Re: Accessing PixBuf pixels

["Dov Grobgeld" <dov grobgeld gmail com>]

It is almost correct except the fact that the rows of a pixbuf may be
padded. You are therefore supposed to use gdk_pixbuf_get_rowstride(pixbuf)
to get the distance between the rows.

Here is e.g. an example of vertically flip an image:

   guint8 *buf = gdk_pixbuf_get_pixels(img);
   gint w = gdk_pixbuf_get_width(img);
   gint h = gdk_pixbuf_get_height(img);
   gint rs = gdk_pixbuf_get_rowstride(img);
   gint row_idx, col_idx;

   for (row_idx=0; row_idx<h/2; row_idx++)
     {
       guint8 *ptr1 = buf+rs * row_idx;
       guint8 *ptr2 = buf+rs * (h-row_idx-1);

       for (col_idx=0; col_idx<w; col_idx++)
         {
           guint8 tmp_r = *ptr1;
           guint8 tmp_g = *(ptr1+1);
           guint8 tmp_b = *(ptr1+2);
           guint8 tmp_alpha = *(ptr1+3);
           *ptr1++ = *ptr2;
           *ptr1++ = *(ptr2+1);
           *ptr1++ = *(ptr2+2);
           *ptr1++ = *(ptr2+3);
           *ptr2++ = tmp_r;
           *ptr2++ = tmp_g;
           *ptr2++ = tmp_b;
           *ptr2++ = tmp_alpha;
         }
     }

Hope this helps.

Regads,

2008/12/18 Luka Napotnik <luka napotnik gmail com>

> Hello. I have some difficulties with manipulation of pixels in the
> GdkPixbuf data buffer. I use the following loop to iterate thought
> each pixel:
>
> -----------------------------------
> n_channels = gdk_pixbuf_get_n_channels(pixbuf);
> guchar *pixel;
> guchar *data = gdk_pixbuf_get_pixels(pixbuf);
>
> for (i = 0; i < width*height; i++) {
>    pixel = buffer + i * n_channels;
>
>    pixel[0] = 100; /* Red channel */
>    pixel[1] = 100; /* Green channel */
>    pixel[2] = 100; /* Blue channel */
>    pixel[3] = 100; /* Alpha channel */
> }
> -----------------------------------
>
> Is this the right way to handle a RGBA buffer?
>
> Greets,
> Luka
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