BBC Screen GIF Conversion Utilities =================================== J.G.Harston - (C)1999, 2010 mdfs.net/Apps/Graphics/Conversion jgh@mdfs.net 70 Camm Street, Walkley, Sheffield S6 3TR BBCtoGIF -------- Syntax: BBCtoGIF -m -s -v Runs on BBC, Master, RISC OS, DOS and Windows. Converts a BBC graphics screen image (MODEs 0 to 6) to a Compuserve GIF (Graphics Interchange Format) file. Will also read a compressed screen image and palette information stored in load and execution addresses by *ScrSave. If no *ScrSave palette information is available then the default palette for the specified screen mode is used. If *ScrSave information is available then the screen mode it specifies is used and the -mode option can be omitted. If the -mode option is used it overrides the mode specified in the *ScrSave information. If the -small switch is given then small resolution screen modes (those other than MODE 0 and MODE 3) are output as 320x256 pixels images instead of 640x512 pixel images. If the -verbose switch is given a countdown of converted pixel lines is displayed. On DOS/Windows if a '.inf' file is present, then the load and execution addresses are read from it. The '.inf' file will also be read on BBC/RISC OS if the source file's load and execution addresses are both zero. On exit OSBYTE 1 or the DOS ERRORLEVEL is set to the return value, which will be 0 if no errors occured. Notes ----- This is an initial version that only outputs uncompressed GIFs. This means that the output file is significantly larger than the BBC input file. A MODE 1 screen dump creates a 330K GIF file. The next step is to implement GIF compression. Usefully, the patent on the LZW compression algorythm has expired, so I don't have to find several thousand dollars for a license. Another cause of bulky output files is that I've only got the GIF generation with a 7-bit palette (128 colours) which uses 8-bit output codes. This means that, for example, a MODE 1 screen with 2-bit pixels (4 colours) has an output file 32 times bigger than needed. Acknowledgements ---------------- Thanks to Jules Richardson and Ed Spittles on the BBC Mailing List for finding where the errors in the early GIF output where. The GIF file format documentation is very difficult to translate into working code. History ------- v0.10 10-Jul-2010 Initial version based on BBCtoBMP v0.20 03-Oct-2012 Mostly working v0.21 03-Oct-2012 Zebra lines correct