Inconsistent PNG file size when manipulating the MagickImage.Quality

[renatoatfugro]

Hi all!

I'm working on a small app where the user can control certain aspects of their image, such as format (Jpeg/Png), Quality (1-100), Color Mode (RGB, GrayScale) and Bit Depth (2-bit, 4-bit and 8-bit options).

The user feeds a Bitmap file, and based on the provided values mentioned above I write the changes to a memory stream print the array length it to inform the user about the estimated size of the output image:

var bitmap = new Bitmap(@"C:\temp\image.bmp");
using var tempStream = new MemoryStream();
bitmap.Save(tempStream, ImageFormat.Bmp);

var image = new MagickImage(tempStream.ToArray())
{
    Format = MagickFormat.Png,
    Quality = 100
};
image.SetBitDepth(8);
image.Grayscale();
using var stream = new MemoryStream();
image.Write(stream);
Console.WriteLine($"Png, Quality 100, Size {stream.ToArray().Length} bytes");

I noticed that this code using Format = MagickFormat.Jpeg gives me a consistent decrease/increase of the file size according to the Quality parameter:

Jpeg, Quality 92, Size 176554 bytes
Jpeg, Quality 93, Size 187915 bytes
Jpeg, Quality 94, Size 201850 bytes
Jpeg, Quality 95, Size 217083 bytes
Jpeg, Quality 96, Size 235019 bytes

However, with Png, it behaves strangely:

Png, Quality 92, Size 366245 bytes
Png, Quality 93, Size 366245 bytes
Png, Quality 94, Size 366245 bytes
Png, Quality 95, Size 297701 bytes
Png, Quality 96, Size 297701 bytes

I'm looking for options for decreasing the size of Png files, and through Quality does not seem to be correct way.

Any suggestions?

[dlemstra]

The quality works different for PNG files. You can find more information here: https://www.imagemagick.org/script/command-line-options.php#quality.

[renatoatfugro]

Hi @dlemstra, thank you for your response! this does clarify my doubts regarding the differences in image size.
so, in the sample I provided, the values for filter-type and compression-level would be:

quality: 92 filter-type: 2 compression-level: 9
quality: 93 filter-type: 3 compression-level: 9
quality: 94 filter-type: 4 compression-level: 9
quality: 95 filter-type: 5 compression-level: 9
quality: 96 filter-type: 6 compression-level: 9

which makes complete sense, given that up to filter-type = 4 we have different strategies applied.
once again, thank you!