Hi, I`m currently on a project that is a sequel and also a port. The previous format had their textures scaled by a factor of 2x when ported for the prequel, so if the original .png had a 128 x 128 size, the new one for the HD counterpart got scaled up to 256 x 256.

No-one on my team seems to have a clue how the textures were scaled up last time, I`ve had some experience mucking around on GIMP and paint.net so I know the basics of such packages.

However, I cannot scale these images to 256 x 256 and make them look good. I`ve been using gimp and tried both linear and cubic filtering but the colors get too fuzzy. The textures are profile shots for dialog in a jrpg, and obviously in an anime style so their hair and skin has `highlights` and that`s where most of the noticable ugliness comes from.

Does anyone recommend a method to retain visual clarity when scaling to 2x the size?

I`ve heard about 2xSaI, hq2x, hq3x and hq4x filtering algorithms, are there any plugins or standalone apps that exist that can be used? Do they support .png transparency?

There are no algorithmic methods for upscaling that universally look good - it's a fundamental imposibility, because all scaling algormithms fundamentally have to "invent" new information to fill in the added bandwidth provided by the higher resolution. Every scaling algorithm is a compromise. In particular, all extant upscaling algorithms have significant issues with large curves, arbitrary curves, and lines of arbitrary slope.

Really good upscaling requires an artist to manually add in new detail.

Probably the simplest way to improve the visual would be upscaling with whatever method you think looks best and then using shaperning filters to make it look more crisp. In Photoshop or GIMP this is called "Unsharp Mask".

Next time you need different sizes for the same texture, try doing the opposite: start creating high-res material that you downscale to whatever you want.

I`ve heard about 2xSaI, hq2x, hq3x and hq4x filtering algorithms, are there any plugins or standalone apps that exist that can be used?

Google, fifth searchresult using "hq4x filter" as keyword, brings up c source code for the filter

and guess what on the download page are compiled binaries.

https://code.google.com/p/hqx/downloads/list

Thank you.