GIF Creation

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GIF File Creator

The following routine can be used with Qbasic or QB64 to create a Graphic Information File image of a program screen.

  • Accommodates _NEWIMAGE screen pages with up to 256 colors and image files loaded with _LOADIMAGE.
  • The maximum screen coordinates are always one pixel LESS than the screen mode's resolution! (SCREEN 13's are 319 and 199)
  • The $INCLUDE text file can be created using Notepad and is REQUIRED when the program is compiled with QB64 ONLY!


'*********************************** DEMO CODE ********************************** 'Save code as a BAS file! Includes the GIFcreate.BI and BM text files. Demo by CodeGuy DEFINT A-Z SCREEN 13 RANDOMIZE TIMER FOR A = 1 TO 40 x = RND * 320 y = RND * 200 c = RND * 256 CIRCLE (x, y), RND * 80, c PAINT (x, y), RND * 256, c NEXT MakeGIF "GIFtemp.gif", 0, 0, _WIDTH - 1, _HEIGHT - 1, 256 'use 319 and 199 in Qbasic 'Use the include file in QB64 only! Hard code the SUB in Qbasic. '$INCLUDE: 'GIFcreate.BM' '************************************ END DEMO *********************************

GIFcreate.BM text $INCLUDE file:

'----------------------------------------------------------------------------- ' GIFcreate.BM Compression Routine v1.00 By Rich Geldreich 1992 ' Converted into one SUB Library routine by Ted Weissgerber 2011 '----------------------------------------------------------------------------- ' For 1 BPP, 4 BPP or 8 BPP images only! 'file$ = save image output filename 'XStart = <-left hand column of area to encode 'YStart = <-upper row of area to encode 'Xend = <-right hand column of area to encode 'Yend = <-lowest row of area to encode " 'NumColors = # of colors on screen: 2(Black & White), 16(SCREEN 12), 256(SCREEN13) ' SUB MakeGIF (file$, Xstart, YStart, Xend, Yend, NumColors) CONST True = -1, False = 0 CONST Table.size = 7177 'hash table's size - must be a prime number! DIM Prefix(Table.size - 1), Suffix(Table.size - 1), Code(Table.size - 1) DIM Shift(7) AS LONG FOR i = 0 TO 7: Shift(i) = 2 ^ i: NEXT 'create exponent array for speed. PWidth% = ABS(Xend - Xstart) + 1 PDepth% = ABS(Yend - Ystart) + 1 'MinX, MinY, MaxX, MaxY are maximum and minimum image coordinates IF Xstart > Xend THEN MaxX = Xstart: MinX = Xend ELSE MaxX = Xend: MinX = Xstart IF Ystart > Xend THEN MaxY = Ystart: MinY = Yend ELSE MaxY = Yend: MinY = Ystart 'Open GIF output file GIF = FREEFILE 'use next free file OPEN file$ FOR BINARY AS #GIF B$ = "GIF87a": PUT #GIF, , B$ 'Put GIF87a header at beginning of file SELECT CASE NumColors 'get color settings CASE 2 'monochrome (B&W) image BitsPixel = 1 '1 bit per pixel StartSize = 3 'first LZW code is 3 bits StartCode = 4 'first free code StartMax = 8 'maximum code in 3 bits CASE 16 '16 colors images SCREENS 7, 8, 9, 12, 13 BitsPixel = 4 '4 bits per pixel StartSize = 5 'first LZW code is 5 bits StartCode = 16 'first free code StartMax = 32 'maximum code in 5 bits CASE 256 '256 color images SCREEN 13 or _NEWIMAGE 256 BitsPixel = 8 '8 bits per pixel StartSize = 9 'first LZW code is 9 bits StartCode = 256 'first free code StartMax = 512 'maximum code in 9 bits END SELECT 'ColorBits = 2 'for EGA ColorBits = 6 'VGA monitors ONLY PUT #GIF, , PWidth% 'put screen's dimensions PUT #GIF, , PDepth% CP = 128 + (ColorBits - 1) * 16 + (BitsPixel - 1) 'pack colorbits and bits per pixel PUT #GIF, , CP Zero$ = CHR$(0) 'PUT a zero into the GIF file PUT #GIF, , Zero$ OUT &H3C7, 0 'start read at color 0 FOR c = 0 TO NumColors - 1 'Get the RGB palette from the screen and put into file R = (INP(&H3C9) * 65280) \ 16128 'C = R * 4.0476190(for 0-255) G = (INP(&H3C9) * 65280) \ 16128 B = (INP(&H3C9) * 65280) \ 16128 red$ = CHR$(R): PUT #GIF, , red$ grn$ = CHR$(G): PUT #GIF, , grn$ blu$ = CHR$(B): PUT #GIF, , blu$ NEXT 'write out an image descriptor sep$ = "," 'image separator PUT #GIF, , sep$ 'write it PUT #GIF, , Minx 'image start locations PUT #GIF, , MinY PUT #GIF, , PWidth% 'store them into the file PUT #GIF, , PDepth% A$ = CHR$(BitsPixel - 1) '# bits per pixel in the image PUT #GIF, , A$ A$ = CHR$(StartSize - 1) 'store the LZW minimum code size PUT #GIF, , A$ CurrentBit = 0: Char& = 0 'Initialize the vars needed by PutCode MaxCode = StartMax 'the current maximum code size CodeSize = StartSize 'the current code size ClearCode = StartCode 'ClearCode & EOF code are the EOFCode = StartCode + 1 'first two entries StartCode = StartCode + 2 'first free code that can be used NextCode = StartCode 'the current code OutBuffer$ = STRING$(5000, 32) 'output buffer; for speedy disk writes Buff& = SADD(OutBuffer$) 'find address of buffer Buff& = Buff& - 65536 * (Buff& < 0) Oseg = VARSEG(OutBuffer$) + (Buff& \ 16) 'get segment + offset >> 4 OAddress = Buff& AND 15 'get address into segment OEndAddress = OAddress + 5000 'end of disk buffer OStartAddress = OAddress 'current location in disk buffer DEF SEG = Oseg GOSUB ClearTree 'clear the tree & output a PC = ClearCode: GOSUB PutCode 'clear code x = Xstart: y = YStart 'X & Y have the current pixel GOSUB GetByte: Prefix = GB 'the first pixel is a special case Done = False 'True when image is complete DO 'while there are more pixels to encode DO 'until we have a new string to put into the table IF Done THEN 'write out the last pixel, clear the disk buffer ' 'and fix up the last block so its count is correct PC = Prefix: GOSUB PutCode 'write last pixel PC = EOFCode: GOSUB PutCode 'send EOF code IF CurrentBit <> 0 THEN PC = 0: GOSUB PutCode 'flush out the last code... PB = 0: GOSUB PutByte OutBuffer$ = LEFT$(OutBuffer$, OAddress - OStartAddress) PUT #GIF, , OutBuffer$ A$ = ";" + STRING$(8, &H1A) 'the 8 EOF chars is not standard, PUT #GIF, , A$ A$ = CHR$(255 - BlockLength) 'correct the last block's count PUT #GIF, LastLoc&, A$ CLOSE #GIF: EXIT SUB '<<<<<<<<<<< End of procedure ELSE 'get a pixel from the screen and find the new string in table GOSUB GetByte: Suffix = GB GOSUB Hash 'is it in hash table? IF Found = True THEN Prefix = Code(Index) 'replace prefix:suffix string with code in table END IF LOOP WHILE Found 'don't stop unless we find a new string PC = Prefix: GOSUB PutCode 'output the prefix to the file Prefix(Index) = Prefix 'put the new string in the table Suffix(Index) = Suffix Code(Index) = NextCode 'we've got to keep track of code! Prefix = Suffix 'Prefix = the last pixel pulled from the screen NextCode = NextCode + 1 'get ready for the next code IF NextCode = MaxCode + 1 THEN 'increase the code size MaxCode = MaxCode * 2 'Note: The GIF89a spec mentions something about a deferred clear code IF CodeSize = 12 THEN 'is the code size too big? PC = ClearCode: GOSUB PutCode 'yup; clear the table and GOSUB ClearTree 'start over NextCode = StartCode CodeSize = StartSize MaxCode = StartMax ELSE CodeSize = CodeSize + 1 'increase code size if not too high (not > 12) END IF END IF LOOP 'while we have more pixels ' 'GOSUB ROUTINES ClearTree: FOR A = 0 TO Table.size - 1 'clears the hashing table Prefix(A) = -1 '-1 = invalid entry Suffix(A) = -1 Code(A) = -1 NEXT RETURN Hash: 'hash the prefix & suffix(there are also many ways to do this...) Index = ((Prefix * 256&) XOR Suffix) MOD Table.size ' Note: the table size(7177 in this case) must be a prime number ' Calculate an offset just in case we don't find what we want first try... IF Index = 0 THEN 'cannot have Table.Size 0! Offset = 1 ELSE Offset = Table.size - Index END IF DO 'loop until we find an empty entry or find what we're lookin for IF Code(Index) = -1 THEN 'is this entry blank? Found = False ' didn't find the string RETURN ELSEIF Prefix(Index) = Prefix AND Suffix(Index) = Suffix THEN Found = True 'found the string RETURN ELSE 'didn't find anything, must retry - this slows hashing down. Index = Index - Offset IF Index < 0 THEN 'too far down the table? wrap back the index to end of table Index = Index + Table.size END IF END IF LOOP PutByte: 'Puts a byte into the GIF file & also takes care of each block. BlockLength = BlockLength - 1 'are we at the end of a block? IF BlockLength <= 0 THEN 'end of block BlockLength = 255 'block length is now 255 LastLoc& = LOC(GIF) + 1 + (OAddress - OStartAddress) 'remember the position BW = 255: GOSUB BufferWrite 'for later fixing END IF BW = PB: GOSUB BufferWrite RETURN BufferWrite: 'Puts a byte into the buffer IF OAddress = OEndAddress THEN 'are we at the end of the buffer? PUT #GIF, , OutBuffer$ 'write it out and OAddress = OStartAddress 'start all over END IF POKE OAddress, BW 'put byte in buffer OAddress = OAddress + 1 'increment position RETURN GetByte: 'This routine gets one pixel from the display GB = POINT(x, y) 'get the "byte" x = x + 1 'increment X coordinate IF x > MaxX THEN 'are we too far? x = Minx 'go back to start y = y + 1 'increment Y coordinate IF y > MaxY THEN Done = True 'flag if too far down END IF RETURN PutCode: 'Puts an LZW variable-bit code into the output file... Char& = Char& + PC * Shift(CurrentBit) 'put the char were it belongs; CurrentBit = CurrentBit + CodeSize 'shifting it to its proper place DO WHILE CurrentBit > 7 'do we have a least one full byte? PB = Char& AND 255: GOSUB PutByte 'mask it off and write it out Char& = Char& \ 256 'shift the bit buffer right 8 bits CurrentBit = CurrentBit - 8 'now we have 8 less bits LOOP 'loop until we don't have a full byte RETURN END SUB


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