Display & Bitplanes

Display & Bitplanes

Black Crypt runs in the Amiga’s EHB (Extra Half-Brite) display mode. This is the single most important thing to understand before decoding any of the game’s graphics.

EHB in one paragraph

EHB uses 6 bitplanes. The first 5 planes give 32 colours (indices 0–31). The 6th plane (plane 5) is a half-bright flag: when it’s set, the hardware shows the colour at half intensity. So the palette is effectively 32 base colours plus 32 half-bright copies (indices 32–63).

BPLCON0 $6200   →  6 bitplanes + EHB
Plane pitch $1F40 = 8000 = 320×200/8  →  resolution is 320×200

Planar bitmaps

The Amiga stores bitmaps planar, not byte-packed. For a 6-bitplane image, each pixel’s colour index is spread across 6 separate planes — one bit per plane. To recover a pixel’s colour you read the same bit position from all 6 planes and combine them:

colour = plane0_bit | (plane1_bit << 1) | ... | (plane5_bit << 5)

Bit order is standard: plane 1 → bit 0 (LSB), plane 6 → bit 5 (EHB half-bright MSB).

Sequential vs word-interleaved

The game stores its bitplanes sequentially — all of plane 0, then all of plane 1, and so on — not word-interleaved. When decoding, advance the source pointer by the plane stride between planes.

The two sprite formats

Almost every sprite in the game uses one of two layouts:

1. 7-plane masked — plane 0 is a 1-bit cookie-cut mask (1 = opaque), planes 1–6 are the 6bpp EHB colour. This is the convention for monster sprites, UI elements, wall decorations, and most bcdfa art.
2. 6-plane opaque — no mask plane; transparency is a specific colour index. This is used for item icons (where “transparency” is colour index 53, RGB 0x222222, byte-identical to the inventory slot it’s blitted onto).

The tile descriptor

Portraits and UI tiles use a fixed descriptor (blitter setup):

+2  source offset   = $60 + index × $240  (96 + index × 576)
+6  plane stride    = $60 (96) = 32×24/8
+14 BLTSIZE         = $0603  (h=24, w=3 words)
+24 width           = 32
+26 height          = 24

The blit loop runs 6 planes with a cookie-cut minterm, advancing the source by one plane stride each time — confirming sequential planar storage.

Ground-truth oracle

Because the game holds its decompressed graphics in chip RAM, emulator savestates (data/blackcrypt/default*.uss) are a powerful oracle: each carries the machine’s entire 2 MB of chip RAM. If your RLE decode is correct, the whole decompressed block appears verbatim somewhere in chip RAM — find() either returns an address or it doesn’t. This is how the 75,600-byte item bank was confirmed at $7D918.

One trap: mapping screenshot RGB back to palette indices is not injective under EHB. Register 22 (0x666) and EHB register 56 (half of 0xCCC) are both RGB 0x666666. Always compare in RGB, not recovered indices.