gdk_pixbuf_add_alpha ()

GdkPixbuf*  gdk_pixbuf_add_alpha            (const GdkPixbuf *pixbuf,
                                             gboolean substitute_color,
                                             guchar r,
                                             guchar g,
                                             guchar b);

Takes an existing pixbuf and adds an alpha channel to it. If the existing pixbuf already had an alpha channel, the channel values are copied from the original; otherwise, the alpha channel is initialized to 255 (full opacity).

If substitute_color is TRUE, then the color specified by (r, g, b) will be assigned zero opacity. That is, if you pass (255, 255, 255) for the substitute color, all white pixels will become fully transparent.

gdk_pixbuf_copy_area ()

void        gdk_pixbuf_copy_area            (const GdkPixbuf *src_pixbuf,
                                             int src_x,
                                             int src_y,
                                             int width,
                                             int height,
                                             GdkPixbuf *dest_pixbuf,
                                             int dest_x,
                                             int dest_y);

Copies a rectangular area from src_pixbuf to dest_pixbuf. Conversion of pixbuf formats is done automatically.

gdk_pixbuf_saturate_and_pixelate ()

void        gdk_pixbuf_saturate_and_pixelate
                                            (const GdkPixbuf *src,
                                             GdkPixbuf *dest,
                                             gfloat saturation,
                                             gboolean pixelate);

Modifies saturation and optionally pixelates src, placing the result in dest. src and dest may be the same pixbuf with no ill effects. If saturation is 1.0 then saturation is not changed. If it's less than 1.0, saturation is reduced (the image is darkened); if greater than 1.0, saturation is increased (the image is brightened). If pixelate is TRUE, then pixels are faded in a checkerboard pattern to create a pixelated image. src and dest must have the same image format, size, and rowstride.

gdk_pixbuf_fill ()

void        gdk_pixbuf_fill                 (GdkPixbuf *pixbuf,
                                             guint32 pixel);

Clears a pixbuf to the given RGBA value, converting the RGBA value into the pixbuf's pixel format. The alpha will be ignored if the pixbuf doesn't have an alpha channel.