Reference · Formats

BS still pictures

partialfrom#5

Five .BS files in /DATA/MENU/TIMS, all 512 x 240: MDEC run-length blocks packed with MPEG-1's AC codes. libpress decodes the codes on the CPU (DecDCTvlc) and hands the result to the MDEC; rugrats_data::bs does both.

Layout

Little-endian.

u16  words      run-length output, in 32-bit words
u16  0x3800     with `words`, the MDEC decode command (15-bit, unsigned)
u16  q_scale
u16  version    2; E3SCR512.BS is 3
bitstream: 16-bit little-endian words, most significant bit first
  per block:  dc:10, then MPEG-1 table B.14 codes with a sign bit,
              "10" end of block, "000001" escape to run:6 level:10

Blocks go Cr, Cb, Y0, Y1, Y2, Y3 per 16 x 16 macroblock; macroblocks run down each 16-pixel column (15 per column), columns left to right.

Decoding

Each block emits q_scale << 10 | dc, then run << 10 | level per code, then 0xfe00; the output is padded to words with 0xfe00. The bitstream's last bits hold less than a block and are not decoded. The MDEC tables are libpress's: the executable keeps them as ready MDEC commands, 0x40000001 and 128 bytes (luma and chroma quant tables, identical) at 0x800c1f20, and 0x60000000 and the 64-entry IDCT scale table at 0x800c1fa4.

Shown

show_still(file) (0x80030010) decodes into VRAM at (0, 256), draws it in 32 x 32 textured quads with colour 0x7f7f7f (0x800c3f9c), so each 5-bit channel is c * 127 >> 7, into both display buffers, and holds until 250 vertical blanks after it began. Only the legal screen is shown this way at boot (main, 0x800186cc).

Unknown

• Version 3's differential DC (only E3SCR512.BS) is not decoded.
• Where THQ512, NICK512 and NSPAC512 are shown, if anywhere (the logos at boot are .STR movies).