Printing Images with /tmp/DEVTERM_PRINTER_IN

Leo3065 · December 14, 2021, 6:07pm

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

clockworkpi/DevTerm/blob/1a1be01dbb8ad8f1812111066b752f09659ec3ca/Code/thermal_printer/devterm_thermal_printer.c#L577

    
      
                  cfg->img->idx = 0;
                }
              }
              
              reset_cmd();
            }
            */
          }
          
          
if(cmdidx > 7){
            // GS v 0 p wL wH hL hH d1…dk 
            if(cmd[0] == ASCII_GS && cmd[1] == 118 && cmd[2] == 48 ) {
               uint16_t width = cmd[4] + cmd[5]*256;
               uint16_t height = cmd[6]+cmd[7]*256;
               uint16_t k;
               k = width * height;
               if(k<= IMAGE_MAX){
                cfg->state = GET_IMAGE;
                cfg->img->num = k;
                cfg->img->idx = 0;
                cfg->img->width = width;

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

korneliuszo · December 14, 2021, 6:29pm

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

poulsbo · February 3, 2022, 5:51am

@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.

jchavarria · June 2, 2022, 3:57am

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