Triangle

[RobLoach]

Add a triangle draw

[RobLoach]

From Chris on discord

package primitives

import "cart/w4"

func Triangle(x1, y1, x2, y2, x3, y3 int) {
    colors := *w4.DRAW_COLORS

    *w4.DRAW_COLORS = *w4.DRAW_COLORS
    if y1 > y2 {
        y1, y2 = y2, y1
        x1, x2 = x2, x1
    }
    if y2 > y3 {
        y2, y3 = y3, y2
        x2, x3 = x3, x2
    }

    if y2 == y3 {
        fillBottomFlatTriangle(x1, y1, x2, y2, x3, y3)
    } else if y1 == y2 {
        fillTopFlatTriangle(x1, y1, x2, y2, x3, y3)
    } else {
        x4 := x1 + int((float32(y2)-float32(y1))/(float32(y3)-float32(y1))*(float32(x3)-float32(x1)))
        fillBottomFlatTriangle(x1, y1, x2, y2, x4, y2)
        fillTopFlatTriangle(x2, y2, x4, y2, x3, y3)
    }

    *w4.DRAW_COLORS /= 0xF
    w4.Line(x1, y1, x2, y2)
    w4.Line(x1, y1, x3, y3)
    w4.Line(x2, y2, x3, y3)

    *w4.DRAW_COLORS = colors
}

func fillBottomFlatTriangle(x1, y1, x2, y2, x3, y3 int) {
    invslope1 := (float32(x2) - float32(x1)) / (float32(y2) - float32(y1))
    invslope2 := (float32(x3) - float32(x1)) / (float32(y3) - float32(y1))

    curx1 := float32(x1)
    curx2 := float32(x1)

    for scanlineY := y1; scanlineY <= y2; scanlineY++ {
        w4.Line(int(curx1), scanlineY, int(curx2), scanlineY)
        curx1 += invslope1
        curx2 += invslope2
    }
}

func fillTopFlatTriangle(x1, y1, x2, y2, x3, y3 int) {
    invslope1 := (float32(x3) - float32(x1)) / (float32(y3) - float32(y1))
    invslope2 := (float32(x3) - float32(x2)) / (float32(y3) - float32(y2))

    curx1 := float32(x3)
    curx2 := float32(x3)

    for scanlineY := y3; scanlineY > y1; scanlineY-- {
        w4.Line(int(curx1), scanlineY, int(curx2), scanlineY)
        curx1 -= invslope1
        curx2 -= invslope2
    }
}