Euclid_Compress (&80653)
Euclid_Expand (&80654)
=> R0 = Compression technique (0 is the only one defined so far).
R1 ! Data block.
R2 ! Last screen base, or 0
<= R1 ! Word beyond compressed data.
R2 ! Screen base of current screen.
The data block is a chunk of memory which holds compressed screen images.
The data block return value points to the word past the end of the frame
just compressed/expanded (i.e. it is ready for the next call if winding a
film forwards).
Euclid_Compress will compress the current graphics window (which must have
it's edges byte aligned) into the data block. The length of the block is
placed in the first and last words of the block (to enable fast
forward/rewind), the next word is set to zero. If the last screen base value
is non-zero, then delta compression takes place between the last screen and
the current screen - the current screen is calculated internally so you do
not need to access the Vdu Variables.
Euclid_Expand will expand the data block into the current graphics window
(see constraints above). If the last screen base value is non-zero then
delta expansion takes place between the last screen and the current screen.
Note that the last screen and the current screen can be the same, in which
case delta expansion will be very fast, assuming the differences between the
frames are small.
Note that attempts to expand a film into the wrong size of window are likely
to end in catastrophe.
Examples
Euclid_Compress and Expand examples
To compress a sequence using 'Normal' compression:
DIM datablock% dblocksize%
DPTR%=datablock%
FOR FRAME%=1 TO FRAME_MAX%
PROCDRAWPICCY
SYS"Euclid_Compress",,DPTR% TO ,DPTR%
NEXT
IFDPTR%>datablock%+dblocksize% THEN ERROR1,"Oops!"
To compress a sequence using 'Delta' compression:
DIM datablock% dblocksize%
DPTR%=datablock:LAST%=0:SCREEN%=1
FOR FRAME%=1 TO FRAME_MAX%
SYS&70,SCREEN%:SCREEN%=3-SCREEN%:SYS&71,SCREEN%:REM Flip screens
PROCDRAWPICCY
SYS"Euclid_Compress",,DPTR%,LAST% TO ,DPTR%,LAST%
NEXT
IFDPTR%>datablock%+dblocksize% THEN ERROR1,"Oops!"
To expand a sequence using 'Normal' decompression:
DIM datablock% dblocksize%:OSCLI"Load Film "+STR$~datablock%
DPTR%=datablock%
FOR FRAME%=1 TO FRAME_MAX%
SYS"Euclid_Expand",,DPTR% TO ,DPTR%
NEXT
(Alternatively REPEAT:SYS"Euclid_Expand",,DPTR% TO ,DPTR%:UNTIL!DPTR%=0)
To expand a sequence using 'Delta' decompression on two screens:
DIM datablock% dblocksize%
DPTR%=datablock:LAST%=0:SCREEN%=1
FOR FRAME%=1 TO FRAME_MAX%
SYS"OS_AMBControl",SCREEN%:SCREEN%=3-SCREEN%:SYS&71,SCREEN%:REM Flip screens
SYS"Euclid_Expand",,DPTR%,LAST% TO ,DPTR%,LAST%
NEXT
To expand on one screen simply remove the screen flip line.
Note that Normal compression and Delta compression are data compatible. This
means that you can use Normal decompression on a Delta compressed screen to
see the differences between frames.
You can run both Normal and Delta films backwards, although in the latter
case you will probably need to run the film forward to find the last frame.
To get to the end of the film use:
DPTR%=datablock%:REPEAT:DPTR%+=!DPTR%:UNTIL!DPTR%=0
Note that it is important that you save the 0 word on the end of a film,
i.e. you should use SYS"OS_File",10,"Film",&D6A,,datablock%,DPTR%+4.
To go to the previous frame use: DPTR%-=DPTR%!-4 (which can be repeated
until DPTR%=datablock%).
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