Flatten a photo or illustration by reducing its number of tonal levels, replacing smooth gradients with bands of flat color. It works like the "Posterize" adjustment in Photoshop or GIMP — each pixel's brightness is rounded to a set number of levels, giving the bold, flat-color look of posters, silkscreen prints, and retro graphics. Internally, each channel (R/G/B) is quantized to N levels (v → round(v/255×(N-1))/(N-1)×255), collapsing the continuous 0–255 range into just N steps. Set "levels" to 2 for the most extreme, near-binary result, and raise it to keep the image closer to the original. Turn on "grayscale (black & white)" to first convert to luminance (L = 0.299×R + 0.587×G + 0.114×B) and then quantize, giving a black-and-white posterization (monochrome tone separation). The preview updates instantly as you change settings without reloading, and the result is saved as a PNG with its transparency (alpha) preserved. Use it to give illustrations and thumbnails a poster look, to simplify colors as a prep step for vector tracing or silkscreen color separation, or to make retro graphic assets. Nothing to install and nothing to upload — loading, converting, and saving all happen locally in your browser, so your image is never sent anywhere.