Graphics¶
Explanation: how a colour attribute occupies a cell, what each change of style
costs, and why the compositor places runs exactly rather than searching for a
placement. The recipes that apply this are Draw an icon,
Draw a chart, Letter on a background and
Compose a frame. The API is sextile.viewdata.blocks,
sextile.viewdata.composition and sextile.viewdata.charting.
The block grid¶
A mosaic cell is a 2 × 3 grid of blocks, so a frame of 40 × 24 cells is 80 × 72 blocks — 78 across in practice, because a graphics colour attribute occupies a cell and attributes reset at the start of every row, so a picture pays one cell on each row it spans.
What a style costs¶
Style carries every attribute the hardware has, because the transitions are not
uniform — which is the argument for handing a compositor a style rather than
writing controls by hand.
Change |
Cells |
Note |
|---|---|---|
foreground colour |
1 |
also chooses the character set |
entering or leaving graphics |
1 |
the colour attribute does both |
contiguous ↔ separated |
1 |
chooses the set; a colour attribute enters it |
flash / steady |
1 |
|
double height / normal |
1 |
and takes the row below |
hold / release graphics |
1 |
|
a background |
3 |
choose the colour, promote it, choose the foreground again |
a background matching the foreground |
2 |
nothing to change back to |
back to black |
1 |
|
conceal |
1 |
and cannot be undone |
A background costs three cells because the hardware has no set-background, only
NEW_BACKGROUND, which makes the current foreground the background. CONCEAL
has no counterpart, so a composition asked to turn it off is refused rather than
drawn wrongly; double height places its run on the row below as well, which is
how the display draws the bottom halves. The measured basis is in
Viewdata encoding.
Placement is exact¶
The compositor reports whether a layout is possible — naming the row, the column and the arithmetic — before a cell is written, and draws nothing if any row fails, so a bad layout never leaves half a frame on a screen. Two runs in one style cost one attribute, not two, because there is no text between them. Placement is a single left-to-right pass rather than a search: an attribute displays as a blank, and a blank in graphics is the no-blocks mosaic, so an attribute may sit anywhere in the gap before its run and there is nothing to search for. It becomes a search only if runs are free to move, which is a different feature.