IDL Coordinate Systems
You can specify coordinates to IDL in one of the following coordinate systems:
DATA Coordinates
This coordinate system is established by the most recent PLOT, CONTOUR, or SURFACE procedure. This system usually spans the plot window, the area bounded by the plot axes, with a range identical to the range of the plotted data. The system can have two or three dimensions and can be linear, logarithmic, or semi-logarithmic. The mechanisms of converting from one coordinate system to another are described below.
DEVICE Coordinates
This coordinate system is the physical coordinate system of the selected plotting device. Device coordinates are integers, ranging from (0, 0) at the bottom-left corner to (Vx –1, Vy –1) at the upper-right corner. Vx and Vy are the number of columns and rows addressed by the device. These numbers are stored in the system variable !D as !D.X_SIZE and !D.Y_SIZE. In a widget base, device coordinates are measures from the upper-left corner
NORMAL Coordinates
The normalized coordinate system ranges from zero (0) to one (1) over each of the three axes.
Almost all of the IDL graphics procedures accept parameters in any of these coordinate systems. Most procedures use the data coordinate system by default. Routines beginning with the letters TV are notable exceptions. They use device coordinates by default. You can explicitly specify the coordinate system to be used by including one of the keyword parameters /DATA, /DEVICE, or /NORMAL in the call.
Understanding Windows and Related Device Coordinates
Images are displayed within a window (Direct Graphics) or within an instance of a window object (Object Graphics). In Direct Graphics, the WINDOW procedure is used to initialize the coordinates system for the image display. In Object Graphics, the IDLgrWindow, IDLgrView, and IDLgrModel objects are used to initialize the coordinate system for the image display.
A coordinate system determines how and where the image appears within the window. You can specify coordinates to IDL using one of the following coordinate systems:
- Data Coordinates — This system usually spans the window with a range identical to the range of the data. The system can have two or three dimensions and can be linear, logarithmic, or semi-logarithmic.
- Device Coordinates — This coordinate system is the physical coordinate system of the selected device. Device coordinates are integers, ranging from (0, 0) at the bottom-left corner to (Vx –1, Vy –1) at the upper-right corner of the display. Vx and Vy are the number of columns and rows of the device (a display window for example).
- Normal Coordinates — The normalized coordinate system ranges from zero to one over columns and rows of the device.
Note
For images, the data coordinates are the same as the device coordinates. The device coordinates of an image are directly related to the pixel locations within an image. Unless otherwise specified, IDL draws each image pixel per each device pixel.
