PngEncoder ignores FilterMethod

Prerequisites

I have written a descriptive issue title
I have verified that I am running the latest version of ImageSharp
I have verified if the problem exist in both DEBUG and RELEASE mode
I have searched open and closed issues to ensure it has not already been reported

ImageSharp version

3.1.4

Other ImageSharp packages and versions

NA

Environment (Operating system, version and so on)

NA

.NET Framework version

net8

Description

When a new PngEncoder is created with custom FilterMethod it always produces the same file size,
i.e. the FilterMethod is being ignored.

Example:

var encoder = new PngEncoder
{
    FilterMethod = PngFilterMethod.Paeth,
};

I found only one usage of this property here

ImageSharp/src/ImageSharp/Formats/Png/PngEncoderCore.cs

Lines 1522 to 1526 in 10e9c49

    
           if (!encoder.FilterMethod.HasValue) 
        
           { 
        
               // Specification recommends default filter method None for paletted images and Paeth for others. 
        
               this.filterMethod = this.colorType is PngColorType.Palette ? PngFilterMethod.None : PngFilterMethod.Paeth; 
        
           }

i.e. filterMethod is assigned only if encoder.FilterMethod is null,
otherwise, it is always 0 (PngFilterMethod.None).

Steps to Reproduce

    SixLabors.ImageSharp.Image img = null!;
    
    var paethFilterStream = new MemoryStream();
    img.Save(paethFilterStream, new PngEncoder
    {
        FilterMethod = PngFilterMethod.Paeth,
    });

    var noneFilterStream = new MemoryStream();
    img.Save(noneFilterStream , new PngEncoder
    {
        FilterMethod = PngFilterMethod.None,
    });

    Assert.NotEquals(paethFilterStream.Length, noneFilterStream.Length);

Images

No response

@melnikov77

@melnikov77 The filter method is used in the release/3.1.x branch here.

ImageSharp/src/ImageSharp/Formats/Png/PngEncoderCore.cs

Lines 574 to 600 in d7ef0e2

    
           private void FilterPixelBytes(ref Span<byte> filter, ref Span<byte> attempt) 
        
           { 
        
               switch (this.filterMethod) 
        
               { 
        
                   case PngFilterMethod.None: 
        
                       NoneFilter.Encode(this.currentScanline.GetSpan(), filter); 
        
                       break; 
        
                   case PngFilterMethod.Sub: 
        
                       SubFilter.Encode(this.currentScanline.GetSpan(), filter, this.bytesPerPixel, out int _); 
        
                       break; 
        
                   case PngFilterMethod.Up: 
        
                       UpFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, out int _); 
        
                       break; 
        
                   case PngFilterMethod.Average: 
        
                       AverageFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, (uint)this.bytesPerPixel, out int _); 
        
                       break; 
        
                   case PngFilterMethod.Paeth: 
        
                       PaethFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, this.bytesPerPixel, out int _); 
        
                       break; 
        
                   default: 
        
                       this.ApplyOptimalFilteredScanline(ref filter, ref attempt); 
        
                       break; 
        
               } 
        
           }

Your test confused me given you are use null! to define the source image.

SixLabors.ImageSharp.Image img = null!;

However, you are correct! We are not assigning the correct filter on encode, your finding actually uncovers a bug in our SIMD accelerated filtering methods where we suffered from an unsigned integer overflow defect!

Many thanks for raising this.

I've opened a PR #2775 with the fix.

Fixed with 3.1.5

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PngEncoder ignores FilterMethod #2771

Prerequisites

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.NET Framework version

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	if (!encoder.FilterMethod.HasValue)
	{
	// Specification recommends default filter method None for paletted images and Paeth for others.
	this.filterMethod = this.colorType is PngColorType.Palette ? PngFilterMethod.None : PngFilterMethod.Paeth;
	}

	private void FilterPixelBytes(ref Span<byte> filter, ref Span<byte> attempt)
	{
	switch (this.filterMethod)
	{
	case PngFilterMethod.None:
	NoneFilter.Encode(this.currentScanline.GetSpan(), filter);
	break;
	case PngFilterMethod.Sub:
	SubFilter.Encode(this.currentScanline.GetSpan(), filter, this.bytesPerPixel, out int _);
	break;

	case PngFilterMethod.Up:
	UpFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, out int _);
	break;

	case PngFilterMethod.Average:
	AverageFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, (uint)this.bytesPerPixel, out int _);
	break;

	case PngFilterMethod.Paeth:
	PaethFilter.Encode(this.currentScanline.GetSpan(), this.previousScanline.GetSpan(), filter, this.bytesPerPixel, out int _);
	break;
	default:
	this.ApplyOptimalFilteredScanline(ref filter, ref attempt);
	break;
	}
	}

PngEncoder ignores FilterMethod #2771

Description

Prerequisites

ImageSharp version

Other ImageSharp packages and versions

Environment (Operating system, version and so on)

.NET Framework version

Description

Steps to Reproduce

Images

Activity

JimBobSquarePants commented on Jul 16, 2024

JimBobSquarePants commented on Jul 24, 2024

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