FLI
Flexible Line Interpretation
A C64 trick that re-triggers the VIC-II's colour fetch on every pixel line instead of every eighth, multiplying the colours available in a bitmap — at the cost of a permanent grey scar down the left of the screen.
FLI — Flexible Line Interpretation — is the C64 trick that buys colour with time.
The VIC-II normally fetches colour information once per character line, every eight
pixel rows. FLI re-triggers that fetch on every pixel row by writing repeatedly
to $d011, so a bitmap can change colour eight times more often down the screen.
What it does it bringing more colors to your bitmap pictures by re-reading color info each pixel-line instead of each char-line (every 8 pixels). This is possible by repeatedly writing to
$d011to re-trigger color fetch.
The FLI bug
The trick has a permanent, visible cost. Forcing the fetch leaves the VIC-II without valid colour data for the first three characters of each line:
This method causes a bug on the left side of the screen, the typical “FLI-bug”, where the color info is set to
$ff, which is translated to bright gray by VIC.
Twelve pixels of the left edge, on every FLI picture ever made. The scene turned the defect into a competition — “the very few ones who manage to have colorful graphics there are considered having wizard skills.”
If you can recognise the grey stripe, you can identify the technique from across a room. It is the most legible fingerprint in C64 graphics.
Who did it first
⚠ Two firsts, because there are two uses.
| Use | Release | Date |
|---|---|---|
| FLI as a moving-colour effect | Solomon/BeyondForce, Charlatan | January 1989 |
| FLI as a still picture | ASP/Blackmail, So-phisticated III | 1989 |
The picture claim is logged with the scene’s own hedge attached — “not 100 percent sure this was the first one, anyone knows better?”
The family
FLI did not stay one trick. Codebase64 carries separate pages for AFLI, IFLI, SHFLI, UFLI and XFLI, and for applications built on it: a 40-byte FLI displayer, 4×4 FLI chunky mode, tech-tech FLI, copper-style FLI plasma, an FLI-FPP scroller, a second-pixel FLI distorter. Crossbow/Crest is credited on the SHIFLI editor.
See also
- FLD — the vertical counterpart, and older
- Raster interrupts — what makes per-line work possible
- Colour clash — the constraint FLI is fighting