The Co-ordinate Frames Associated with a Picture

Each picture in the graphics database has associated with it several co-ordinate Frames. Some of these describe positions within the displayed data array, and others describe the corresponding positions on the graphics device. Each Frame is referred to by a Domain name (see here for more information about co-ordinate Frames and Domains).

The following Domains may be used to specify positions within any picture:

GRAPHICS

-- This gives positions in millimetres from the bottom left corner of the graphics device.

BASEPIC

-- This gives positions within a normalised co-ordinate system spanning the BASE picture (i.e. the entire graphics device). The units on both axes are set so that either the width or the height of the graphics device, whichever is smaller, is set to 1.0 (put another way, a unit square would fit in the picture and span the shorter axis). The bottom-left corner of the graphics device is (0, 0).

NDC

-- Normalised Device Co-ordinates, which are similar to BASEPIC but specifies positions in a normalised co-ordinate system in which the bottom-left corner has co-ordinates (0, 0) and the top-right corner has co-ordinates (1, 1). Thus, for a non-square device the scales on the two axes will be different.

CURPIC

-- similar to BASEPIC except that it covers only the specified picture. Thus, the bottom-left corner of each picture is at (0, 0) and the shorter dimension of each picture has length 1.0.

DATA pictures have additional co-ordinate Frames  inherited from the WCS information associated with the displayed data. The details of the available Frames will depend on the application that created the picture and the nature of the data. For instance, if an image is displayed in which the current Frame is a SKY Frame (calibrated in RA/DEC for instance), then the Domains GRID, PIXEL, AXIS and SKY will also be available.

Pictures created by older applications that do not yet support WCS information will not have all of these Frames. They will still have GRAPHICS, BASEPIC, NDC and CURPIC Frames, but the only additional Frames will be:

AGI_WORLD

- This corresponds to AGI `world' co-ordinates.

AGI_DATA

- This corresponds to AGI `data' co-ordinates.

The interpretation of both these Frames will depend on the application that created the picture, and should be described in the associated documentation. Note, the AGI_DATA Frame may not always be present. This again depends on the application.