Photo to ASCII with the cell shape measured

Divide the font's advance width by its line-height, both in em. A monospace advance is usually near 0.60 em, so a terminal set solid gives about 0.60, a browser pre element left at line-height normal gives about 0.50, and a page styled at 1.45 leading gives about 0.41 — and the list on this page computes exactly those three from a live measurement of the face your browser is using rather than from the round numbers. If the destination uses a different font, the quickest check is to convert once, paste, and see whether a circle in the picture came out as an oval: taller than wide means the ratio you used was too high.

Because its lightness is bunched and you converted straight through. A typical photograph puts most of its cells inside about a third of the scale — no true black, no true white — so a mapping that reads lightness at face value indexes into a third of the ramp and never touches the rest. The histogram under the result shows this as a single narrow hump. Switching to the stretched mapping fixes it by putting the black point at the low percentile and the white point at the high one; the levels-reached figure tells you how much of the ramp you recovered, and on a flat photograph it typically goes from four levels to all ten.

24-bit escapes work in essentially every terminal released since the mid-2010s, and 256-color escapes work considerably further back. The failure mode when they do not is unmistakable and harmless: the escape sequences print as visible text, so you get lines beginning with a bracket and a run of semicolons rather than a colored picture. If that happens, drop to the 256-color mode first and to plain glyphs second. Piping the file through anything that strips control characters — a chat box, a form field, most bug trackers — also removes the color and leaves the glyphs intact.

Because those sixteen have no fixed color. The first eight are the original terminal colors and the next eight their bright variants, and what they actually paint is whatever the terminal's own theme sets them to — the same escape gives one red in a Solarized profile and a different red in Dracula. Matching a photograph against them would be matching against a color that cannot be known in advance. The 240 that remain are defined by arithmetic rather than by taste: a 6x6x6 cube on the levels 0, 95, 135, 175, 215 and 255, and 24 grays at 8 plus 10 times the step.

Stretching moves the two ends of the range; equalization redistributes everything in between. It places each cell by its rank rather than by its value, so every glyph level ends up with roughly the same number of cells whatever shape the original histogram had. That rescues a photograph so flat that even a stretch leaves it bunched, and it does one thing you have to watch for: a large area of nearly uniform tone — a clear sky, a studio backdrop, a wall — gets its tiny variations pulled apart into visible banding, because the ranking has no way to know those differences were noise.

Between three and eight times, depending on the mode and how busy the picture is. Every run of cells that share a color costs one escape — up to nineteen bytes at 24-bit, up to eleven at 256 — and a photograph changes color often enough that runs are short, so the escapes outweigh the glyphs by a wide margin. Emitting one escape per run rather than one per character is what keeps it to that; the naive form is worse again. The exact figure for your picture is printed under the result as the difference between the glyph-only size and the size of what the copy button hands over.