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Reading Paint Files
Volume Number:1
Issue Number:10
Column Tag:Basic School

"Reading Paint Files"

By Dave Kelly, Hybrids Engineer, MacTutor Editorial Board

This month we will explore how to read MacPaint files via MSBASIC. We will start by generally dissecting a paint file.

The 8"X10" MacPaint pictures that we are used to seeing are represented by 576 X 720 pixels. That's 414,720 pixels which would require 51,840 bytes to store directly to disk. The first 512 bytes (first block) contain the brush and fill pattern information. Fortunately, the bit map is compressed a row at a time in order to conserve disk space or we really wouldn't have much room to do anything else. Therefore there is a block of encoded pixels for each of the 720 rows. There are two flavors of encoding these entries, a straight bitmap and a run-length encoded sequence.

PATTERN BYTES

The first 512 bytes start with 00 00 00 02. Then after these four bytes are the 38 patterns that you see when you open your MacPaint document. The patterns may be edited with the pattern editor provided in MacPaint and your custom patterns may then be used as needed. Each of the 38 patterns are represented by 8 bytes each where each byte represents a row in the pattern grid. The pattern rows are mapped from left to right and top to bottom. The order of the patterns is the same as appear at the bottom of the MacPaint screen. The patterns are not encoded. The remainder of the first 512 bytes are filled with zeros.

BITMAP FLAVORS

Both flavors start with a one byte count. In the bitmap flavor the count indicates the number of pixel bytes which will follow minus one. As an example of this see figure 1 (Flavor 1) below:

Fig. 2 Program Output

In the example, the pattern 0100101011010010 (where a 1 represents a black pixel and a 0 represents a white pixel) would be encoded as 014AD2. The first byte (01) is the count minus one, therefore the count is 2 bytes. The following 2 bytes represent the data (4AD2).

The second flavor is composed of a sequence of 8 bits which is repeated. The count indicates the number of times which the sequence is repeated. For the second flavor the count byte (first byte) is set negative. Negative binary numbers have the first bit set to indicate that it is negative. In this case, the first byte is the absolute value of the of the first byte minus one. For example FDFF would represent 4 bytes of "FF", or "FF FF FF FF". FD is 11111101. The absolute value (negative) is 00000010 = 3. Therefore the count is 3+1 = 4 bytes. A row that is entirely blank is represented by B900 where B9 = -71. The inverse is 71. Add one to get the count (71+1=72 bytes of the 8-bit pattern "00". There are 72 bytes (72X8 = 576 pixels) in one row of a MacPaint document. The pattern may be any combination of ones and/or zeros. This would be most useful in coding patterns of bits which are repeated.

The tricky part is determining which of the flavors to use. In reading the file this is not too much of a problem because we can always look at the first bye. If it is negative then it is of the second flavor. Writing a MacPaint file is not quite as simple but can be done if you plan ahead.

BASIC PROGRAMS

I'm sure that you have probably seen the public domain program which will read the top left corner of a MacPaint document and display it on the screen. The program below called Paint Pokes will read any MacPaint file and display it on the screen using the Prof. Mac Pokes procedures that I have shown before (see Apr. 85 MacTutor, pg. 34). The problem is that this method of printing on the screen is extreamly slow. I would still like to convert the paint file (or a portion of it) into a BASIC PICTURE$ which could be printed to the screen much quicker, but so far I have not been able to understand how PICTURE$ (or the clipboard for that matter) is encoded. The problem is that BASIC does not recognize anything that has been poked onto the screen. Because of this the screen GET and screen PUT won't work for saving the picture via BASIC for later use. Anyone have the answer??

Paint Pokes asks for the paint file to be displayed and then reads the file and pokes it onto the screen. Press the mouse button at the end to continue when the display has finished (or when you have finished looking at it). The program then returns to BASIC.

Pattern Editor is a BASIC program which allows you to edit the patterns used in MacPaint. The pattern editor built into MacPaint is really more useful, but the BASIC pattern editor demonstrates the format of the patterns stored in a paint file. To use the pattern editor run the program and select the desired menus. First load in a Paint document. The entire document is read in so that it can be completely written back to disk again a with modified set of patterns. Select 'Display Pattern' to show the patterns as they are now defined. Choose 'Edit Pattern' to edit a particular pattern. Use the mouse to select the desired pattern and then a grid will appear and allow you to change the bits. To change the pattern as you are editing it choose 'Display Pattern' and the patterns will be re-drawn using your new pattern. If you display the pattern, the pattern will now be changed for good, you can't undo what has been displayed unless you want to re-edit the same pattern back to the original pattern. You may revert back to the original pattern set by loading the paint document again or you may want to save the changes to a new Paint document.

