arc: Re-add arc support

1 files changed, 39 insertions, 0 deletions

diff --git a/virtual-programs/display/arc.folk b/virtual-programs/display/arc.folk
new file mode 100644
index 00000000..e2412bf0
--- /dev/null
+++ b/virtual-programs/display/arc.folk
@@ -0,0 +1,39 @@
+# Example:
+#   When $this has region /r/ {
+#     lassign [region centroid $r] x y
+#     Wish to draw an arc with x $x y $y start 0 arclen 1 thickness 3 radius 100 color green
+#   }
+
+Wish the GPU compiles pipeline "arc" {{vec2 center float start float arclen float radius float thickness vec4 color} {
+    float r = radius + thickness;
+    vec2 vertices[4] = vec2[4](
+         center - r,
+         vec2(center.x + r, center.y - r),
+         vec2(center.x - r, center.y + r),
+         center + r
+    );
+    return vertices[gl_VertexIndex];
+} {
+    #define M_TWO_PI 6.283185307179586
+    start = clamp(start, 0, M_TWO_PI);
+    arclen = clamp(arclen, 0, M_TWO_PI);
+
+    float dist = length(gl_FragCoord.xy - center) - radius;
+    float angle = atan(-(gl_FragCoord.y - center.y), gl_FragCoord.x - center.x);
+
+    // Shift angle from [-pi, pi) to [0, 2*pi]
+    angle = (angle < 0) ? (angle + M_TWO_PI) : angle;
+    float end = start + arclen;
+
+    return ((dist < thickness && dist > 0.0) &&
+            ((end < M_TWO_PI && angle > start && angle < end) || 
+            (end >= M_TWO_PI && (angle > start || angle < end-M_TWO_PI)))) ? color : vec4(0, 0, 0, 0);
+
+}}
+
+When /someone/ wishes to draw an arc with /...options/ {
+    dict with options {
+        Wish the GPU draws pipeline "arc" with arguments \
+            [list [list $x $y] $start $arclen $radius $thickness [getColor $color]]
+    }
+}