Can you tell me how to ensure high performance of writing to the stream?

[mshakurov]

We noticed that saving the images we created (png format) to MemoryStream for some encoder settings is slow for our tasks.
The simplest tests show amazing things.

For example, download the map tile obtained from OSM at https://tile.openstreetmap.org/6/11/17.png .
This will be a 256*256 png image, file size 13,829.

Let's upload this file in a simple way: var img = Image.Load( fileName );

Then, let's look at some img instance parameters: Metadata.HorizontalResolution: 96 x 96, Metadata.ResolutionUnits: PixelsPerInch, PixelType.BitsPerPixel BitsPerPixel: 32.

And now let's try to save this instance to MemoryStream using the simple SaveAsPng method:

var encoder = new PngEncoder { BitDepth = PngBitDepth.Bit4, ColorType = PngColorType.Palette };
using Stream ms = new MemoryStream( /*capacity*/ );
img.SaveAsPng( ms, encoder );

This saving works very slowly.
Regardless of PngBitDepth, if ColorType is set to PngColorType.Palette, then saving is very slow.
If you do not use Palette, but, for example, RgbWithAlpha, then the recording will be much faster, but the size of the stream also grows significantly.

The main question:
What advice would you give to get a very high speed saving of the image from the example: a) to MemoryStream in png format, b) with minimal size increase?

[JimBobSquarePants]

What do you mean by very slow?
ImageSharp is considered fast in comparison to other libraries. In your example you're asking the library to reduce the color palette count to enable a few colors. This is an intensive operation.

[mshakurov]

• lets consider - they all png
• I'm not a guru in graphics, but your library shows the following:
  PixelsPerInch - 96 x 96
  BitsPerPixel: 32
• I will prepare a very simple test, and I will post it here

[JimBobSquarePants]

Thanks. A test image would be great also.

[mshakurov]

TestImageSharpNet6.zip

A simple console application... Test image is included, but the application can download it from OSM tile web service.

[JimBobSquarePants]

Thanks.

Looks like the images are palette based which will always require quantization. If you test using the new v3.1 alpha (see our readme for the feed details) we have optimized the pipeline to reuse the same palette if available. That should be much faster.

For benchmarking you really shouldn't use anything other than BenchmarkDotnet. Anything else will be wildly inaccurate.