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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+************
+Introduction
+************
+
+Some video capture devices can sample a subsection of a picture and
+shrink or enlarge it to an image of arbitrary size. Next, the devices
+can insert the image into larger one. Some video output devices can crop
+part of an input image, scale it up or down and insert it at an
+arbitrary scan line and horizontal offset into a video signal. We call
+these abilities cropping, scaling and composing.
+
+On a video *capture* device the source is a video signal, and the
+cropping target determine the area actually sampled. The sink is an
+image stored in a memory buffer. The composing area specifies which part
+of the buffer is actually written to by the hardware.
+
+On a video *output* device the source is an image in a memory buffer,
+and the cropping target is a part of an image to be shown on a display.
+The sink is the display or the graphics screen. The application may
+select the part of display where the image should be displayed. The size
+and position of such a window is controlled by the compose target.
+
+Rectangles for all cropping and composing targets are defined even if
+the device does supports neither cropping nor composing. Their size and
+position will be fixed in such a case. If the device does not support
+scaling then the cropping and composing rectangles have the same size.