diff --git a/c++/contourgl/clcontext.cpp b/c++/contourgl/clcontext.cpp
index 6183921..d5a60a8 100644
--- a/c++/contourgl/clcontext.cpp
+++ b/c++/contourgl/clcontext.cpp
@@ -80,6 +80,7 @@ ClContext::ClContext(): err(), context(), queue() {
 	queue = clCreateCommandQueue(context, devices.front(), CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE, NULL);
 	assert(queue);
 
+	//hello();
 }
 
 ClContext::~ClContext() {
diff --git a/c++/contourgl/clrender.cpp b/c++/contourgl/clrender.cpp
index 9bcf1eb..447aab2 100644
--- a/c++/contourgl/clrender.cpp
+++ b/c++/contourgl/clrender.cpp
@@ -18,6 +18,7 @@
 #include <cassert>
 
 #include <algorithm>
+#include <iostream>
 
 #include "clrender.h"
 #include "measure.h"
@@ -102,10 +103,15 @@ void ClRender::send_surface(Surface *surface) {
 		surface_format.image_channel_order = CL_RGBA;
 		surface_format.image_channel_data_type = CL_FLOAT;
 
-		surface_image = clCreateImage2D(
+		cl_image_desc surface_desc = { };
+		surface_desc.image_type = CL_MEM_OBJECT_IMAGE2D;
+		surface_desc.image_width = surface->width;
+		surface_desc.image_height = surface->height;
+
+		surface_image = clCreateImage(
 			cl.context, CL_MEM_READ_WRITE,
-			&surface_format, surface->width, surface->height,
-			0, NULL, NULL );
+			&surface_format, &surface_desc,
+			NULL, NULL );
 		assert(surface_image);
 
 		size_t origin[3] = { };
@@ -141,7 +147,9 @@ Surface* ClRender::receive_surface() {
 		cl.err |= clEnqueueReadImage(
 			cl.queue, surface_image, CL_FALSE,
 			origin, region, 0, 0, surface->data,
-			prev_event ? 1 : 0, &prev_event, NULL );
+			prev_event ? 1 : 0,
+			prev_event ? &prev_event : NULL,
+			NULL );
 		assert(!cl.err);
 
 		cl.err |= clFinish(cl.queue);
@@ -212,7 +220,7 @@ void ClRender::path(int start, int count, const Color &color, bool invert, bool 
 		&scount,
 		NULL,
 		prev_event ? 1 : 0,
-		&prev_event,
+		prev_event ? &prev_event : NULL,
 		&path_event );
 	assert(!cl.err);
 
diff --git a/c++/contourgl/polyspan.cpp b/c++/contourgl/polyspan.cpp
index 2e39124..1d21dc2 100644
--- a/c++/contourgl/polyspan.cpp
+++ b/c++/contourgl/polyspan.cpp
@@ -755,7 +755,7 @@ void Polyspan::draw_line(Real x1, Real y1, Real x2, Real y2) {
 	}
 }
 
-Real Polyspan::extract_alpha(Real area, bool evenodd) const {
+Real Polyspan::extract_alpha(Real area, bool evenodd) {
 	if (area < 0)
 		area = -area;
 
diff --git a/c++/contourgl/polyspan.h b/c++/contourgl/polyspan.h
index bb19648..aa4d86b 100644
--- a/c++/contourgl/polyspan.h
+++ b/c++/contourgl/polyspan.h
@@ -140,7 +140,7 @@ public:
 	void draw_scanline(int y, Real x1, Real y1, Real x2, Real y2);
 	void draw_line(Real x1, Real y1, Real x2, Real y2);
 
-	Real extract_alpha(Real area, bool evenodd) const;
+	static Real extract_alpha(Real area, bool evenodd);
 };
 
 #endif