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base:dysp_d017 [2016-04-22 14:14] – compyxbase:dysp_d017 [2016-04-22 14:36] – compyx
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 The trick to variable Y-positioning is to stretch the first line of a sprite until we reach the Y-position where we want the sprite to display, then displaying 19 lines of the sprite (not stretching the sprite(s)), and then stretching the last line of the sprite until the loop ends. The trick to variable Y-positioning is to stretch the first line of a sprite until we reach the Y-position where we want the sprite to display, then displaying 19 lines of the sprite (not stretching the sprite(s)), and then stretching the last line of the sprite until the loop ends.
  
-Suppose we would want to display three sprites, each one one pixel lower than the next. 'A' means sprite 0, 'B' sprite 1 and so on. A[xx] means which line of the sprite we display, 00 is the empty sprite line which we use to stretch until we display the actual sprite data (00-19), line 20 is the empty line we use to complete the loop.+Suppose we would want to display three sprites, each one one pixel lower than the next. 'A' means sprite 0, 'B' sprite 1 and so on. A[xx] means which line of the sprite we display, 00 is the empty sprite line which we use to stretch until we display the actual sprite data (01-19), line 20 is the empty line we use to complete the loop.
  
 <code> <code>
 Sprites            $d017             Display Sprites            $d017             Display
 ---------------    --------------    --------------------- ---------------    --------------    ---------------------
-A00   B00   C00    %11111111  $ff+A00   B00   C00    %11111111  $ff    (nothing, we stretch the first, empty, line
 A01   B00   C00    %11111110  $fe    AAA            A01   B00   C00    %11111110  $fe    AAA           
 A02   B01   C00    %11111100  $fc    AAA  BBB A02   B01   C00    %11111100  $fc    AAA  BBB
Line 80: Line 80:
 A20   B19   C18    %11111001  $f9         BBB  CCC  A20   B19   C18    %11111001  $f9         BBB  CCC 
 A20   B20   C19    %11111011  $fb              CCC A20   B20   C19    %11111011  $fb              CCC
-A20   B20   C20    %11111111  $ff+A20   B20   C20    %11111111  $ff    (nothing, we stretch the last, empty, line
 </code> </code>
  
-=== Limitations ====+==== Generating the $d017 values ==== 
 + 
 +Generating the correct $d017 value is quite simple, but takes some cycles. 
 + 
 +  * fill $d017 table with $ff (all sprites are stretched) 
 +  * for each SPRITE: 
 +    - get Y-position of SPRITE 
 +    - mask out bits in $d017 table, starting at Y-position, for 19 lines 
 +  * next SPRITE 
 + 
 +In pseudo-code, this would look like this: 
 +<code> 
 +        ; fill $d017 table with $ff 
 +        ldx #0 
 +        lda #$ff 
 +-       sta $1000,x 
 +        inx 
 +        cpx #64 
 +        bne - 
 + 
 +        ; render sprite 0 
 +        ldy sprite0_y 
 +        ldx #0 
 +-       lda $1000,x 
 +        and #%11111110    ; don't stretch sprite 0 
 +        sta $1000,x 
 +        iny 
 +        inx 
 +        cpx #19           ; repeat 19 times 
 +        bne - 
 + 
 +        ; more code ... 
 + 
 +        ; render sprite 7 
 +        ldy sprite7_y 
 +        ldx #0 
 +-       lda $1000,x 
 +        and #%01111111    ; don't stretch sprite 7 
 +        sta $1000,x 
 +        iny 
 +        inx 
 +        cpx #19 
 +        bne - 
 +</code> 
 + 
 +==== Limitations ====
  
 This of course means we can only use 19 pixels high sprites, lines 0 and 20 are cleared so we don't see the stretched sprite data. This of course means we can only use 19 pixels high sprites, lines 0 and 20 are cleared so we don't see the stretched sprite data.
Line 90: Line 135:
  
 ===== The Code ===== ===== The Code =====
 +
 +Putting it all together, we end up with the code below. I use two sinus tables for the X-movement, giving us 344 pixels total movement (actually a little less to avoid the DMA sprite access bug in the right border) and a single sinus for the Y movement.
 +
 +Again, this code is not up to (my) demo standards: change the border color to non-black and you'll see the border-opening bugs at the start of the routine.
  
 <code 6502tasm> <code 6502tasm>
base/dysp_d017.txt · Last modified: 2016-04-26 13:52 by compyx