Re: Drawing higher depth images

[Olexiy Avramchenko <olexiy ctech cn ua>]

David Necas (Yeti) wrote:
> Is it possible to use Gdk drawing functions (gdk_draw_arc(),
> etc.) to draw images of higher depth than system and save
> them?
>
> I can save GdkPixbufs.  If there were drawing primitives in
> gdk-pixbuf, it would be everything I need.  However I can
> draw only on GdkDrawables.
>
> To use gdk_pixbuf_get_from_drawable() I need a colormap.
>
> To create a colormap I need a visual.
>
> And I can get only visuals with depths returned by
> gdk_query_depths(), that is system visual.  If it's 16 or
> even 8 bpp, I have to use that.
>
> Or is there any other method to draw true color images?
> (Except using other libraries like Cairo, which currently is
> no option for me).
There's one trick:
1. Create a pixmap with depth of 32.
2. Create gc for the pixmap.
3. Set the 32-bit colour you want with gdk_gc_set_foreground() 
(0xAABBGGRR for the little-endian boxes, 0xRRGGBBAA for the big-endian 
ones).
4. Use any some GDK drawing primitives to draw on pixmap.
5. Create image from pixmap.
6. Create pixbuf with alpha-channel from image.
7. Do whatever you want with pixbuf.

I think (didn't check, however, with a new GDK) that you'll not be able 
to draw a Pango layout to such pixmap with gdk_draw_layout(). The 
possible solution is to use X11 calls (they should work ok) or to render 
layout manually.

If you want to directly draw to such pixbuf with GDK calls, you can 
create shared pixmap with XShm extension. In this case steps will be 
like this:
1. Create shared pixmap width depth of 32 via MIT-SHM extension.
2. Create foreign pixmap to make GDK happy.
3. == 2 from the steps above
4. == 3 ^^^
5. == 4 ^^^
6. Create pixbuf from the data, stored in shared memory.
7. Do whatever you want with both pixbuf and pixmap.
This will work in the local case (application and X server run on the 
same box).

PS: you also can try to setup another X server with framebuffer in the 
system memory, but this is somewhat exotic.

PPS: as John said you can try libart (if this is more suitable option 
than cairo), but with GnomeCanvas. Canvas can be easily hacked (guys, 
please dont flame on the word *easily*) in the way to redirect output to 
the pixbuf. The main advantage in this case is that you can draw text in 
the modern way.

PPPS: sorry for the late reply :)

--- the sample
#include <gdk/gdk.h>

#define WIDTH   128
#define HEIGHT  128

int
main(int argc, char **argv)
{
        GdkGC           *gc;
        GdkColor        colour;
        GdkPixmap       *pixmap;
        GdkImage        *image;
        GdkPixbuf       *pixbuf;

        gdk_init(&argc, &argv);

        pixmap = gdk_pixmap_new(NULL, WIDTH,HEIGHT, 32);
        g_return_val_if_fail(pixmap != NULL, 1);

        gc = gdk_gc_new(pixmap);
        g_return_val_if_fail(gc != NULL, 1);

        colour.pixel = 0;
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_rectangle(pixmap, gc, TRUE, 0,0, -1,-1);

        colour.pixel = 0xff000000;	/* black */
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_line(pixmap, gc, 0,0, WIDTH/2,HEIGHT/2);

        colour.pixel = 0xffff0000;	/* blue */
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_line(pixmap, gc, WIDTH,0, WIDTH/2,HEIGHT/2);

        colour.pixel = 0xff00ff00;	/* green */
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_line(pixmap, gc, 0,HEIGHT, WIDTH/2,HEIGHT/2);

        colour.pixel = 0xff0000ff;	/* red */
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_line(pixmap, gc, WIDTH,HEIGHT, WIDTH/2,HEIGHT/2);

        colour.pixel = 0xff80CCff;
        gdk_gc_set_foreground(gc, &colour);
        gdk_draw_arc(pixmap, gc, FALSE, 10,10, WIDTH-20,HEIGHT-20, 
0,360*64);

        image = gdk_image_get(pixmap, 0,0, WIDTH,HEIGHT);
        g_return_val_if_fail(image != NULL, 1);

        pixbuf = gdk_pixbuf_new_from_data(
                image->mem,
                GDK_COLORSPACE_RGB,
                TRUE,
                8,
                image->width, image->height,
                image->bpl,
                NULL, NULL
        );
        g_return_val_if_fail(pixbuf != NULL, 1);

        gdk_pixbuf_save(pixbuf, "test.png", "png", NULL, NULL);

        return 0;
}
the sample ---

	Olexiy