Re: MIT-SHM example

["Rose Cumming" <rose_alice_cumming hotmail com>]

Paul, what do you say about this?

From: Olexiy Avramchenko <ath beast stu cn ua>
To: Patrice St-Gelais <patrice st-gelais oricom ca>
Cc: "gtk-app-devel-list gnome org" <gtk-app-devel-list gnome org>
Subject: Re: Hints needed about drawing lines and arcs
Date: Wed, 06 Nov 2002 12:25:00 +0200

Patrice St-Gelais wrote:

> Hi all,
>
> I am working on an application which needs to process16 bit greyscale
> images.
>
> The image is stored in a kind of raw buffer (actually an unsigned short
> int*).  It is displayed by drawing it in a GdkPixmap* with
> gdk_draw_gray_image after cutting the depth to 8 bits, then emitting an
> "expose_event" for the gtk_drawing_area.  This event launches a callback
> which transfers the pixmap to the drawing area gdk_window with
> gdk_draw_pixmap.
>
> I want to draw lines and arcs of variable thickness in the 16 bits
> buffer, without reinventing the wheel.
>
> GDK offers some drawing primitives, but as I understand them, they allow
> drawing in "drawables" like GDK pixmaps or windows, which are structures
> on the X Server side.  I cannot easily, or at least efficiently,
> retrieve the result of the drawing in the client side with the intent to
> combine it with other client side images or to save it, for instance, in
> a PNG file - or can I?  Furthermore, if I could, it wouldn't be a 16
> bits image, or am I wrong?
>
> Any advice on using GTK/GDK for this, or on any alternate solution,
> would be welcomed.

I'm using MIT-SHM X extension for this purpose. As result I can use both GDK
drawing primitives and raw memory access.

You have to:
1) Create a shared image *XShmCreateImage()*. This gives you raw access.
2) Get SHM pixmap of this image *XShmCreatePixmap()*. This gives you X
drawable.
3) Use GDK function *gdk_pixmap_foreign_new()* to obtain usual GDK pixmap.

*Note* this will only work when you run your app on the same box with
Xserver.

You can read about MIT-SHM at:
http://ftp.xfree86.org/pub/XFree86/4.2.1/doc/mit-shm.TXT

Olexiy

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