sending image data to printer directly

[fz315]

Hi,everyone:
This slip of code print out a image(bmp/jpg format) by
sending image data to printer directly. I can't understand well about "send surface" section, could someone give me some clue?
Thanks and Merry Christmas!
------------------
void PrintSurface (int hDevice, SDL_Surface *surface, char justify)
{
Uint8 byte;
int bit;
SDL_Color pixel;
int i, pc;
int r, c;

**==================================================
** Start bit image printing.
**==================================================
*/
write (hDevice, "\x1B\x2A\x62", 3);

byte = surface->h % 256;
write (hDevice, &byte, sizeof (byte));
byte = surface->h / 256;
write (hDevice, &byte, sizeof (byte));

for (r = 0; r < surface->h; r++)
{
byte = 0x00;
bit = 7;
pc = 0;


/*
**==================================================
** Justify line.
**==================================================
*/
if (surface->w < DotsPerLine[Model])
{
switch (toupper (justify))
{
case 'C':
pc = (DotsPerLine[Model] - surface->w) / 2;
break;

case 'R':
pc = DotsPerLine[Model] - surface->w;
break;

default:
pc = 0;
break;
}

byte = 0x00;
for (i = 0; i < (pc / 8); i++)
{
write (hDevice, &byte, sizeof (byte));
}

bit = 7 - (pc % 8);
}


/*
**==================================================
** Send surface.
**==================================================
*/
for (c = 0; (pc < DotsPerLine[Model]) && (c < surface->w); c++, pc++)
{
pixel = GetPixel (surface, c, r);
if ((pixel.r < 128) && (pixel.g < 128) && (pixel.b < 128))
{
byte |= (0x01 << bit);
}
bit--;
if (bit < 0)
{
write (hDevice, &byte, sizeof (byte));
bit = 7;
byte = 0x00;
}
}


/*
**==================================================
** Finish out line.
**==================================================
*/
if (bit != 7)
{
write (hDevice, &byte, sizeof (byte));
pc += bit + 1;
}

byte = 0x00;
for (bit = (DotsPerLine[Model] - pc) / 8; bit > 0; bit--)
{
write (hDevice, &byte, sizeof (byte));
}
}
}

 

[xwb]

I assume this is a monochrome printer. It doesn't make sense otherwise.

First the bit pattern - this is a bit strange as it is reversed every 8 bits. If X represents a set pixel and O represents an unset pixel, the pattern XXOOXOXO will produce 0x53.

The if statement checks whether any of the colours have an intensity greater than 128. If they do, the pixel is set, if they don't the pixel is not set.

You can see roughly what it is going to do on the screen by changing the for loop to

      for (c = 0; (pc < DotsPerLine[Model]) && (c < surface->w); c++, pc++)
      {
         pixel = GetPixel (surface, c, r);
         if ((pixel.r < 128) && (pixel.g < 128) && (pixel.b < 128))
         {
            printf ("O");
         }
         else
         {
            printf (" ");
         }
       }