SL2 — Next Layer 2 screens
SL2 stores native bitmap bytes followed by an optional palette. This differs from NXI, which prepends the palette. An optional 128-byte +3DOS header is preserved.
| Explicit mode | Bitmap bytes | Storage order | Optional palette bytes |
|---|---|---|---|
| 256×192, 8-bit | 49152 | Row-major | 256 RGB332 / 512 RGB333 |
| 320×256, 8-bit | 81920 | Column-major | 256 RGB332 / 512 RGB333 |
| 640×256, 4-bit | 81920 | Column-major packed pixel pairs | 16 RGB332 / 32 RGB333 |
The high nibble is the left pixel. Width, height and depth describe the selected native layout; they are not fields invented in a header. Palette-free 320/640 files have identical lengths, so select the mode explicitly. No dimensions or bit depth are guessed.
Reading and writing
Use Sl2Format: Standard, Wide, or HighResolution; ForDimensions selects the corresponding singleton.
var screen = Sl2Format.Wide.Read(bytes);
var image = screen.Image; // Unpacked, top-left-origin, row-major indexed view.
var header = screen.Header; // Null for a headerless file.
screen.Write(stream); // Preserves native bytes, including header metadata and priority bits.Sl2File exposes byte-backed Header, PixelData and Palette. Plus3DosHeader exposes issue/version, complete size, BASIC type/length/parameters and checksum. The header signature, soft EOF, checksum and complete file length are validated. Reserved bytes and other metadata survive reading/writing. An exact raw layout takes precedence if its pixel bytes happen to start with PLUS3DOS.
Palettes and conversion
ScreenPaletteEncoding selects no palette, RGB332 or RGB333. RGB333's second byte permits the low blue bit and the Layer 2 priority bit; other bits must be zero. ScreenPalette exposes the encoding, derived colours and GetPriority.
Without stored palette bytes, the convenience image uses the default RGB332 palette: the missing blue bit is the OR of the two stored blue bits. This is an interpretation, not a claim about the palette currently loaded on a running machine. Supply an embedded palette when those colours differ. Creation without palette bytes requires that default palette; it never silently drops arbitrary colours.
var screen = new Sl2File(indexedImage, Sl2Format.Standard,
ScreenPaletteEncoding.Rgb333, includeHeader: true);
var converter = new ImageToSl2Converter(BmpFormat.Instance, Sl2Format.Standard,
ScreenPaletteEncoding.Rgb333, includeHeader: true);
var converted = converter.Convert(bmp);Creation requires matching dimensions, fitting indices and exactly 16/256 opaque colours. Palette order and duplicate slots remain intact; colours are rounded to the nearest representable hardware levels. No implicit quantization, reindexing, resizing or palette generation occurs. Explicit conversion re-creates the requested layout and does not preserve source header metadata or palette priority flags; use native read/write for lossless copies. Screen-to-BMP conversion is registered; incoming converters are explicit because target type alone cannot choose mode/palette/header options.
Use ZXSpectrumNextResourceFileFormats.WithScreens(layer2, lowResolution) for discovery; the unqualified list excludes SL2/SLR. Existing OakIO compression wrappers can wrap the whole file. Truncated bitmaps, partial/unrecognized palette lengths, unsupported modes and invalid headers are rejected. Header/palette combinations extend the browser's basic no-palette screen variant; not every loader accepts every variant. No arbitrary-size dumps or bank padding are guessed.