Allow implementing custom brush types

[mysticfall]

For now, IBrush is supposed to return an instance of BrushApplicator, but both its constructor and indexer are defined with an internal modifier, which makes it practically impossible to create a custom brush implementation.

It can prevent users from adopting ImageSharp in case they need a type of brush that the library doesn't support, as described in SixLabors/ImageSharp#969, for example.

[JimBobSquarePants]

Hi @mysticfall

We've currently been keeping those APIs internal as we expect some churn as we separate out and optimize the drawing portion of the library, however we would be very happy work with you if you would like to implement a PR against the main library and add the PathGradientBrush described in SixLabors/ImageSharp#969

[antonfirsov]

Yeah those API-s are very unstable, many changes are expected, we try to avoid breaking changes, so we keep them hidden.

[antonfirsov]

@mysticfall SixLabors/ImageSharp#967 might be of your interest to see the whole picture.

[mysticfall]

@antonfirsov Thanks for the pointer. I'm still undecided what to do with the situation, as serializing textures was initially only a small part of the API which I've been writing.

Not sure if I could justify spending much time in creating such an implementation that can be accepted to be merged (i.e. with studying conventions, writing tests, and etc.) and postponing the main project until it gets released.

I'm not complaining, and I'd love to contribute if I could. It's just that I can only spend only small amount of time for my own project for which texture serialization is but a small part.

I will take a serious look at the code to see if I can make a PR quickly though.

[antonfirsov]

@mysticfall yeah we are quite strict with both code style and test, but we can support you with those, if you make up your mind to contribute. I'm even happy to push code changes to your PR branch, if it's not too much & I have the time.

Benefit: we will maintain the code afterwards, and keep it in the refactor chain we plan.

[mysticfall]

@antonfirsov I opened a pull request for SixLabors/ImageSharp#969, and I think I need some help or an advice as to what to do with test fixtures which belongs to a different repository(submodule).

I'd appreciate if you could help me resolving this problem.

[JimBobSquarePants]

@mysticfall Will have a good look for you as soon as possible. Thanks so much for your work so far!

[antonfirsov]

@mysticfall thanks a lot for investing your time! Looks like @JimBobSquarePants has it under control, but ping me if I can help.

[antonfirsov]

There is not much work left:

We need to make BrushApplicator constructor protected:

ImageSharp.Drawing/src/ImageSharp.Drawing/Processing/BrushApplicator.cs

Line 23 in d8960b8

internal BrushApplicator(Configuration configuration, GraphicsOptions options, ImageFrame<TPixel> target)

We shall consider changing Apply to consume RowInterval instead of y:

ImageSharp.Drawing/src/ImageSharp.Drawing/Processing/BrushApplicator.cs

Line 77 in d8960b8

public abstract void Apply(Span<float> scanline, int x, int y);

[JimBobSquarePants]

Yeah if we can use RowInterval that’d be much preferable

[JimBobSquarePants]

@antonfirsov Found issues with using RowInterval. Looks like we're stuck working one line at a time.

ImageSharp.Drawing/src/ImageSharp.Drawing/Processing/Processors/Text/DrawTextProcessor{TPixel}.cs

Lines 155 to 160 in 423bf06

	for (int row = firstRow; row < end; row++)
	{
	int y = startY + row;
	Span<float> span = buffer.GetRowSpan(row).Slice(offsetSpan);
	currentApp.Apply(span, startX, y);
	}

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

Lines 87 to 132 in 423bf06

	while (scanner.MoveToNextPixelLine())
	{
	if (scanlineDirty)
	{
	scanline.Clear();
	}
	scanlineDirty = scanner.ScanCurrentPixelLineInto(minX, 0, scanline);
	if (scanlineDirty)
	{
	int y = scanner.PixelLineY;
	if (!graphicsOptions.Antialias)
	{
	bool hasOnes = false;
	bool hasZeros = false;
	for (int x = 0; x < scanline.Length; x++)
	{
	if (scanline[x] >= 0.5)
	{
	scanline[x] = 1;
	hasOnes = true;
	}
	else
	{
	scanline[x] = 0;
	hasZeros = true;
	}
	}
	if (isSolidBrushWithoutBlending && hasOnes != hasZeros)
	{
	if (hasOnes)
	{
	source.GetPixelRowSpan(y).Slice(minX, scanlineWidth).Fill(solidBrushColor);
	}
	continue;
	}
	}
	applicator.Apply(scanline, minX, y);
	}
	}
	}
	finally