Render a PPM image in an ANSI terminal with ppmtoterm

By the end of this guide, you will have rendered a small PPM image directly in a terminal and know how to scale it when the result is too large, too tall or visually inaccurate. The examples use the Netpbm package installed on this machine, version 2:11.05.02-1.1build1.

Allow about 10 minutes. You need a shell, the ppmtoterm command and a terminal that implements ANSI ISO 6429 colour control sequences. No root access is needed. The command writes terminal control sequences and image characters to standard output, so test it in a terminal rather than redirecting it to a text file.

1. Check the installed command

First confirm that the command is present and see the package version. The program has no ppmtoterm-specific switches. It accepts one optional PPM filename, or reads a PPM image from standard input.

$ command -v ppmtoterm
/usr/bin/ppmtoterm
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1
$ ppmtoterm --help
ppmtoterm: Use 'man ppmtoterm' for help.

The help message is not a list of options because there are no options specific to this program. Netpbm's common options may still be recognised, but this guide keeps to the documented filename and standard-input interface.

Checkpoint: the input and terminal are ready

You should have a real executable at /usr/bin/ppmtoterm, and your terminal should be able to display ANSI colour changes. If the command is missing, install the distribution's Netpbm package through your normal software-management process. That is an administrative action and is deliberately not included as a copy-and-paste command here.

2. Create a tiny test image

Use a plain-text PPM file for a predictable first test. This image is two pixels wide and one pixel high: red followed by green. The file is disposable and stays in the current directory.

$ cat > /tmp/ppmtoterm-test.ppm <<'PPM'
P3
2 1
255
255 0 0 0 255 0
PPM

Here, P3 identifies an ASCII PPM, 2 1 gives its width and height, and 255 is the maximum channel value. The six numbers are the red, green and blue channels for the two pixels.

Do not use sudo for this test. The only state change is a temporary file in /tmp. Remove it later with rm -- /tmp/ppmtoterm-test.ppm if you want to tidy up; that removes only this named test file.

3. Render the PPM at the current cursor

Pass the filename as the optional argument:

$ ppmtoterm /tmp/ppmtoterm-test.ppm

The result is not ordinary text. The program emits ANSI escape sequences to select approximate colours and uses one terminal character for each input pixel. For the two-pixel test, the visible image is only two character cells wide, so it may be easy to miss. A reset sequence is emitted after the image.

The image starts wherever the cursor is when the command begins. Every later row starts at column 0. This means a prompt or previous output can appear beside the first row, while later rows overwrite from the left edge. Run the command on a clean prompt, or use a shell line break before it if the placement matters.

4. Feed the image through standard input

You can avoid a named input file by piping PPM data into the command. This is useful when another Netpbm program produces the image.

$ printf 'P3\n2 1\n255\n255 0 0 0 255 0\n' | ppmtoterm

The output should again be a pair of coloured character cells followed by a terminal reset. To verify that the pipeline itself succeeded, check the producer and consumer status in a simple shell pipeline:

$ printf 'P3\n2 1\n255\n255 0 0 0 255 0\n' | ppmtoterm > /dev/null
$ printf 'exit status: %s\n' "$?"
exit status: 0

Redirecting to /dev/null is suitable for checking the exit status, but it discards the ANSI image. Redirecting to a text file produces escape sequences rather than a portable screenshot or plain-text drawing.

5. Fit a real image to the terminal

ppmtoterm does not resize the input. It produces one output character for every input pixel, so a large image can fill the terminal and scroll past immediately. Use Netpbm's pamscale before ppmtoterm. For example, scale an existing image to a width of 80 pixels:

$ pamscale -width 80 input.ppm | ppmtoterm

The exact height is calculated from the source image's dimensions. If the result is still too tall, choose a smaller width or set a height as well, while keeping the image's intended proportions in mind. The manpage specifically points to pamscale and pamcut for making an image fit; use pamcut when cropping is preferable to shrinking.

A terminal character is normally taller than it is wide, often by about a factor of two. Without compensation, a square source image can look tall. To make a 20 by 20 image appear closer to square, the documented approach is to squash it vertically or stretch it horizontally by about two. One practical starting point is:

$ pamscale -width 40 -height 20 input.ppm | ppmtoterm

Adjust those dimensions for your font and terminal. Scaling changes only the data in the pipeline; it does not modify input.ppm.

6. Set expectations for colour and image choice

The output palette is restricted. Netpbm maps each input RGB value to the generated palette using the minimum Cartesian distance between the RGB vectors. The closest available colour is therefore an approximation, not a faithful copy of the source.

Cartoons, logos and other images with a few plain colours usually work best. Photos and gradients can look markedly different, especially when their colours are bright or numerous. For a better first result, choose an image whose channel intensities are near zero, half maximum or maximum. This is a limitation of the terminal palette, not evidence that the PPM file is corrupt.

If you need a monochrome line drawing, ppmtoterm is the wrong tool. The related ppmtoascii combines pixels into characters and represents their rough brightness, while pbmto4425 targets black-and-white output on terminals that support its line-drawing characters.

7. Diagnose a failed or confusing render

• If the command says the file cannot be opened, check the path with ls -- /path/to/image.ppm. The input must be a readable PPM image, not a PNG or JPEG renamed to end in .ppm.
• If only escape-looking text appears, the output is being viewed outside an ANSI-capable terminal or has been redirected. Return to an interactive terminal for the visual test.
• If the first row is offset but later rows start at the left edge, that is the documented cursor behaviour. Move the cursor to the desired starting position before invoking the command.
• If the image scrolls away, scale it with pamscale or crop it with pamcut before rendering.
• If colours look wrong, try a small, flat-colour image. More terminal colour settings cannot restore detail that the generated palette cannot represent.