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fb.go

@@ -3,6 +3,7 @@ package main
 import (
 	"encoding/binary"
 	"fmt"
+	"math"
 	"path/filepath"
 	"unsafe"
 
@@ -449,6 +450,31 @@ func (fb *framebuffer) rect(x0, y0, w, h int, c rgb) {
 	}
 }
 
+// Distance to the rectangle the corner radius sweeps around, which is zero
+// across the whole flat middle and grows only near a corner. Taking coverage
+// from that rather than from a plain inside test keeps the curves smooth
+// instead of stepped.
+func (fb *framebuffer) roundRect(x0, y0, w, h, r int, c rgb) {
+	// A radius past half the shorter side has no meaning and would put the
+	// swept rectangle inside out, which matters while something is growing from
+	// nothing.
+	r = min(r, min(w, h)/2)
+	ix0, iy0 := float64(x0+r), float64(y0+r)
+	ix1, iy1 := float64(x0+w-1-r), float64(y0+h-1-r)
+	for y := y0; y < y0+h; y++ {
+		for x := x0; x < x0+w; x++ {
+			fx, fy := float64(x), float64(y)
+			dx := math.Max(math.Max(ix0-fx, fx-ix1), 0)
+			dy := math.Max(math.Max(iy0-fy, fy-iy1), 0)
+			cov := float64(r) - math.Sqrt(dx*dx+dy*dy) + 0.5
+			if cov <= 0 {
+				continue
+			}
+			fb.blend(x, y, c, uint8(math.Min(cov, 1)*255))
+		}
+	}
+}
+
 // Blends src over the existing pixel, with cov as 0-255 coverage.
 func (fb *framebuffer) blend(x, y int, c rgb, cov uint8) {
 	if x < 0 || y < 0 || x >= fb.w || y >= fb.h || cov == 0 {