feat: render .pdn thumbnails.

[Sola-ris]

Summary

Add support for rendering .pdn thumbnails.
Fixes #1147

.pdn files have an XML header that contains the thumbnail as a base64 encoded .png file.
The file layout is the following:

• 4 byte magic number (PDN3)
• 24-bit little-endian int specifying the header's length
• The XML header
• The actual image data which we don't care about

The header itself looks like this:

<pdnImage width="800" height="600" layers="1" savedWithVersion="5.109.9343.2610">
    <custom>
        <thumb png="base64 string" />
    </custom>
</pdnImage>

Sources for the file format

• http://justsolve.archiveteam.org/wiki/Paint.NET_image
  • Apparently there's a compresed version of the file format, but I couldn't get Paint.NET to produce that one
• https://github.com/rivy/OpenPDN
  • https://github.com/rivy/OpenPDN/blob/master/src/Data/Document.cs#L64-L120
  • https://github.com/rivy/OpenPDN/blob/master/src/DocumentWorkspace.cs#L2594-L2610
• https://github.com/addisonElliott/pypdn/blob/master/pypdn/reader.py#L142-L143
• The test file opened in a text editor

Before

After

File used for testing

pdn.zip (The actual file is archived since GitHub doesn't allow uploading .pdn files)

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable

[TrigamDev]

.pdn files have an XML header that contains the thumbnail as a base64 encoded .png file.

Well that's convenient

[TrigamDev]

Transparent backgrounds appear to be interpreted as pure black in the thumbnail preview, but not in the sidebar preview.

[Sola-ris]

I'll take a look

[Sola-ris]

Regular PNGs have a background added, if they're in RGBA. I've given the thumbnail the same treatment

[TrigamDev]

I haven't noticed any other images in my library with a transparent background having a background added. And I'd argue that if it is a behavior in other file types, it should be fixed as it's very unintuitive and unwanted behavior.

[Sola-ris]

Happens for regular images and exr image.
I have no opinion on it, but I don't think it's something that should be changed as part of this issue.

[TrigamDev]

Yeah I agree that it should be a separate issue..
Thanks for your work on the thumbnail rendering btw, didn't expect someone to start working on these so quickly.

[Sola-ris]

You're welcome :D