JpegBitmapEncoder default QualityLevel issue

[OlexandrPavliuk]

Description

If to set QualityLevel of JpegBitmapEncoder to 75, the quality level of a generated image will be 90, not 75.

This happens because SetupFrame method checks if the value of the level is a default one (that is 75) and doesn't set ImageQuality property of the native encoder.

But, according to https://learn.microsoft.com/en-us/windows/win32/wic/jpeg-format-overview the default value of the quality in the native encoder is not 0.75, but 0.9.

Reproduction Steps

Execute the following code:

using var imageStreamSource = new FileStream("input.png", FileMode.Open, FileAccess.Read, FileShare.Read);

var decoder = BitmapDecoder.Create(imageStreamSource, BitmapCreateOptions.None, BitmapCacheOption.OnLoad);

var frame = decoder.Frames[0];

var encoder = new JpegBitmapEncoder()
{
    QualityLevel = 75,
};

encoder.Frames.Add(decoder.Frames[0]);

using var fileStream = new FileStream("output.jpg", FileMode.Create);
encoder.Save(fileStream);

Expected behavior

Saved JPEG image has Quality Level 75

Actual behavior

Saved JPEG image has Quality Level 90

Regression?

No response

Known Workarounds

No response

Impact

No response

Configuration

No response

Other information

No response

[miloush]

Note that the default quality of the encoder used to be 0.75 in the past.

[h3xds1nz]

The docs are even better from the past:

https://web.archive.org/web/20120912064436/http://msdn.microsoft.com/en-us/library/gg430026(v=vs.85).aspx