Deciphering the SHP format...

[MarkSheppard]

I think the decompression code works out as this:

Read Character

R = Character % 2
B = Character / 2

###########

EOF Test:
If Character EQUAL TO -1 break, this is End of SHP.

##########

TEST ONE:

IF B equals 0 AND R equals 1
(I think Char = 1, which returns B = 0.5 and R = 1)

We have a huge string of blank pixels coming up.

Read the next character in the file. That character is the number of pixels that we paint as the transparent background color.

############

TEST TWO:

If B equals 0

End of SHP Line -- I think this used to mark the end of each row in the SHP file, i.e. incremeting from y = 0 to y =1 and so on.

###########

TEST THREE:

IF R equals 0

Read the next character in the file; that character is the color index number from the palette.

We then paint a number of pixels equal to "B" with that color we just grabbed.

##############

TEST FOUR:

IF B NOT ZERO and R EQUAL ONE

Read the next [b] bytes as color numbers?

Basically this says that the next B characters are direct image colors; probably used to save space when you have a lot of different characters, to avoid the use of space wasting couplets.

//---------------------------------------------------------------------------
// read the shp data (for icon n) into the pseudo image
// this does not use the header info for lines, but reads
// until it finds KEY BYTE 88 Next byte 0.
void __fastcall read_SPicon_shp(FILE *inf, int n)
{
   int ch,b,r,i,l=0;
   long fpos;

   fseek(inf,icon_add[n]+HEADER_SIZE,0);
   pix_pos=0;
   far_right=0;
   far_bottom=0;
   do
   {
     // read data  and decode
     ch=fgetc(inf);

     //==== this section checks for end =====
     if (ch==-1) break;// for last image this is end of shp
     fgetpos(inf,&fpos);
     if ((n<(icons-1)) && fpos>icon_add[n+1]) break;
     // ==== end of end check ===========
     r=ch%2;
     b=ch/2;
	 
     if (b==0 && r==1) // a skip over
     {
        ch=fgetc(inf);
        for (i=0; ifar_right) far_right=pix_pos;
     }
	 
     else if (b==0)   // end of line
     {
       ++l;
       pix_pos=0;
     }
     else if (r==0) // a run of bytes
     {
       ch=fgetc(inf); // the color #
       for (i=0; ifar_right) far_right=pix_pos;
     }
     else // b!0 and r==1 ... read the next b bytes as color #'s
     {
       for (i=0; ifar_right) far_right=pix_pos;
     }
   } while (true); //exit this loop with break
   far_bottom=l;
}