Re: A request for future gdk_scale_pixbuf...

[Havoc Pennington <hp redhat com>]

Hi,

Have a look at the implementation of scale_simple:

GdkPixbuf *
gdk_pixbuf_scale_simple (const GdkPixbuf *src,
                         int              dest_width,
                         int              dest_height,
                         GdkInterpType    interp_type)
{
  GdkPixbuf *dest;

  g_return_val_if_fail (src != NULL, NULL);
  g_return_val_if_fail (dest_width > 0, NULL);
  g_return_val_if_fail (dest_height > 0, NULL);

  dest = gdk_pixbuf_new (GDK_COLORSPACE_RGB, src->has_alpha, 8, dest_width, dest_height);
  if (!dest)
    return NULL;

  gdk_pixbuf_scale (src, dest,  0, 0, dest_width, dest_height, 0, 0,
                    (double) dest_width / src->width,
                    (double) dest_height / src->height,
                    interp_type);

  return dest;
}

This is the function people would normally actually choose to use. The
others are sort of an exercise in overgenericity, though they may be
useful from time to time.
 
> b) the API documentation, as written, implies that the function
> actually reallocates the source pixbuf and changes the pixels inside
> it. I am guessing that it really just reads the source pixbuf line by
> line and performs an implicit transformation before blitting the
> result to the destination pixbuf, but that's not what the
> documentation says :)

I can see how the docs are confusing, I'll clarify them.
 
> I was hoping that it worked like the drawing code in (say) OS/2, where
> you specify a source rect within the source bitmap and a destination
> rect within the destination bitmap. That way, everything is always an
> integer.

scale_simple() only shows you integers. (It uses floats internally,
but if things ever come out wrong just file a bug report and we'll fix
it - they shouldn't come out wrong.)

In GTK 2 you can create a "subpixbuf" of the source image, in order to
scale only a subregion of it. gdk-pixbuf 0.x standalone lib doesn't
have this though, so you have to use the full gdk_pixbuf_scale() to
scale only a subregion.

> I guess it's possible to wrap the function to get the behavior I
> desire, but I'm still working with trying to understand how it works :/

scale_simple() source code should make it fairly clear how to do the
simple thing, I hope.

Havoc