Of course Thundercraft is right; the best solution would be for someone to develop a 24x24 tileset. But if people want to keep experimenting with various scaling options, might I point them to this algorithm, which was specifically designed for scaling pixel art. Unfortunately, it supports neither transparency nor a scale factor of 1.5. However, these shortcuts could be overcome: Transparency would have to be extracted, scaled separatedly, and then added back in; also, you would probably scale by 3x and then downscale to 0.5 (with bilinear filtering?). Unfortunately I'm stuck on the transparency problem. As a teaser, here's a version of Phoebus scaled to 3x with lost transparency:
Thanks for telling me about hqx I had never heard of it before.
I just released Tileset Resizer v2.0 which includes functions that use hqx to resize tilesets as well as improving the old resizing function.
Here is how I did it.
Because hqx can not resize images with a alpha channel resize_tileset_hqx and resize_tileset_hqx_tilebytile split the image to its rgb and alpha channels and saves them to be resized by hqx.
These resized images are recombined after resizing. Because hqx can only resize images to 2x, 3x, or 4x size the recombined image is then downsized to the desired final size using a user specified interpolation method. The functions pick the best amount to resize using hqx automatically.
A picture illustrating this process is shown below. Warning it is large.
My best result for the current Phoebus tileset is show below resized to 24x24 tiles with resize_tileset_hqx_tilebytile with spline interpolation used to downsize the hqx resize.
More examples of the results can be found in the Tileset Resizer thread.
I had to split the image in 3 horizontal sections because imgur wouldn't take such a large png without greatly degrading the image quality by converting it to jpg.
I use
Fotki to post my images for my Tileset Resizer samples because its free and according to them "Well, we really don’t have any strict limits on file size. You can upload very, very large files, no problem. Just make sure you use appropriate uploading tool for big files.".
Just make sure you link to the download original file link and you can post the full size images anywhere.