xgb-draw: point interpolation

prototypes/xgb-draw.go

@@ -1,7 +1,6 @@
 // Network-friendly drawing application based on XRender.
 //
 // TODO
-//  - interpolate motion between points
 //  - use double buffering to remove flicker
 //    (more pronounced over X11 forwarding)
 //  - maybe keep the pixmap as large as the window
@@ -183,7 +182,7 @@ func main() {
 	//  - http://ssp.impulsetrain.com/porterduff.html
 	//  - https://keithp.com/~keithp/talks/renderproblems/renderproblems/render-title.html
 	//  - https://keithp.com/~keithp/talks/cairo2003.pdf
-	drawLineTo := func(x, y int16) {
+	drawPointAt := func(x, y int16) {
 		_ = render.Composite(X, render.PictOpConjointOver,
 			brushid, render.PictureNone, pixmaskpictid,
 			0, 0, 0, 0, x-brushRadius, y-brushRadius,
@@ -203,6 +202,55 @@ func main() {
 			pixWidth, pixHeight)
 	}
 
+	// Integer version of Bresenham's line drawing algorithm
+	drawLine := func(x0, y0, x1, y1 int16) {
+		dx, dy := x1-x0, y1-y0
+		if dx < 0 {
+			dx = -dx
+		}
+		if dy < 0 {
+			dy = -dy
+		}
+
+		steep := dx < dy
+		if steep {
+			// Flip the coordinate system on input
+			x0, y0 = y0, x0
+			x1, y1 = y1, x1
+			dx, dy = dy, dx
+		}
+
+		var stepX, stepY int16 = 1, 1
+		if x0 > x1 {
+			stepX = -1
+		}
+		if y0 > y1 {
+			stepY = -1
+		}
+
+		dpr := dy * 2
+		delta := dpr - dx
+		dpru := delta - dx
+
+		for ; dx > 0; dx-- {
+			// Unflip the coordinate system on output
+			if steep {
+				drawPointAt(y0, x0)
+			} else {
+				drawPointAt(x0, y0)
+			}
+
+			x0 += stepX
+			if delta > 0 {
+				y0 += stepY
+				delta += dpru
+			} else {
+				delta += dpr
+			}
+		}
+	}
+
+	var startX, startY int16 = 0, 0
 	drawing := false
 	for {
 		ev, xerr := X.WaitForEvent()
@@ -242,12 +290,14 @@ func main() {
 					[]xproto.Rectangle{{Width: pixWidth, Height: pixHeight}})
 
 				drawing = true
-				drawLineTo(e.EventX, e.EventY)
+				drawPointAt(e.EventX, e.EventY)
+				startX, startY = e.EventX, e.EventY
 			}
 
 		case xproto.MotionNotifyEvent:
 			if drawing {
-				drawLineTo(e.EventX, e.EventY)
+				drawLine(startX, startY, e.EventX, e.EventY)
+				startX, startY = e.EventX, e.EventY
 			}
 
 		case xproto.ButtonReleaseEvent: