I tried my luck on the bullet holes...

@questseeker:
Interesting ideas! I didn't know that you can achieve a rotation by just shifting rows and columns. That sounds incredible efficient! Do you happen to know any piece of software that implements it so I could see how it looks? Wonder how it compares to nearest neighbour sampling... same goes for your "hundreds of votes" approach: I still think that I could achieve the same effect cheaper but if you'd have an example where this technique is used we could just compare the results?

Also, while I don't expect perfect results I hope to end up with a set of algorithms that produce acceptable results where hand retouching is not necessary. In the scenario I want to use it that would just not be possible. An individual pixel that flickers between visibility and invisibilty depending on the current rotation would be a pretty huge flaw.
But your idea of using a reference to compare against might help here. How about also interpolating the image with a standard approach (that actually mixes close colors together) then, by comparing the local neighbourhood in that smooth-filtered image with the result where we don't mix numbers and based on the amount of difference (error) we reconsider our choice of pixels to reduce the summed local difference in comparision with the reference. Hmmm...

yrizoud, that visualization you posted is quite nice. Makes discussing the matter a lot easier!

But, first here's one of mine:



As you can see one of the properties of the upscaling algorithm is that they "rounden" the image. Without upscaling diagonals would appear jagged (due to the low resolution) and the artist relies on the eye of the viewer to reconstruct the image. With the right upscaling algorithm the "real" shape of the depicted thing is approximated. And this improves the quality when rotating, transforming and rescaling the image. A chunk of pixels that is meant to have a certain shape is more likely to end up in that shape after the transformation!

My progress is a lot slower then I hoped for (due to a lot of distractions, not because I don't believe in it's doability) but in my most recent build you can freely transform (left mouse button) and rotate (right mouse button) the image allready, and I think that it allready domonstrates that using an upsaled source image does improve the final quality.

PixelPunch003.rar



It's far from perfect but there's still one step in the transformation process that I didn't adress yet. Sampling. Your illustration and explaination show perfectly what causes the artefacts. Now let's say for every green square you count the number of white and black sub-pixels to make a decision. That would get rid of most of the artefacts except where there are diagonals in the source image that introduce jaggedness because of the low resolution in which case (hopefully) the upscaling will kick in.

P.S.: None of the artworks shown in the screenshots are done by me. But they all have been posted on the forums so I hope it's okay for me to use them in the screenshots.

This reminded me of what you were planning to do! Looks pretty nice on first glance...
http://rpgjs.com/

The main difference for me when comparing my program with RotSprite is that I have the source code, so I can really shape it like I need it. And I've a lot more planned than what RotSprite currently offers. For example I hope to offer a WYSIWYG interface and allow scaling and rotating at the same time. It's just a fun side project for me but I wan't to improve my knowledge about that topic, see where I can take it. Because if RotSprite is the best tool available so far there seems to be room for improvement.^^

Another nice thing about it is that it should be easy to compile it to Linux and Mac.

When I first heard about the retargeting algorithm I thought it would be a great step for image processing software but I've yet to see it used in any of the programs I use! And thanks for the other tip... "AlienSkin BlowUp" never heard of it but it looks pretty sophisticated. But I can't find any background infos about how their algorithms work.