	2.0326 us
GDI_DrawLines_SolidThin	False	31.2110 us	0.7240 us
Skia_DrawLines_DashedThick	False	216.3211 us	7.8385 us
GDI_DrawLines_DashedThick	False	198.5229 us	5.8046 us
Skia_DrawLines_DashedThin	False	216.9377 us	9.8053 us
GDI_DrawLines_DashedThin	False	85.8987 us	1.6362 us
Skia_Clear	True	689.3211 us	18.8872 us
GDI_Clear	True	1,882.9171 us	54.4645 us
Skia_DrawLines_SolidThick	True	925.7412 us	27.4917 us
GDI_DrawLines_SolidThick	True	331.8494 us	16.4880 us
Skia_DrawLines_SolidThin	True	39.8524 us	1.3069 us
GDI_DrawLines_SolidThin	True	297.3545 us	3.2218 us
Skia_DrawLines_DashedThick	True	646.9674 us	2.7239 us
GDI_DrawLines_DashedThick	True	341.4178 us	1.2708 us
Skia_DrawLines_DashedThin	True	128.6211 us	1.1245 us
GDI_DrawLines_DashedThin	True	503.6172 us	1.7782 us

As it can be seen from the table (see the case when AntiAlias = True), Skia_DrawLines_SolidThick (stroke width equals 2 px) method is 2.78 times slower than its GDI+ counterpart GDI_DrawLines_SolidThick. Next, the performance of thick dashed strokes is a bit better, namely, Skia_DrawLines_DashedThick is 1.89 times slower than GDI_DrawLines_DashedThick.
At the same time, SkiaSharp is able to draw thin solid lines 7.46 times faster than GDI+. The difference in drawing thin dashed lines is 3.9 times respectively.

Do you have any explanation for such performance degradation of Skia when drawing thick lines?

Thank you in advance.

[Shawn Riordan]

It might help if your drawLines() calls included info about point count and other details.

[E. Greg Daniel]

Do you know if your test uses Skia's CPU or GPU backend? Also, just for sanity check, are you running a release build for both Skia and GDI. I know skia has a lot of debug code that can be slow if run in that way.

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[Mike Klein]

From the code in the Github repo, it looks like it's drawing with software (I don't see the GPU backend exposed in SkiaSharp).  I imagine users of SkiaSharp do not build Skia themselves, so it's likely the equivalent of Release all across the board.

[Maxim Rylov]

I ran the benchmark only with release builds, SkiaSharp has been configured to use the CPU backend.

The DrawLine method looks as follows [Benchmark]

public void Skia_DrawLines_SolidThick()

{

foreach (LineSegment ls in lineSegments)

skCanvas.DrawLine(ls.X0, ls.Y0, ls.X1, ls.Y1, skPaintSolidThick);

}

where lineSegments is an array containing just one element LineSegment(-10F, -10F, 1200F, 1200F).

[Matthew Leibowitz]

I think we are building a release version. 

I build with MSVC using these env vars:

GYP_GENERATORS="skia_arch_type='x86' skia_gpu=1"

GYP_DEFINES="ninja,msvs"

I then build with msbuild Release configuration. Does this do it, or is there some release bit that needs to go into the generators?