Programmer 7500/MAX_PROGRAMMERS.iso/PASCAL/MCGA#04.ZIP/LESSON04.TXT

Text File | 1992-04-13 | 10.7 KB | 347 lines

                           MCGA Graphics Tutorial
                                 Lesson #4
                                by Jim Cook

Now for the fun stuff.........
Whenever you design a graphics package you have to decide what format to
store your images in.  There are too many to count on both hands, and
for the topic of this lesson I have to come up with one that is easy to
handle and can be represented quickly.  Here's what I've come up with...
NO format.  We are going to basically emulate the PutImage and GetImage
found in TP's BGI.  I chose this because it's easiest to understand and
it can be put on the screen very quickly.  As a matter of fact we can
put a complete screen image up 32 times per second on my 386/33mhz.
Most of the games we write will rely on a much smaller animation scale,
but may involve up to 100 animations at a time.

OK, you may ask, how do I save an image to disk in an uncompressed
format?  Good question, most paint programs do not directly support this
format, but most do support PCX files.  We will write a program that
puts a .PCX (or it's smaller cousin .PCC) graphic on the screen.  From
here we will cut the chunk of the picture we want and save it to the
disk.  In order to accomplish all of this, we need to understand PCX
files.  Fasten your seatbelts!

  PCXHeader   =  record
                   Signature      :  Byte;    { $0A }
                   Version        :  Byte;    { varies }
                   Encoding       :  Byte;    { $01 }
                   BitsPerPixel   :  Byte;    { $08 for 1 byte per pixel }
                   XMin,YMin,                 { Upper-left corner (0,0)  }
                   XMax,YMax      :  Integer; { Lower-right corner }
                   HRes,VRes      :  Integer; { resolution 320 x 200 }
                   Palette        :  Array [0..47] of byte;  { colors }
                   Reserved       :  Byte;
                   Planes         :  Byte;
                   BytesPerLine   :  Integer;
                   PaletteType    :  Integer;
                   Filler         :  Array [0..57] of byte;
                 end;

This is the first 128 bytes of any PCX (or PCC) file.  The Signature is
always $0A and the Encoding is always $01, at least for now.  The rest
of the header is self-explanatory.  Well, there is one catch, when this
format was developed, the geniuses at ZSoft didn't plan for pictures
with 256 colors or more.  There are 3 bytes per color in MCGA mode so
the measely 48 bytes set aside in the header only allows for 16 colors.
ZSoft ended up putting the 768 bytes of palette data at the end of the
file and the Palette area in the header is available real estate.

Immediately following the header is the body of the PCX file.  The body
is compressed by a technique called run-length compression.  For many of
you, this is the first time you have looked at compression so I'll take
this slow.  It is not important to understand this for the purpose of
our graphic library, but it is vital if you work with other formats.
Most use some type of run-length compression.  TIFF and GIF use LZW
compression, by the way.

This is the flowchart (ugh) for unpacking one row of a PCX file:

           -------->  Read in a byte X
         /                   |
       /           Are the two high bits set?
     /          [ 1  1  ?  ?  ?  ?  ?  ? ]  <- binary representation of X
   /               / Yes                \ No
   |        X := X and $3F         Write X to screen
   |              |                      |
   ^       Get next byte Y               |
   |              |                      |
   |       Write byte Y to the          /
   |       screen X times             /
   |              \                 /
    \                Done with row?
      \                | No   | Yes
        \              /      |
          -----<------        | Exit

That's how to unpack 1 row of a PCX, to unpack the whole picture:

                For I := 1 to NumRows do UnpackRow;

Along with some housekeeping to keep track of advancing down the screen
memory.  I know you want the code so here goes:

