Problem with gdk.Pixbuf in gtk# Mono - mono

Question

I'm creating a small drawing program in Mono gtk# and using the Cairo graphics library. I'm coding and compiling on a MacOs X system. I have a drawable object which I put into Pixbuf at a certain time and then retrieve it later into the drawable object! The idea is to take a "snapshot" of the image in the drawable and then draw on top of it.
The problem is that when I put the Pixbuf back into the drawable it looks obscure, all yellow with stripes and it looks like a portion of the image is missing.
UPDATE: I ran the program on my linux and windows machines and there it works flawlessly! So this error is only on MacOs X.
Here's the code:
// use: gmcs -pkg:gtk-sharp-2.0 -pkg:mono-cairo ttv1.cs
using Gtk;
using Cairo;
using System;
public class Draw : Window
{
DrawingArea canvas;
public Gdk.Pixbuf pixbuf;
public Draw() : base("teikniteink")
{
canvas = new DrawingArea();
canvas.ExposeEvent += canvasExposed;
DeleteEvent += delegate { Application.Quit();};
KeyPressEvent += onKey;
SetDefaultSize(400,400);
SetPosition(WindowPosition.Center);
Add(canvas);
ShowAll();
}
private void onKey(object o, KeyPressEventArgs args)
{
switch (args.Event.Key)
{
case Gdk.Key.w:
Console.WriteLine("Key Pressed {0}", args.Event.Key);
// Send to Pixbuf
pixbuf = Gdk.Pixbuf.FromDrawable(canvas.GdkWindow, Gdk.Colormap.System,0,0,0,0,400,400);
// Save to output.png
pixbuf.Save ("output.png", "png");
break;
case Gdk.Key.e:
Console.WriteLine("Key Pressed {0}", args.Event.Key);
Gdk.GC g = new Gdk.GC(canvas.GdkWindow);
// Retrive from pixbuf
canvas.GdkWindow.DrawPixbuf (g,pixbuf,0,0,0,0,-1,-1,Gdk.RgbDither.Normal,0,0);
break;
}
}
private void canvasExposed(object o, ExposeEventArgs args)
{
using (Cairo.Context ctx = Gdk.CairoHelper.Create(canvas.GdkWindow))
{
PointD start = new PointD(100,100);
PointD end = new PointD(300,300);
double width = Math.Abs(start.X - end.X);
double height = Math.Abs(start.Y - end.Y);
double xcenter = start.X + (end.X - start.X) / 2.0;
double ycenter = start.Y + (end.Y - start.Y) / 2.0;
ctx.Save();
ctx.Translate(xcenter, ycenter);
ctx.Scale(width/2.0, height/2.0);
ctx.Arc(0.0, 0.0, 1.0, 0.0, 2*Math.PI);
ctx.Restore();
ctx.Stroke();
}
}
public static void Main()
{
Application.Init();
new Draw();
Application.Run();
}
}
It would be very much appreciated if someone knows whats going on here and can point me in the right direction to fix it.

Answer 1

I triggered the same problem in this manner:
gw = gtk_widget_get_window(GTK_WIDGET(GLOBALS->mainwindow));
if(gw)
{
gdk_drawable_get_size(gw, &w, &h);
cm = gdk_drawable_get_colormap(gw);
if(cm)
{
dest = gdk_pixbuf_new(GDK_COLORSPACE_RGB, FALSE, 8, w, h);
if(dest)
{
dest2 = gdk_pixbuf_get_from_drawable(dest, gw, cm, 0, 0, 0, 0, w, h);
if(dest2)
{
succ = gdk_pixbuf_save (dest2, *GLOBALS->fileselbox_text, "png", &err, NULL);
}
}
}
}
The gdk_pixbuf_get_from_drawable() function when the source drawable is a Quartz window has issues, specifically in how _gdk_quartz_image_copy_to_image() services it. In short, 256 bit vertical strips are converted but the conversion routine assumes the pixels are 24-bit RGB rather than 32-bit RGBA. The following patch fixed the problem for me:
--- gtk+/gdk/quartz/gdkimage-quartz.c 2011-12-03 14:24:03.000000000 -0600
+++ gtk+664894/gdk/quartz/gdkimage-quartz.c 2013-10-15 18:52:24.000000000 -0500
## -150,6 +150,10 ## _gdk_quartz_image_copy_to_image (GdkDraw
data = [rep bitmapData];
size = [rep size];
+ int bpr = [rep bytesPerRow];
+ int wid = size.width;
+ int bpx = bpr/wid;
+
for (y = 0; y < size.height; y++)
{
guchar *src = data + y * [rep bytesPerRow];
## -158,12 +162,15 ## _gdk_quartz_image_copy_to_image (GdkDraw
{
gint32 pixel;
+ if (bpx == 4) // fix gdk_pixbuf_get_from_drawable "yellow stripes"
+ pixel = src[0] << 16 | src[1] << 8 | src[2];
+ else
if (image->byte_order == GDK_LSB_FIRST)
pixel = src[0] << 8 | src[1] << 16 |src[2] << 24;
else
pixel = src[0] << 16 | src[1] << 8 |src[2];
- src += 3;
+ src += bpx;
gdk_image_put_pixel (image, dest_x + x, dest_y + y, pixel);
}
I don't know if this was fixed in future versions of the GTK OSX source. I use my own for producing binaries of gtkwave as I have some necessary patches that were seemingly never integrated into the jhbuild source tree long ago.
-Tony