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--- base:16x16_char_matrix [2015-08-31 13:46] – fixed circular grid joriszwart.nl
+++ base:16x16_char_matrix [2024-02-18 15:32] (current) – [Extentions] icepic
@@ Line 1: / Line 1: @@
+====== Basics ======
+The VIC has the ability to display not only the charsets build into the rom,
+but also custom made ones.
+For each character 8 bytes are reserved so they represent 8x8 pixels.
+The Petscii Character '@' uses the first 8 bytes,
+the 'a' the second 8 bytes etc.
+====== What is it about ======
+Contrary to the bitmap mode these 8x8 pixel blocks can be freely positioned
+on the screen. Advantageously the blocks can be ordered in a way so that
+the y coordinate equals the byte offset in the charset.
+Character '@' represents the y coordinates 0-7, character 'a' the
+coordinated 8-15 etc.
+==== A 16x16 matrix looks like this ====
+{{base:16x16_char_display_small.jpg|}}
+==== Stupid effect using 16x16 character mode ====
+{{base:16x16stupideffect.jpg|}}
+====== Extentions ======
+If you code an effect with rotation, for example a vector,
+then in a 16x16 square the edge chars are often not used at all.
+If so, it is better to order the chars in a circular grid like this:
+{{:base:circular-chars.png|}}
+This has the advantage of covering a bigger area.
+====== Code ======
+===== Code for generating 16x16 rectangular grid =====
+This code uses the ca65 assembler.
+<code>
+ichar
+       ldx #0
+initic
+       txa
+       ldy #0
+i2
+       sta scrn,y
+       clc
+       adc #icharhigh
+       iny
+       cpy #icharwidth
+       bne i2
+       lda i2+1
+       clc
+       adc #scrwidth
+       sta i2+1
+       bcc *+5
+       inc i2+2
+       inx
+       cpx #icharhigh
+       bne initic
+       rts
+</code>
+===== Code for plotting a pixel to the 16x16 grid =====
+The coordinates are passed in the X and Y registers.
+<code>
+plotpixel   lda #<charset
+            sta $fb
+            ; ptr = (x / 8) * 128
+            txa
+            lsr                     ; x / 8
+            lsr
+            lsr
+            lsr                     ; * 128 (16-bit)
+            ror $fb
+            adc #>charset
+            sta $fc
+            ; mask = 2 ^ (x & 3)
+            txa
+            and #%00000111
+            tax
+            lda ($fb),y
+            ora bitmask,x
+            sta ($fb),y
+            rts
+bitmask     .byte $80,$40,$20,$10,$08,$04,$02,$01
+</code>