Unit Lib04;

interface

type
  PCXHeaderPtr=  ^PCXHeader;
  PCXHeader   =  record
                   Signature      :  Char;
                   Version        :  Char;
                   Encoding       :  Char;
                   BitsPerPixel   :  Char;
                   XMin,YMin,
                   XMax,YMax      :  Integer;
                   HRes,VRes      :  Integer;
                   Palette        :  Array [0..47] of byte;
                   Reserved       :  Char;
                   Planes         :  Char;
                   BytesPerLine   :  Integer;
                   PaletteType    :  Integer;
                   Filler         :  Array [0..57] of byte;
                 end;

Procedure DisplayPCXPas (X,Y:Integer;Buf:Pointer);
Procedure DisplayPCX    (X,Y:Integer;Buf:Pointer);

implementation

uses
  Dos;

Procedure ExtractLine (BytesWide:Integer;Var Source,Dest:Pointer);
var
  DestIdx,
  SourceIdx   :  Integer;
  InCode,
  RunCount    :  Byte;
begin
  DestIdx := 0;
  SourceIdx := 0;

  While DestIdx < BytesWide do begin
    InCode := Mem [Seg(Source^):Ofs(Source^)+SourceIdx];
    Inc (SourceIdx);

    If (InCode and $C0) = $C0 then begin
      RunCount := InCode and $3F;
      InCode := Mem [Seg(Source^):Ofs(Source^)+SourceIdx];
      Inc (SourceIdx);
      FillChar (Mem[Seg(Dest^):Ofs(Dest^)+DestIdx],RunCount,InCode);
      Inc (DestIdx,RunCount);
      end
    Else begin
      Mem [Seg(Dest^):Ofs(Dest^)+DestIdx] := InCode;
      Inc (DestIdx);
      end;
    end;
  If Odd (BytesWide) then Source := Ptr(Seg(Source^),Ofs(Source^)+SourceIdx+2)
                     else Source := Ptr(Seg(Source^),Ofs(Source^)+SourceIdx);
  Dest   := Ptr(Seg(Dest^),Ofs(Dest^)+DestIdx);
end;

Procedure ExtractLineASM (BytesWide:Integer;Var Source,Dest:Pointer);
var
  DestSeg,
  DestOfs,
  SourceSeg,
  SourceOfs   :  Word;
begin
  SourceSeg := Seg (Source^);
  SourceOfs := Ofs (Source^);
  DestSeg   := Seg (Dest^);
  DestOfs   := Ofs (Dest^);

  asm
    push  ds
    push  si

    mov   ax,DestSeg
    mov   es,ax
    mov   di,DestOfs     { es:di -> destination pointer }
    mov   ax,SourceSeg
    mov   ds,ax
    mov   si,SourceOfs   { ds:si -> source buffer }

    mov   bx,di
    add   bx,BytesWide   { bx holds position to stop for this row }
    xor   cx,cx

  @@GetNextByte:
    cmp   bx,di          { are we done with the line }
    jbe   @@ExitHere

    lodsb                { al contains next byte }

    mov   ah,al
    and   ah,0C0h
    cmp   ah,0C0h
    jne   @@SingleByte
                         { must be a run of bytes }
    mov   cl,al
    and   cl,3Fh
    lodsb
    rep   stosb
    jmp   @@GetNextByte

  @@SingleByte:
    stosb
    jmp   @@GetNextByte

  @@ExitHere:
    mov   SourceSeg,ds
    mov   SourceOfs,si
    mov   DestSeg,es
    mov   DestOfs,di

    pop   si
    pop   ds
  end;

  If Odd(BytesWide) then Source := Ptr (SourceSeg,SourceOfs+2)
                    else Source := Ptr (SourceSeg,SourceOfs);

  Dest := Ptr (DestSeg,DestOfs);
end;

Procedure DisplayPCX (X,Y:Integer;Buf:Pointer);
var
  I,NumRows,
  BytesWide   :  Integer;
  Header      :  PCXHeaderPtr;
  DestPtr     :  Pointer;
  Offset      :  Word;
begin
  Header := Ptr (Seg(Buf^),Ofs(Buf^));
  Buf := Ptr (Seg(Buf^),Ofs(Buf^)+128);
  Offset := Y * 320 + X;
  NumRows := Header^.YMax - Header^.YMin + 1;
  BytesWide := Header^.XMax - Header^.XMin + 1;
  For I := 1 to NumRows do begin
    DestPtr := Ptr ($A000,Offset);
    ExtractLineASM (BytesWide,Buf,DestPtr);
    Inc (Offset,320);
    end;
end;

