004
Four ways to double an image
Which upscaler can I trust to double an image without redrawing it?
Part 01
One image, four upscalers
I upscaled one Krea 2 image to 2× with four methods to decide which one I use for each kind of work. I measured each result and then looked at the crops, because the numbers alone reward the method that changes the least.
Krea 2 · txt2img · seed 460701 · 1024×12805 attempts
The face and the hair, at 2×
The same 320×400 area of the test image, taken at each method’s own scale and brought to the same size. The numbers come from scaling each result back down to 1024×1280 and comparing it with the original.
- OriginalReference
The starting point: a 1024×1280 image. This crop is stretched from the original, so it is the softest.
Original · 1024×1280 - LanczosTest
The neutral baseline: it doubles the size and adds no detail. Good for a plain file copy.
PSNR 39.17 dB · mean error 1.136 · edge energy 3.806 - AnimeSharpBetter
The most faithful: it keeps the drawing, the identity and the line, and makes the edges clearly sharper.
PSNR 44.20 dB · mean error 0.752 · edge energy 4.674 - SeedVR2 3BDiscarded
It smooths a lot and changes the face and the small shapes more than the others. Not the right one for this style.
PSNR 28.08 dB · mean error 5.042 · edge energy 2.816 - SeedVR2 7BMy pick
It recovers definition with less drift than the 3B, but it is still generative: it reinterprets slightly. The best finish for a semi-real anime portrait.
PSNR 32.45 dB · mean error 3.251 · edge energy 3.925
5 attempts
The helmet and the hand
Hands, accessories and text are where a generative upscaler can redraw what was there. I look at them separately.
- OriginalReference
Helmet and hand in the original: small details that a generative upscaler can redraw.
Original · 1024×1280 - LanczosTest
Same details, just bigger.
PSNR 39.17 dB · mean error 1.136 · edge energy 3.806 - AnimeSharpBetter
The helmet, the hand and the trousers keep their shapes; the edges get crisper.
PSNR 44.20 dB · mean error 0.752 · edge energy 4.674 - SeedVR2 3BDiscarded
The softest of the four in this area too.
PSNR 28.08 dB · mean error 5.042 · edge energy 2.816 - SeedVR2 7BMy pick
More definition in the helmet and the fabric. This is where I check hands and accessories, because it is the method that can invent.
PSNR 32.45 dB · mean error 3.251 · edge energy 3.925
What I learned
AnimeSharp is the safe choice when I need to keep the drawing exactly. It had the highest fidelity (44.20 dB) and the sharpest edges. I use it for flat anime, chibi, posters and line art.
Attempts: AnimeSharp, AnimeSharp
For semi-real anime, SeedVR2 7B Sharp gives the best finish if I accept a slight reinterpretation. With noise at zero and LAB color correction it drifted less than the 3B (32.45 dB against 28.08 dB).
Attempts: SeedVR2 7B, SeedVR2 3B
The metrics measure similarity, not beauty. Scaling each result back down and comparing it with the original rewards the least change, so I also look at the crops, and at the hands and accessories first.
Attempts: Lanczos, SeedVR2 7B
A saved workflow can break quietly after a node update. My old SeedVR2 workflow had its values shifted (resolution, max_resolution, batch_size, uniform_batch_size and color_correction), which may explain earlier soft results and unexpected sizes. I made two corrected copies and kept the original.
If the image has text or readable symbols, I use AnimeSharp or Lanczos: a strong generative upscaler can redraw the glyphs.