The GdkPixBuf class contains methods to scale pixbufs, to scale pixbufs and alpha blend against an existing image, and to scale pixbufs and alpha blend against a solid color or checkerboard. Alpha blending a checkerboard is a common way to show an image with an alpha channel in image-viewing and editing software.

Note that in these functions, the terms ‘alpha blending’ and ‘compositing’ are used synonymously.

Since the full-featured functions gdk_pixbuf_scale(), gdk_pixbuf_composite(), and gdk_pixbuf_composite_color() are rather complex to use and have many arguments, two simple convenience functions are provided, gdk_pixbuf_scale_simple() and gdk_pixbuf_composite_color_simple() which create a new pixbuf of a given size, scale an original image to fit, and then return it.

If the destination pixbuf was created from a read only source, these operations will force a copy into a mutable buffer.

Scaling and alpha blending functions take advantage of MMX hardware acceleration on systems where MMX is supported. If GdkPixbuf is built with the Sun mediaLib library, these functions are instead accelerated using mediaLib, which provides hardware acceleration on Intel, AMD, and Sparc chipsets. If desired, mediaLib support can be turned off by setting the GDK_DISABLE_MEDIALIB environment variable.

The alpha blending function used is:

Cd = Cs·As + Cd(1-As)

where Cd is the destination pixel color, Cs is the source pixel color, and As is the source pixel alpha.

NOTE: It is recommended to use Cairo for scaling and compositing, by using the contents of a GdkPixbuf pixel buffer as the data for a Cairo image surface.