Procedure DisplayPCXPas (X,Y:Integer;Buf:Pointer);
var
  I,NumRows,
  BytesWide   :  Integer;
  Header      :  PCXHeaderPtr;
  DestPtr     :  Pointer;
  Offset      :  Word;
begin
  Header := Ptr (Seg(Buf^),Ofs(Buf^));
  Buf := Ptr (Seg(Buf^),Ofs(Buf^)+128);
  Offset := Y * 320 + X;
  NumRows := Header^.YMax - Header^.YMin + 1;
  BytesWide := Header^.XMax - Header^.XMin + 1;
  For I := 1 to NumRows do begin
    DestPtr := Ptr ($A000,Offset);
    ExtractLine (BytesWide,Buf,DestPtr);
    Inc (Offset,320);
    end;
end;
end.

That's the unit to put a PCX on the screen.  There are two routines to
put up a picture.  One is written entirely in Pascal and the other is an
assembly/Pascal hybrid.  I was very suprised to find the hybrid
performed as well as the 100% assembly routine I wrote.  Following is a
routine to put the unit to the test.

Program MCGATest;

uses
  Crt,Dos,MCGALib,MCGAPCX;

var
  Stop,
  Start       :  LongInt;
  Regs        :  Registers;
  PicBuf      :  Pointer;
const
  NumTimes    = 10;

Procedure LoadBuffer (S:String;Buf:Pointer);
var
  F           :  File;
  BlocksRead  :  Word;
begin
  Assign (F,S);
  Reset (F,1);
  BlockRead (F,Buf^,65000,BlocksRead);
  Close (F);
end;

Function Tick : LongInt;
begin
  Regs.ah := 0;
  Intr ($1A,regs);
  Tick := Regs.cx shl 16 + Regs.dx;
end;

Procedure ShowAndTell;
var
  Ch     :  Char;
begin
  TextMode (3);
  WriteLn ('We just displayed a PCX file to the screen ',NumTimes,' times.');
  WriteLn ('Routine took ',(Stop-Start)/NumTimes:6:4,' ticks or ');
  WriteLn ((Stop-Start)/18.2/NumTimes:4:3,' seconds per image!');
  WriteLn ('That''s about ',18.2/((Stop-Start)/NumTimes):6:4,' times per second.');
  Repeat Until Keypressed;
  While Keypressed do Ch := Readkey;
end;

Procedure Control;
var
  I :  Integer;
begin
  SetGraphMode ($13);
  LoadBuffer ('E:\NAVAJO.PCX',PicBuf);
  Start := Tick;
  For I := 1 to NumTimes do
    DisplayPCXPas (0,0,PicBuf);
  Stop := Tick;
  ShowAndTell;

  SetGraphMode ($13);
  Start := Tick;
  For I := 1 to NumTimes do
    DisplayPCX (0,0,PicBuf);
  Stop := Tick;
  ShowAndTell;
end;

Procedure Init;
begin
  Randomize;
  GetMem (PicBuf,65500);
end;

Begin
  Init;
  Control;
End.

Please run the above program and drop me a line on how it performs.  I
am very curious to learn how it behave on different machines.  Leave me
a message showing CPU, speed and how many images per second you
obtained.  Kinda like this:

                          <----- 386, 33 mhz ----->
        Pascal method ............................. 0.8585 per second
        Pascal/Asm hybrid.......................... 8.6667 per second

The first thing you will notice is, we are not setting the palette.  The
picture will appear in an ugly color unless the PCX you try to display
is drawn using the default palette.  Also, it does not support pictures
larger then 65500 bytes.  I'll finish up this discussion next time.

Hack on,
jim