ColourTrans_ReturnFontColours ?

Can anybody confirm that this SWI is broken?

On my Pi, homebrew RISC OS 5.21 dating from around November 2014, the ColourTrans_ReturnFontColours SWI is broken – it is returning the appropriate background and foreground colours as zero and actually setting the specified colours!

Surely this is the job of the SWI ColourTrans_SetFontColours? As it happens, swapping ReturnFontColours for SetFontColours does exactly the same thing; and for what it is worth, both SWIs return the foreground/background colours as zero.

MODE 28
SYS "Font_FindFont",,"Corpus.Bold", (20*16), (20*16), 0, 0 TO fnt%

SYS "ColourTrans_ReturnFontColours", fnt%, 0, &FFFFFF00, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Monochrome?", (1<<4), 256, 512
SYS "ColourTrans_ReturnFontColours", fnt%, 0, &FF00FF00, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Magenta?", (1<<4), 256, 384
SYS "ColourTrans_ReturnFontColours", fnt%, 0, &FFFF0000, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Cyan?", (1<<4), 256, 256

SYS "ColourTrans_SetFontColours", fnt%, 0, &FFFFFF00, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Monochrome?", (1<<4), 720, 512
SYS "ColourTrans_SetFontColours", fnt%, 0, &FF00FF00, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Magenta?", (1<<4), 720, 384
SYS "ColourTrans_SetFontColours", fnt%, 0, &FFFF0000, 14 TO a,b,c,d
PRINT b,c,d
SYS "Font_Paint", fnt%, "Cyan?", (1<<4), 720, 256

SYS "Font_LoseFont", fnt%

Update: On proper release of RISC OS 5.23 dated 5th July (with ZeroPain) – same behaviour.

This is one of those nasty situations where the OS doesn’t really deal with 256 colour (let alone true colour) properly. The PRM entry for Font_SetFontColour reveals what’s going on – in >= 256 colour modes, the colour values given in R1 and R2 are indices from 0-15 into the palette that’s set by Font_SetPalette. So the only way that ColourTrans_ReturnFontColours/ColourTrans_SetFontColours can work for true colour values is if it also modifies the font manager’s 16 entry palette. Looking at the code, ColourTrans will use the lower 4 bits of R1 and R2 to work out which font manager palette indices to use – so if you use non-zero values for them then you should be able to get ColourTrans_ReturnFontColours to work with up to 16 different colours at a time.