Routines
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B2C5: Calculate Pixel Coordinates
Used by the routines at DrawObject, CalculateSpriteSize, DropObject, CreateCreature and RunHelperSource.
Works out the pixel position of an object on the screen from its x, y and z positions. The pixel x position is x + y - 80 and the pixel y position (from the bottom of the screen) is half of y - x + 80, plus z, less 68, each with the adjustments of the object added. The pixel x position is set to zero if it would be below zero.
Input
IX The address of the object
Output
F The carry flag is set if the sprite is on the screen
Work out the pixel x position.
CalculatePixelCoordinates B2C5 LD A,(IX+$01) Fetch the x position of the object.
B2C8 ADD A,(IX+$02) Add the y position.
B2CB SUB $80 Subtract 80 to centre the room on the screen.
B2CD ADD A,(IX+$12) Add the horizontal sprite offset.
B2D0 JR C,CalculatePixelCoordinates_SetX Jump to CalculatePixelCoordinates_SetX if the result is not below zero.
B2D2 XOR A Set A to 00.
CalculatePixelCoordinates_SetX B2D3 LD (IX+$1A),A Write the screen x co-ordinate of the object.
Work out the pixel y position.
B2D6 LD A,(IX+$02) Fetch the y position of the object.
B2D9 SUB (IX+$01) Subtract the x position.
B2DC ADD A,$80 Add 80.
B2DE SRL A Halve it.
B2E0 ADD A,(IX+$03) Add the height of the object.
B2E3 SUB $68 Subtract 68 to put the floor at the bottom of the screen.
B2E5 ADD A,(IX+$13) Add the vertical sprite offset.
B2E8 LD (IX+$1B),A Write the screen y co-ordinate of the object.
B2EB CP $C0 Compare it with C0, the number of lines on the screen.
B2ED RET Return, with the carry flag set when the sprite is on the screen.
View the equivalent code in;
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