Thanks go to Bob Denny for his analysis of MacPaint file format.

'    Paint Pokes
'    By Dave Kelly
'    ©MACTUTOR 1985

start=108288!     'Set up beginning screen addr.
ending=130175!      'Set up ending screen addr.
mac512=(512-128)*1024    'Set up addr. for 512K Mac
IF FRE(0)>100000! THEN start=start+mac512
ending=ending+mac512
screen=start
WINDOW 1,"",(0,0)-(512,342),3
x$=FILES$(1,"PNTG")  'Get a MacPaint file
IF x$="" THEN quit   'No selection,  quit
HIDECURSOR
OPEN x$ FOR INPUT AS #1
    FOR i%= 1 TO 512  'Disgard the first 512 bytes
        x$=INPUT$(1,#1)
    NEXT i%
    pixel%=0
    WHILE NOT EOF(1)
    count=ASC(INPUT$(1, #1))
    IF count<&H80 THEN GOSUB type1 ELSE GOSUB type2  'Check if high bit 
is set
    WEND
CLOSE #1

wait.for.mouse.click:
    pause:IF MOUSE(0)>0 THEN  quit
    GOTO pause

Quit: MENU RESET:SHOWCURSOR
WINDOW 1,"Output Window",(2,40)-(510,340),1
END

Pokescreen:
    IF pixel%>511 THEN RETURN
    POKE screen,ASC(byte$)
    screen=screen+1
    IF screen >=ending THEN wait.for.mouse.click
    RETURN


type1:  'first flavor
    FOR i% = 1 TO count+1
        byte$=INPUT$(1,#1)  'Read a byte
        GOSUB Pokescreen
        pixel%=pixel%+8 'Count pixels printed
    NEXT i%
    IF pixel%>=576 THEN pixel%=0   'line is full
RETURN

type2:  'second flavor
    byte$=INPUT$(1,#1)    ' Read a byte
    FOR i%=1 TO (&H101-count)
        GOSUB Pokescreen
        pixel%=pixel%+8 'Count pixels printed
    NEXT i%
    IF pixel%>=576 THEN pixel%=0  'line is full
RETURN


***************************************
'    Pattern Editor
'    By Dave Kelly
'    ©MACTUTOR 1985

start=108288! 'Set up beginning screen addr.
ending=130175!      'Set up ending screen addr.
mac512=(512-128)*1024    'Set up address for 512K Mac
IF FRE(0)>100000! THEN start=start+mac512:ending=ending+mac512
screen=start:editor%=0:NewYork=2
Bold=1:Plain=0: DEFINT i,j,k
DIM pattern%(512), k$(1),Bound0%(72),  Bound1%(72),Bound2%(72), 
 Bound3%(72),Bstatus%(72),  
WINDOW 1,"",(2,24)-(510,150),3
FOR i%=1 TO 5
    MENU i%,0,0,""  'Erase old menus
NEXT i%
MENU 1,0,1,"File"
MENU 1,1,1,"Load Paint patterns"
MENU 1,2,0,"Save Paint patterns"
MENU 1,3,1,"Quit"
MENU 2,0,0,"Patterns"
MENU 2,1,1,"Display Patterns"
MENU 2,2,1,"Edit a Pattern"

