Sprite shrinking

Shrinking (also known as scaling, reduction, and "zooming") is a hardware feature of the NeoGeo which allows to scale down sprites with per-pixel accuracy in 2 dimensions.

It can be seen as a simple subsampling of the original graphics since there's no smoothing at all.

Vertical shrinking

Vertical shrinking is set by an 8-bit value indicating the final height of the sprite's graphics. This value is set in VRAM, in the SCB2.

Range: $FF is the tallest, $0 is the smallest.

To know which line to skip, the GPU uses a lookup table stored in the L0 ROM.

Vertical shrink propagates in chained sprites.

Caveat: Display window

Note that vertical shrinking does not affect the tile height setting of the sprite. The graphics are shrunk inside the defined window.

• 

  Sprite height set to 4 tiles, graphics intentionally filling 4 tiles

• 

  The same sprite updated to be only 3 tiles high without clearing the 4th (yellow) tile

• 

  The sprite will shrink this way, showing the garbage yellow tile from the previous setup

Caveat: Last line repeat

If the display window of a vertically shrinked sprite is taller than the shrunk graphics, the remaining lines will be filled with repeats of the last line of the last tile (the bottom line of the tile at map index 15).

SNK recommends leaving that line fully transparent if this causes problems.

Horizontal shrinking

The horizontal shrinking is set by a 4-bit value indicating the final width of the sprite graphics +1 (giving 1 to 16 pixels wide sprites). This value is also set in SCB2.

Range: $F is full size (all 16 pixels shown), $0 is the smallest (only 1 pixel shown).

Horizontal shrink does not propagate in chained sprites. Values for each sprite of a block have to be distributed to obtain the total desired block width.

Pixel skipping for rendering is done with a "bit matrix" lookup table inside the GPU. This table is not in the L0 ROM.

For each shrinking value on a sprite line, 1 means the pixel will be drawn and 0 means it will be skipped (info from MAME's source, matches the real hardware):

Horizontal shrink effect on a multicolored tile, showing how pixels are skipped.

• 0: 0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0 (15 pixel skipped, 1 remaining)
• 1: 0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0 (14 pixels skipped...)
• 2: 0,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0
• 3: 0,0,1,0,1,0,0,0,1,0,0,0,1,0,0,0
• 4: 0,0,1,0,1,0,0,0,1,0,0,0,1,0,1,0
• 5: 0,0,1,0,1,0,1,0,1,0,0,0,1,0,1,0
• 6: 0,0,1,0,1,0,1,0,1,0,1,0,1,0,1,0
• 7: 1,0,1,0,1,0,1,0,1,0,1,0,1,0,1,0
• 8: 1,0,1,0,1,0,1,0,1,1,1,0,1,0,1,0
• 9: 1,0,1,1,1,0,1,0,1,1,1,0,1,0,1,0
• A: 1,0,1,1,1,0,1,0,1,1,1,0,1,0,1,1
• B: 1,0,1,1,1,0,1,1,1,1,1,0,1,0,1,1
• C: 1,0,1,1,1,0,1,1,1,1,1,0,1,1,1,1
• D: 1,1,1,1,1,0,1,1,1,1,1,0,1,1,1,1
• E: 1,1,1,1,1,0,1,1,1,1,1,1,1,1,1,1 (...1 pixel skipped)
• F: 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1 (no pixels skipped, full size)

Centering

Centering can be required since the "anchor point" of sprites is fixed at their top left.

The X offset is -((hshrink+1)/2). The Y offset is -(vshrink+1).