How do I remove upscaling errors in paint.NET

Question

I'm working on a simple cover art and I initially designed it like this:

This image is 400 x 400 pixels. However, now I have upscaled it and added some cool stuff to the image to make it look like this:

This image is 1920 x 1080 as it's the right size for video's. I've also have some other sizes for various other uses.

The problem is that there are little upscaling arteffects from upscaling the small image. As you can see all of the squares in the bottom image have little "spikes" instead of a straight line. I tried removing these spikes by hand using the pen tool in paint.net however it's slow and I was hoping it could be done better.

As far as I know you cannot diagonally select inside paint.net otherwise I could use that to select all the spikes and then delete them.

Is there a simple way to delete the spikes and smothe the squares inside the image in paint.net?

Answer 1

Get free G'MIC filter pack. The same is available for GIMP and Krita. Make a polygonal selection which covers your problematic squares. Apply G'MIC filter Artistic > Cutout. It simplifies edges and reduces colors. My screenshot is from Krita:

Cutout rounds easily corners, too. Be sure your selection covers no texts nor other already acceptable. Here's the result. My selection contained only the big black square:

**Not asked:**Fixing with filters generally do not give perfect results in resolution problems, the only perfect method is to draw the shape using the final resolution. It maybe is a good moment to get into the vector domain, as already suggested and avoid future problems like this. Redrawing you work in a vector program cannot take more than 20 minutes if one can use the program. Complex programs like Illustrator or Inkscape can seem frightening and the cost of Illustrator is a real obstacle for many people. But there exists simpler programs for beginners and there's low cost commercial stuff which can satisfy also professional users.