ON MENU GOSUB menu.selection
MENU ON
pause:GOTO pause

menu.selection:
    menunumber=MENU(0)
    menuitem=MENU(1):MENU:MENU OFF
    IF menunumber=1 THEN ON menuitem GOSUB read.paint.file,    
 write.paint.file, Quit
    IF menunumber=2 THEN ON menuitem GOSUB Display.patterns,   
 Edit.pattern
    MENU ON:RETURN

read.paint.file:
    x$=FILES$(1,"PNTG")    'Get a MacPaint file
    IF x$="" THEN RETURN  'No selection,                        then 
forget it.
    MENU 1,0,0:MENU 2,0,0
    HIDECURSOR
    TEXTFONT(NewYork):TEXTSIZE(14)
    TEXTFACE(Bold)
    CLS:LOCATE 4,1
PRINT"Now Reading Macpaint document.....           Please wait."
    OPEN x$ FOR INPUT AS #1 LEN= 2000
    ERASE k$
    DIM k$(LOF(1)+1)
    FOR i%= 1 TO 512
        k$(i%)=INPUT$(1,#1)
    NEXT i%
    WHILE NOT EOF(1)
        k$(i%)=INPUT$(1,#1)
        i%=i%+1
    WEND
    number.of.bytes=i%
CLOSE #1
SHOWCURSOR:CLS
MENU 2,0,1:MENU 1,0,1:MENU 1,2,1
BEEP:RETURN 

write.paint.file:
TEXTFONT(NewYork):TEXTSIZE(14)
TEXTFACE(Bold)
CLS:LOCATE 4,1
PRINT "Now writing new MacPaint document....                   Please 
wait."
x$=FILES$(0,"Choose a new filename:") 'Get a                         
                  Paint file
IF x$="" THEN RETURN  'No selection, then                            
                                                  forget it.
MENU 2,0,0:MENU 1,0,0
OPEN x$ FOR OUTPUT AS #1
    FOR i1%=1 TO number.of.bytes
        PRINT #1, k$(i1%);
    NEXT i1%
CLOSE #1
NAME x$ AS x$,"PNTG"
MENU 1,0,1:MENU 2,0,1
LOCATE 4,1:PRINT SPACE$(80)
BEEP:RETURN

Quit: MENU RESET:SHOWCURSOR
WINDOW CLOSE 2
WINDOW 1,"Output Window", (2,40)-(510,340),1
END

Display.patterns:
'Poke the display onto the screen
HIDECURSOR:MENU 1,0,0:MENU 2,0,0
a1=start+64*50+3
b1=3
'These loops poke the patterns to the screen
FOR qloop =0 TO 9 STEP 8
FOR pat% = 0 TO 18
    FOR i%=  (pat%*8)+1 TO(8*pat%)+8
        row=i%+4
        POKE a1+((row MOD 8+qloop)*64)+            (b1*pat%), ASC(k$(row))
        POKE a1+((row MOD 8+qloop)*64)+            (b1*pat%)+1, ASC(k$(row))
        POKE a1+((row MOD 8+qloop)*64)+            (b1*pat%)+2, ASC(k$(row))
    NEXT i%
NEXT pat%
NEXT qloop
a1=start+64*70+3
FOR qloop=0 TO 9 STEP 8
FOR pat% = 19 TO 38
    FOR i%= (pat%*8)+1 TO (8*pat%)+8
        row=i%+4
        POKE a1+((row MOD 8+qloop)*64)+            (b1*(pat%-19)), ASC(k$(row))
        POKE a1+((row MOD 8+qloop)*64)+            (b1*(pat%-19))+1, 
ASC(k$(row))
     POKE a1+((row MOD 8+qloop)*64)+ (b1*(pat%-19))+2, ASC(k$(row))
    NEXT i%
NEXT pat%
NEXT qloop
SHOWCURSOR:MENU 1,0,1:MENU 2,0,1
RETURN

Choose.pattern:  'Pattern% = selected pattern #
SHOWCURSOR:WINDOW 1
IF editor%=1 THEN makeselection
GOSUB Display.patterns
makeselection:
LOCATE 5,1:PRINT"Please use mouse to select pattern"
Edit.pattern:
GOSUB Choose.pattern
IF editor%=1 THEN editor
InitEditor:
WINDOW 2,"Pattern Editor",  (10,160)-(500,320),2
x=20:y=20:offsetx=0:offsety=0:editor%=1
TEXTFONT(NewYork):TEXTSIZE(14)
TEXTFACE(Bold)
LOCATE 5,1
PRINT"Please wait.... Initializing Editor."
PICTURE ON
FOR j= 0 TO 7
    FOR k=7 TO 0 STEP -1
        rectangle%(0)=y+offsety
        Bound0%((j*8)+k)=rectangle%(0)
        rectangle%(1)=x+offsetx
        Bound1%((j*8)+k)=rectangle%(1)
        rectangle%(2)=y+offsety+12
        Bound2%((j*8)+k)=rectangle%(2)
        rectangle%(3)=x+offsetx+12
        Bound3%((j*8)+k)=rectangle%(3)
        Bstatus%((j*8)+k)=0:offsetx=11+offsetx
        FRAMERECT(VARPTR(rectangle%(0)))
    NEXT k
    offsety=11+offsety:offsetx=0
NEXT j
PICTURE OFF
grid$=PICTURE$

