Image Display Orientation
Display orientation refers to whether row number increases upward or downward. The orientation also affects the sense in which angles are measured. This is important to know because, as described below, images in different formats are displayed in different directions.
FITS format
images are displayed displayed with the origin (first pixel) in the
lower left corner. This is the convention used in all
scientific software applications that support FITS format. You can
also reverse the display direction to
accommodate unconventional FITS images.
Other formats,
including TIFF, BMP, and JPEG do not specify the display
orientation. Mira follows the tradition of displaying these
graphics formats with the origin (first pixel) in the upper left
corner.
Since the orientations are different between FITS and the other formats, a FITS image and, for example, a TIFF image of the same field of view, will be displayed differently. To appear with the same orientation on the screen, row number will increase upward in the FITS image but will increase downward in the TIFF image. Suppose we have an image that is 1000 x 1000 pixels both in FITS format and in TIFF format. The different orientations means that a pixel at coordinate (500,100) in the FITS image will be at coordinate (500,900) in the TIFF image.
Because of the orientation differences, if a FITS image appears upside down, then at some time in its past history it was not saved with normal orientation of a FITS format image.