Extending VDU 19 to set the alpha channel

[Hated Moron]

A notable omission from the capabilities of VDU 19 is the option of setting the alpha (opacity) channel as well as RGB; at present it's only possible to set fully-opaque colours.

Because the command must always be the same length (6 bytes) there's no possibility of adding an extra parameter. However the high-bit of the logical colour is currently unused (this is used to flag the background colour in VDU 17 and VDU 18, which implies a logical colour range of no more than 0-127).

So I'm wondering about this as a possibility:

Code: Select all

VDU 19, logical+128, alpha, red, green, blue

Would this introduce any compatibility issues?

[Ric]

Not for me

[Hated Moron]

On 11/02/2024 10:27, J.G.Harston wrote (cross-posted from the Discussion Group):

I've spent a hour or so with a cup of tea checking documentation and
existing VDU driver code, and it looks like it will be fine. All
pre-existing correctly written code will be sending l<64 to VDU 19(*),
and using bit 7 safely segregates the additional function from
existing ranges.

Also, &80+n has a neat similarity to the use of &80+n in other functions.

Thank you, that's very helpful (I was hoping you would chime in).

(*)Other than an implementation I have that uses VDU 19,255 to set the
border colour, but I think that will also be fine as you would not be
using VDU 19 in a display with more than 64 colours

It's not impossible (BBCSDL can have 128 logical colours, so &80+127 is 'in range') but unlikely, and it's probably the case that platforms on which you can set the border colour don't overlap with platforms on which you can have >64 logical colours and set the alpha/opacity.