Why an anime upscaler needs a different model
Photographs are textured almost everywhere: skin, fabric, foliage and sensor noise all carry fine detail. Anime and most digital illustration work the other way round. Large areas hold a single flat colour, shapes are bounded by hard outlines of fairly even weight, cel shading adds one or two hard-edged shadow tones, and soft transitions appear only where they were drawn on purpose, in skies, hair highlights and glows. A model that expects photographic texture can treat a clean fill as something that ought to have grain, leaving faint mottling in flat areas or ripples beside lines.
That is why the anime upscaler runs on a different engine from the rest of the site. Basic, Photo and Screenshot are presets on one model, Real-ESRGAN x4plus hosted on fal.ai, with face enhancement turned off. The anime upscaler instead uses RealESRGAN_x4plus_anime_6B, a Real-ESRGAN network trained on anime art, which is at home with flat colour, clean line art and cel shading. Like any upscaler, it invents new pixels by predicting detail, firms up edges, and cleans blur, JPEG blocking and noise up to a point. It does not redraw missing parts of a picture, and it does not colourise.
Outlines, flat fills and gradients up close
Stretch a frame with ordinary resizing and a two-pixel outline turns into a soft four-pixel band with stepped curves. The anime upscaler aims for a sharp edge at the new size instead, and the difference shows most in hair, eyes and the silhouette against the background. Flat fills carrying JPEG blocking, the faint 8×8 squares and speckles that compressed files collect around lines, get cleaned up to a degree, so skin and clothing read as solid colour again.
Gradients are harder. If a sky or a glow was saved with visible banding, those steps are part of the file, and the model has no record of how smooth the original was, so expect crisper steps rather than a restored gradient. Detail at the edge of the source resolution, like thin hatching, single hairs or tiny lettering in a speech bubble, has to be guessed, and the model may merge or simplify it. Inspect those areas at 100% zoom before keeping the result.
Manga pages and screentone
Screentone is the printed pattern of dots or lines that manga uses for shading. In a scan or a downsized digital page, the dots can end up smaller than a pixel or out of step with the pixel grid, which produces moiré: the wavy or checkered interference that shows up in grey areas. Inked black-and-white pages are line art, so the anime upscaler suits them; how the tones come out depends on what survived in the file.
Where each dot spans several pixels, there is a real pattern for the model to sharpen. Where the dots have already been averaged into flat grey or into moiré, nothing is left to recover: expect smooth grey, or moiré that comes out sharper rather than gone. Page size matters as well. A 2400×3500 scan is 8.4 megapixels, above the anime upscaler's ceiling of about 6, so it is reduced to roughly 2030×2960 before processing. If the tones matter, upscale the page in halves or panel by panel, so that no part is shrunk first.
Picking 2x or 4x under the 6-megapixel ceiling
In the anime upscaler, a file larger than about 6 megapixels is brought down to that size in your browser first, and the result is exactly 2x or 4x of whatever goes in. A 1920×1080 frame is 2.07 megapixels, comfortably under the limit, so 2x gives exactly 3840×2160 and 4x gives 7680×4320.
Choose 4x for small sources that must fill a big space, such as thumbnails, avatars and 720p frames headed for a 4K screen. Choose 2x when the source is already 1080p or larger: an illustration close to the ceiling comes out at about 24 megapixels at 2x and around 96 at 4x, far more than most screens or prints can use.
Anime upscaler targets: 4K wallpapers, phone screens and prints
For a desktop wallpaper, a 1080p frame at 2x lands exactly on 3840×2160, the usual 4K size. A 720p frame at 4x overshoots to 5120×2880; downsize it to 3840×2160 in any image editor for a clean fit. For a phone lock screen, crop first, because the crop is what needs the pixels: a tall 9:16 slice of a 1080p frame is about 608×1080, which becomes 1216×2160 at 2x and 2432×4320 at 4x. Check your phone's screen resolution and pick the smaller scale that clears it.
Prints need more planning. A common target for prints viewed up close is 300 pixels per inch, so an A4 page (8.27×11.69 inches) wants about 2480×3508 pixels, which a 1240×1754 illustration reaches at exactly 2x. A 3840×2160 result covers about 12.8×7.2 inches at that density; posters seen from across a room get away with less. Everything an anime upscaler adds is predicted, so inspect a large print file at full size before ordering.
When Photo or Basic mode fits better
Not every image with anime characters in it is flat-shaded. Realistic 3D renders with lit materials, photos of figures, plushies and merchandise, and painterly illustrations built on brush texture or paper grain all contain texture you probably want to keep, and a build suited to flat colour can smooth some of it away. For those, try Photo or Basic. Face enhancement is off in every mode, so a figure's face is never redrawn by a separate face model.
There is a trade-off. At 2x, Photo and Basic also take a 1920×1080 render at full size and return exactly 3840×2160, and they keep fine grain and painted texture. The anime upscaler gives cleaner lines and flatter colour from the same file but may smooth that texture away. When an image sits between the two styles, run a 2x in each mode and compare them in the slider before deciding.
Upscaling someone else's art: respect the artist's rights
Much of what people want to enlarge was made by someone else: fan art, official key visuals, episode frames, commissioned pieces. Upscaling changes the pixel count, not who made the picture or who holds the rights to it. Before you print, sell or repost an upscaled version, check the licence or the artist's stated terms, and ask when they are unclear. Keep signatures and attribution intact, and say that the file is an upscale instead of presenting it as the original high-resolution art.
The tool does not remove watermarks, and cropping one away to feed an image in is exactly the kind of use to avoid. For your own work the answer is simpler: if the source canvas or a vector file still exists, export it at the size you need, which beats any anime upscaler. Upscaling earns its place when the only copy left is small or compressed.