Alien 8: Routine at C63D

C63D: Calculate Sprite Size

Used by the routine at FlagOverlappingObjects.

Works out where an object is on the screen and how much of the screen its sprite covers. The width in bytes is one more than the width of the sprite if the sprite does not start on a byte boundary, because it has to be shifted across.

Input
IX	The address of the object

Widen the sprite if it is shifted.

CalculateSpriteSize

C63D

CALL CalculatePixelCoordinates

Call CalculatePixelCoordinates.

C640

CALL GetSpriteAddress

Call GetSpriteAddress.

C643

LD A,(IX+$1A)

Fetch the pixel x position.

C646

AND %00000111

Keep only bits 0-2.

C648

LD A,(DE)

Fetch the width of the sprite in bytes.

C649

INC DE

Move on to the height.

C64A

JR Z,CalculateSpriteSize_Store

Jump to CalculateSpriteSize_Store if they were zero.

C64C

INC A

Increment A.

CalculateSpriteSize_Store

C64D

AND %00001111

Keep only bits 0-3.

C64F

LD (IX+$18),A

Store the width in bytes.

C652

LD A,(DE)

Fetch the height of the sprite in lines.

C653

LD (IX+$19),A

Store the height in lines.

C656

RET

Return.

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