Spectrum 512

• Company: Trio Inc. / Antic Software
• Extensions: .spu, .spc, sps

Spectrum 512 was an image drawing program for the Atari ST.

16-bit words are stored in big-endian order.

Spectrum 512 Uncompressed

• Extension: .spu

typedef spectrum512_file {
 uint16_t data[16000];             /* first 80 words (first scanline) is unused and should be zeroes */
 uint16_t palettes[199 * 3 * 16];  /* 199 scanlines, 3 palettes per scanline, 16 entries per palette */
} spectrum512_file;

Spectrum 512 Compressed

• Extension: .spc

typedef spectrum512_compressed_file {
 uint16_t magic;                   /* "SP" */
 uint16_t reserved;                /* always 0 */
 uint32_t data_length;             /* length of compressed image data */
 uint32_t palette_length;          /* length of compressed palettes */
 uint8_t data[];
 uint8_t palettes[];
} spectrum512_compressed_file;

Image data is compressed by RLE.

   0 <= x <= 127        x+1 literals
-128 <= x <=  -1        -x+2 times next byte

Each plane is stored separately, i.e. 199 scanlines of plane 0, 199 scanlines of plane 1, et cetera.

Just like uncompressed .spu files, there are 199 * 3 palettes of 16 entries. Each palette is compressed separately. A compressed palette:

typedef compressed_palette {
 uint16_t map;                    /* bit = 1, color is present; bit = 0, color is black (0x0000) */
 uint16_t colors[];
} compressed palette;

Bit 0 of map refers to palette entry 0 and so on. Bit 0 and bit 15 should always be zero. The background is always black and palette entry 15 is unused. This results in 15 * 3 + 1 = 46 colors per scanline.

Spectrum 512 Smooshed

• Extension: .sps

typedef spectrum512_smooshed_file {
 uint16_t magic;                  /* "SP" */
 uint16_t reserved;               /* always 0 */
 uint32_t data_length;
 uint32_t palette_length;
 uint8_t data[];
 uint8_t palettes[];
} spectrum512_smooshed_file;

The image data is compressed similar to .spc files.

There's a second variant where the data is not encoded plane by plane and line by line, but in byte wide vertical stripes (for each plane (for each column (scanline 1-199))).

It is unknown how (or impossible) to distinguish the two variants.

The palettes are compressed similar to .spc files, but rather than uint16_t values, variable length codes are stored. Each palette consists of a 14-bit map which marks which entries will follow (entries 0 and 15 are skipped and set to black). After the map, zero or more codes of 9-bits follow.

Palettes

Spectrum 512 files use three palettes per scanline. Because of the Atari ST's limitted graphics capabilities, a trick was used to change the hardware color palette twice per scanline. Given an x-coordinate and a color index, the following algorithm returns the corresponding spectrum palette index (0-47).

int find_index(unsigned int x, unsigned int c) {
    unsigned int t = 10 * c;

    if (c&1) t -= 5;
    else     t++;

    if (x < t)      return c;
    if (x >= t+160) return c + 32;
    return c + 16;
}

Image Data

Uncompressed image data (.spu files) is stored in a planar format.