From 649cffa1e37d4f39b30e45dbcbc87188dc77727c Mon Sep 17 00:00:00 2001
From: Thomas Friedel <tomee6@gmail.com>
Date: Sun, 25 Mar 2018 15:42:23 +0200
Subject: [PATCH] clipping fixed

---
 canvas.go              |  5 ++++-
 paths.go               | 26 +++++++++++++++++---------
 sdlcanvas/sdlcanvas.go |  2 +-
 3 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/canvas.go b/canvas.go
index bde28bb..9ccb115 100644
--- a/canvas.go
+++ b/canvas.go
@@ -262,7 +262,10 @@ func LoadGL(glimpl GL) (err error) {
 	gli.Enable(gl_BLEND)
 	gli.BlendFunc(gl_SRC_ALPHA, gl_ONE_MINUS_SRC_ALPHA)
 	gli.Enable(gl_STENCIL_TEST)
-	gli.StencilFunc(gl_EQUAL, 1, 0x00)
+	gli.StencilMask(0xFF)
+	gli.Clear(gl_STENCIL_BUFFER_BIT)
+	gli.StencilOp(gl_KEEP, gl_KEEP, gl_KEEP)
+	gli.StencilFunc(gl_EQUAL, 0, 0xFF)
 
 	return
 }
diff --git a/paths.go b/paths.go
index fa885dc..c956f99 100644
--- a/paths.go
+++ b/paths.go
@@ -465,17 +465,12 @@ func (cv *Canvas) Clip() {
 func (cv *Canvas) clip(path []pathPoint) {
 	cv.activate()
 
-	gli.ColorMask(false, false, false, false)
-	gli.StencilFunc(gl_ALWAYS, 2, 0xFF)
-	gli.StencilOp(gl_REPLACE, gl_REPLACE, gl_REPLACE)
-	gli.StencilMask(0x02)
-	gli.Clear(gl_STENCIL_BUFFER_BIT)
-
 	var triBuf [1000]float32
 	tris := triBuf[:0]
 	tris = append(tris, 0, 0, float32(cv.fw), 0, float32(cv.fw), float32(cv.fh), 0, 0, float32(cv.fw), float32(cv.fh), 0, float32(cv.fh))
+	baseLen := len(tris)
 	tris = triangulatePath(path, tris)
-	if len(tris) > 0 {
+	if len(tris) <= baseLen {
 		return
 	}
 
@@ -488,10 +483,23 @@ func (cv *Canvas) clip(path []pathPoint) {
 	gli.Uniform2f(sr.canvasSize, float32(cv.fw), float32(cv.fh))
 	gli.EnableVertexAttribArray(sr.vertex)
 
-	gli.DrawArrays(gl_TRIANGLES, 0, 6)
-	gli.StencilFunc(gl_ALWAYS, 0, 0xFF)
+	gli.ColorMask(false, false, false, false)
+
+	gli.StencilMask(0x04)
+	gli.StencilFunc(gl_ALWAYS, 4, 0x04)
+	gli.StencilOp(gl_REPLACE, gl_REPLACE, gl_REPLACE)
 	gli.DrawArrays(gl_TRIANGLES, 6, int32(len(tris)/2-6))
 
+	gli.StencilMask(0x02)
+	gli.StencilFunc(gl_EQUAL, 0, 0x06)
+	gli.StencilOp(gl_KEEP, gl_INVERT, gl_INVERT)
+	gli.DrawArrays(gl_TRIANGLES, 0, 6)
+
+	gli.StencilMask(0x04)
+	gli.StencilFunc(gl_ALWAYS, 0, 0x04)
+	gli.StencilOp(gl_ZERO, gl_ZERO, gl_ZERO)
+	gli.DrawArrays(gl_TRIANGLES, 0, 6)
+
 	gli.DisableVertexAttribArray(sr.vertex)
 
 	gli.ColorMask(true, true, true, true)
diff --git a/sdlcanvas/sdlcanvas.go b/sdlcanvas/sdlcanvas.go
index 858da41..94d9690 100644
--- a/sdlcanvas/sdlcanvas.go
+++ b/sdlcanvas/sdlcanvas.go
@@ -38,7 +38,7 @@ func CreateCanvasWindow(w, h int, title string) (*Window, *canvas.Canvas, error)
 	sdl.GL_SetAttribute(sdl.GL_BLUE_SIZE, 8)
 	sdl.GL_SetAttribute(sdl.GL_ALPHA_SIZE, 8)
 	sdl.GL_SetAttribute(sdl.GL_DEPTH_SIZE, 0)
-	sdl.GL_SetAttribute(sdl.GL_STENCIL_SIZE, 1)
+	sdl.GL_SetAttribute(sdl.GL_STENCIL_SIZE, 8)
 	sdl.GL_SetAttribute(sdl.GL_DOUBLEBUFFER, 1)
 	sdl.GL_SetAttribute(sdl.GL_MULTISAMPLEBUFFERS, 1)
 	sdl.GL_SetAttribute(sdl.GL_MULTISAMPLESAMPLES, 4)