Reading Specific Parts of Sprites

[KevKelley]

Just wondering out loud here. I had played around with pointers to reuse things like the score sprites and had wondered - is it possible to read specific parts of the sprite, like if a pixel is on or off?

[+Karl G]

I assume you mean with bB? The answer would be yes, but you would have to access it via assembly as a pointer. Here's examples that should work with the standard kernel or the multisprite kernel to load either the first byte or the last byte of the player 0 sprite into temp1, and from there you can check the bits manually in bB as normal.

 

Since sprites are stored upside down, getting the first (top) byte of the sprite is done by getting the last one stored:

    asm
    ; load player 0 height into Y register
    ldy player0height
    ; decrement Y by 1
    dey				
    ; load top of player 0 sprite into A register
    lda (player0pointer),y	
    ; store A register into temp1 variable
    sta temp1				
end

 

Getting the last (bottom) byte of the player0 sprite is nearly the same, but the Y register is set to 0:

    asm
    ; load 0 into Y register
    ldy #0
    ; load bottom of player 0 sprite into A register
    lda (player0pointer),y	
    ; store A register into temp1 variable
    sta temp1				
end

 

Let me know if that answers your question.

[KevKelley]

I guess more specifically I was wondering if it were possible to use a sprite for multiple variables, like have a sprite that is 20 pixels high but set player height to 10. So perhaps you look at the 11th row to read that value as a number.

 

I had no real immediate use for it but was wondering if it was something possible. 

[+Gemintronic]

You could probably do something based on Al_Nafurs technique for sharing sprites.  The sprite data is just a plain old DATA statement at that point.  You could probably then treat the DATA statement as a static array and bobs your uncle.

 

..but, in truth I still barely understand how it works or sprite data pointers in general.