editor:
    WINDOW 2
    TEXTFONT(NewYork):TEXTSIZE(14)
   TEXTFACE(Bold)
    LOCATE 5,1
   PRINT"Please wait for Setup of Editor."
    GOSUB Bitstatus
    ' set up new cursor
    GOSUB print.pic
    LOCATE 2,26:PRINT"Define New Pattern"
    GOSUB Print.message
    GOSUB Draw.Datapixels
    GOSUB Define
    CLS:BUTTON CLOSE 1
    TEXTFONT(NewYork):TEXTSIZE(14)
    TEXTFACE(Bold)
    LOCATE 5,1:PRINT"Please wait."
    GOSUB Set.pattern
    LOCATE 5,1:PRINT SPACE$(70)
    GOSUB Display.patterns
    RETURN

print.pic: CLS:PICTURE,grid$ 'print the grid 
    TEXTFONT(NewYork)
    TEXTSIZE(14)
    TEXTFACE(Bold)
    RETURN

Print.message:
    TEXTFACE(Plain)
    TEXTSIZE(12)
    LOCATE 4,35
    PRINT"Click              to  continue"                           
                  'This space ^^^^belongs here
    BUTTON 1,1,"OK",(310,40)-(350,80),1
    RETURN

Draw.Datapixels:
    FOR i= 0 TO 63
        IF Bstatus%(i)=1 THEN rectangle%(0)=Bound0%(i):
 rectangle%(1)=Bound1%(i):
 rectangle%(2)=Bound2%(i):
 rectangle%(3)=Bound3%(i):
 PAINTRECT(VARPTR(rectangle%(0))):
 FRAMERECT(VARPTR(rectangle%(0)))
 'end if
    NEXT i
    RETURN

mousepress:
    GOSUB getpixel   'see which pixel is selected
    IF pixel%=64 THEN  RETURN
    IF Bstatus%(pixel%)=0 THEN     Bstatus%(pixel%)=1          ELSE Bstatus%(pixel%)=0
    rectangle%(0)=Bound0%(pixel%)
    rectangle%(1)=Bound1%(pixel%)
    rectangle%(2)=Bound2%(pixel%)
    rectangle%(3)=Bound3%(pixel%)
    INVERTRECT(VARPTR(rectangle%(0)))
    FRAMERECT(VARPTR(rectangle%(0)))
    RETURN

getpixel:
    pixel%=64
    FOR i = 0 TO 63
        IF Bound0%(i)<MOUSE(2) AND Bound2%(i)>MOUSE(2) AND     
 Bound1%(i)<MOUSE(1) AND  Bound3%(i)>MOUSE(1) THEN             pixel%=i:i=64
    NEXT i
    RETURN

Define:
    WHILE DIALOG(0)<>1
    IF MOUSE(0)>0 THEN GOSUB  mousepress
    IF MENU(0)=2 AND MENU(1)=1 THEN  GOSUB Set.pattern:        GOSUB 
Display.patterns
    WEND
    BEEP:RETURN

Bitstatus: 'get status of each grid row
    FOR j=  0 TO 7
        row=j+((pattern%-1)*8)+5
        t%=ASC(k$(row))
        FOR k=7  TO 0 STEP -1
        IF t%<2^k THEN Bstatus%(j*8+k)=0
        IF t%>=2^k THEN   Bstatus%(j*8+k)=1:t%=t%-2^k
        endloop:NEXT k
    NEXT j
RETURN

Set.pattern: 'set bits to match bit status
    FOR j=  0 TO 7
        row=j+((pattern%-1)*8)+5
        t%=0
        FOR k=7  TO 0 STEP -1
            t%=(Bstatus%(j*8+k)*2^k)+t%
        NEXT k
        k$(row)=CHR$(t%)
    NEXT j
RETURN
 
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