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diff --git a/docs/scaling-compositing.md b/docs/scaling-compositing.md new file mode 100644 index 000000000..3abc75e73 --- /dev/null +++ b/docs/scaling-compositing.md @@ -0,0 +1,44 @@ +Title: Scaling and compositing + +---- + +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 [method@GdkPixbuf.Pixbuf.scale], +[method@GdkPixbuf.Pixbuf.composite], and [`method@GdkPixbuf.Pixbuf.composite_color`] +are rather complex to use and have many arguments, two simple +convenience functions are provided, [`method@GdkPixbuf.Pixbuf.scale_simple`] +and [`method@GdkPixbuf.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][cairo] for scaling and +compositing, by using the contents of a `GdkPixbuf` pixel buffer as the +data for a Cairo image surface. + +[cairo]: https://www.cairographics.org |