Pixel Coordinate Definition

The pixel coordinate is a number that identifies the location of a pixel in the image array. You must know how pixel coordinates are defined to answer the following types of questions: Where is column 42? Is column 42 actually 42 columns from the edge or is it 43 columns from the edge? is the edge at pixel 0 or at pixel 1? The FITS standard defines a protocol for this, which Mira adopts not just for FITS images, but for all image formats. As shown below, there are some differences in the way this works for different formats.


Pixel number in any direction starts at pixel number 1. This means that the first column is labeled "column 1" and the first row is labeled "row 1". The first pixel of a 2-D image is therefore labeled "pixel 1,1". As a result the maximum pixel number in any direction is equal to the number of pixels in that direction. For example, an image having 1000 columns runs from column 1 to column 1000. Traditionally, graphics file formats have started at pixel 0, which means that a 1000 column image runs from column 0 through column 999. However, this is a convention in the graphics community and not an enforced standard. Thus, to keep everything uniform inside Mira , all images use the FITS definition so that an image axis with n elements (pixels) runs from pixel 1 through pixel n, inclusive.



If a FITS format image opens upside down, open the Open dialog and check the Flip FITS Images option, then re-open the image.

Related Topics

Image Coordinate System, Sub Pixel Coordinate Definition, Image Display Orientation, FITS Image Orientation