Add some code structure for future triangle rendering

wip-hugo/routines/line/line.inc

@@ -1,6 +1,7 @@
 ;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
 
 ;;Not values but register position in memory
+        ;; F4 - EB
         X_end = $EF
         Y_end = $F0
         dx = $F3

wip-hugo/routines/triangle/triangle.inc

@@ -0,0 +1,17 @@
+;;The triangles 3 corner-positions, they may be in any given order.
+A_X = $EA
+A_Y = $E9
+B_X = $E8
+B_Y = $E7
+C_X = $E6
+C_Y = $E5
+;; Differense in height (after these points have been sorted look in triangle.s for more detail)
+AB = $E4
+BC = $E3
+
+;; These come from mem.inc
+;; Takes up FF - F5
+X_pos = $FC
+Y_pos = $FB
+byte_to_paint = $FE ; Byte with one 1 that corisponds to a pixel.
+btp_mem_pos = $F9   ; 16-bit value (uses FA), byte to paint memory position

wip-hugo/routines/triangle/triangle.s

@@ -0,0 +1,80 @@
+.proc triangle
+;; Subroutine for drawing a filld triangle.
+;; It takes in values from A_X, A_Y, B_X, B_Y C_X, C_Y, see triangle.inc
+        .include "triangle.inc"
+
+;; Algorithm
+;; (i) For height n it steps through Bresenham's line algorithm to know where the triangle starts and ends at height n.
+;; (ii) It draws all pixel in this given range.
+;; (iii) It change if the algorithm has hit a corner and need to change line!
+;; Implementation
+;; In the implementation the triangle split into 2 souch that (iii) can be redused to: check if the triangle has ended.
+;; Each side of the sub-triangle can have 4 posiboul states (look at how line is implemented)
+
+;;Lets first sort the edges such as
+;; (i) A is at the highest point
+;; (ii) B is at the middle point
+;; (iii) C is at the lowest point
+;; Lets also calculate the height-differense for the new points:
+;; * AB for differense betwean A and B
+;; * BC for differense betwean B and C
+
+;;AB
+        SEC
+        LDA B_Y
+        SBC A_Y
+        STA AB
+        BCC AB_overflow_end
+;; A_Y > B_Y
+AB_overflow:
+        EOR #$ff ; Fix byte underflow
+        ;; swop A and B
+        LDX B_Y
+        LDY A_Y
+        STX A_Y
+        STY B_Y
+        LDX B_X
+        LDY A_X
+        STX A_X
+        STY B_X
+AB_overflow_end:
+;;BC
+SEC
+LDA C_Y
+SBC B_Y
+STA BC
+BCC BC_overflow_end
+;; B_Y > C_Y
+BC_overflow:
+        EOR #$ff ; Fix byte underflow
+        ;; swop A and B
+        LDX C_Y
+        LDY B_Y
+        STX B_Y
+        STY C_Y
+        LDX C_X
+        LDY B_X
+        STX B_X
+        STY C_X
+        ;; If this is the case A and B may be in the wrong order. Lets redo stuff from the start
+        JMP triangle
+BC_overflow_end:
+
+draw_upper_triangle:
+# This triangle has 10 posibol cases, see table bellow
+# ----------------------------------------------------
+# | name left_side               right_side          |
+# ----------------------------------------------------
+# | RR   right-sloaping-steep    right-sloaping-steep|
+# | rR   right-sloaping          right-sloaping-steep|
+# | lR   left-sloaping           right-sloaping-steep|
+# | LR   left-sloaping-steep     right-sloaping-steep|
+# | rr   right-sloaping          right-sloaping      |
+# | lr   left-sloaping           right-sloaping      |
+# | Lr   left-sloaping-steep     right-sloaping      |
+# | ll   left-sloaping           left-sloaping       |
+# | Ll   left-sloaping-sleep     left-sloaping       |
+# | LL   left-sloaping-sleep     left-sloaping-sleep |
+# ----------------------------------------------------
+RTS
+.endproc

wip-hugo/routines/triangle/triangle_test.s

@@ -0,0 +1,38 @@
+.scope triangle_test
+;; Test program that tries to draw a triangle
+        .include "triangle.inc"
+        LDA #$33
+        STA A_X
+        STA A_Y
+        LDA #$55
+        STA C_X
+        STA C_Y
+        LDA #$77
+        STA B_X
+        LDA #$20
+        STA B_Y
+
+;       jmp skipp_edges ; Skip drawing the edges
+;;Draw the edges (only for reference)
+edges:
+        X_end = $EF
+        Y_end = $F0
+        ;X_pos = $FC
+        ;Y_pos = $FB
+;;; A -- B
+        Mov_16 X_pos, Y_pos, A_X, A_Y
+        Mov_16 X_end, Y_end, B_X, B_Y
+        jsr line
+;;; A -- C
+        Mov_16 X_pos, Y_pos, A_X, A_Y
+        Mov_16 X_end, Y_end, C_X, C_Y
+        jsr line
+;;; B -- C
+        Mov_16 X_pos, Y_pos, C_X, C_Y
+        Mov_16 X_end, Y_end, B_X, B_Y
+        jsr line
+skip_edges:
+;; lets draw the example triangle
+jsr triangle
+jmp exit
+.endscope

wip-hugo/source.s

@@ -6,11 +6,13 @@
 .include "routines/memory/mem.inc"
 ;;Code to run
 .include "STARTUP.s"
-.include "routines/line/line_test.s"
+;.include "routines/line/line_test.s"
 ;.include "routines/memory/memcpy_test.s"
+.include "routines/triangle/triangle_test.s"
 exit:
 jmp exit
 .include "routines/line/line.s"
+.include "routines/triangle/triangle.s"
 .include "routines/memory/pixel_draw.s"
 .include "routines/memory/memset.s"
 .include "routines/memory/memcpy.s"