Printing Images with /tmp/DEVTERM_PRINTER_IN

DevTerm
1

When I was reading the source code for printer for undocumented commands, I found a command for printing images:

The command for image mode is as the following, followed by the raw image data where each bit corresponds to a pixel:
GS(\x1d) v 0 p wL wH hL hH, where:

  • Width is (wL+256*wH)*8 pixels (because it’s in bytes)
  • Height is (hL+256*hH) pixels (because it’s in lines)

There is an upper limit of allowed image size of 9224 bytes (73792 pixels), which when used with full width of 384 dots, can fits a 384*192 image. For the image data, every 8 pixels are grouped in a byte, with the left most pixel being the LSB. Groups are arranged left to right for each line. The image data can be generated with Python:

img_data = PIL.ImageOps.invert(img.convert('L')).convert('1').tobytes()

Results:

  • Left: 1st attempt that went terribly wrong due to not adding the header properly
  • Middle: several attempts that got the size parameter wrong
  • Right: 3 successful attempts (32*32, 64*64, 256*256) with several failed ones in between

20211215_013931
20211215_0139311464×2600 261 KB

5 Likes
2

See: Devterm printer kernelspace driver - no length limit and grayscale possible.

4 Likes
3

@Leo3065 If you’d be interested in sharing more details of how you printed those images from python, it would be cool to see and experiment with.

4

Sorry, I’m very much new with this, could you explain how to print an image, like, for dummies? I truly don’t understand the instructions you gave, I guess I’m not in that level yet :sweat_smile:

5

Actually you need to divide by 8pixels (as each byte represents 8 dots), image data provided by OP works.

Here is example, how you can generate command for printing png in python (slashes are integer divisions, not comments):

import PIL
from PIL import Image
file1 = "/tmp/calligraphy.png"
def makestr(f):
    img = Image.open(f)
    img_data = PIL.ImageOps.invert(img.convert('L')).convert('1').tobytes()
    w = img.width // 8
    h = img.height
    wL = (w % 256).to_bytes(1,'big')
    wH = (w // 256).to_bytes(1,'big')
    hL = (h % 256).to_bytes(1,'big')
    hH = (h // 256).to_bytes(1,'big')
    data = "\x1dv0p".encode('ascii') +wL + wH + hL + hH + img_data
    return data
open("h1.txt", 'wb').write(makestr(file1))

Then execute the following command in the shell :

cat h1.txt > /tmp/DEVTERM_PRINTER_IN
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