Certain PNGs with transparent background are corrupted when written to filepath with JPG extension

[atantapr]

Magick.NET version

Magick.NET-Q8-AnyCPU 13.10.0

Environment (Operating system, version and so on)

Windows 10, .NET Framework 4.8

Description

I have encountered 3 PNG images with transparent background which are corrupted when they are loaded by IM and then written to a new path with the JPG extension instead of PNG. This is how we normally change the output filetype when processing a file with IM, and I am not aware of a different method to make IM output the same file with a different filetype. We have not been able to reproduce this with any other PNGs with transparent background so far, and it's unclear to me what might be unique about these files that is causing the corruption.

The attached ZIP contains the original PNG files and the resaved JPG files which display the corruption. The first one becomes visibly blurred around the edges of the original image, and the background becomes white. The other two have their background become black instead of the expected white.

Can this be resolved with a code change on our end, or can it be fixed in Magick.NET / ImageMagick?

AffectedPNGsWithTransparentBackground.zip

Steps to Reproduce

MagickImage image = new MagickImage();
image.Read(pngPath);
image.Write(jpgPath);

[dlemstra]

This is not wrong but caused by your input image. This is happening because the alpha channel is turned off. The same will happen if you call image.Alpha(AlphaOption.Off and save your image as a png file or any other format. I would advise you to call image.ColorAlpha(MagickColors.White) first before writing the image because not all transparent pixels have a while color "behind them".

[atantapr]

@dlemstra That does seem to resolve the issue for these images, even the blurriness on the first image. Is it possible to identify with IM whether the image's transparent pixels have a white color "behind them"? The MagickImage.BackgroundColor property says it's white, not sure what else I should be looking at.

[atantapr]

@dlemstra I have not yet found a property on MagickImage that allows me to determine when we should call image.ColorAlpha(MagickColors.White) to ensure the image has a white background when it is saved as JPG and the transparency is removed (alpha channel is disabled). Do you have any input on this?

[dlemstra]

You should always use the ColorAlpha method if you want to be sure that you get a white background when you turn the alpha channel off. There is no property that can be used to determine if you need to do this.