Used by the routine at DRAW_SPRITE.

The graphic (+0) indexes GRAPHICS for the sprite. A sprite is a width byte -- bits 0-3 the width in bytes, bit 7 set while its data is stored upside down, bit 6 while it is stored mirrored -- a height byte, and then a mask byte and an image byte for each byte of each row, the bottom row first. Bits 7 and 6 of the object's flags (+7) say how it wants to be drawn. Where they differ from the sprite's, the data is turned in place and the sprite's bit toggled, so an object that keeps facing one way costs nothing after the first time, while two objects sharing a sprite and facing opposite ways turn it back and forth every time each is drawn.

Upside down swaps whole rows end for end, the first with the last, working inwards. Mirroring (MIRROR_SPRITE_DATA, also entered by MIRROR_SPRITE) reverses the order of each row's mask and image pairs and the bits of every byte, through REVERSE_TABLE: a row's pairs are read, reversed, onto the stack and popped back in the opposite order.

Nothing in the game sets bit 7 of a record's flags. Every write to that byte was read: the records are cleared or copied from templates that hold 0 there; the rest set or clear bits 0, 1 and 5, set bit 6 from the view or the facing, take bits 6 and 7 from a table whose four bytes have bit 7 clear (FACING_LOOKS), or copy another record's flags. So the upside-down half, and the multiply it uses (HL_EQUALS_DE_X_A), never run -- which the build's sessions agree with -- and every sprite's bit 7 stays clear. Knight Lore's objects do turn upside down; Pentagram's routine is the same as this one, and Alien 8's nearly so.