# Pixel Coordinate ConventionsΒΆ

In Photutils, integer pixel coordinates fall at the center of pixels and
they are 0-indexed, matching the Python 0-based indexing. That means the
first pixel is considered pixel `0`

, but pixel coordinate `0`

is
the *center* of that pixel. Hence, the first pixel spans pixel values
`-0.5`

to `0.5`

.

For a 2-dimensional array, `(x, y) = (0, 0)`

corresponds to
the *center* of the bottom, leftmost array element. That means
the first pixel spans the `x`

and `y`

pixel values from
`-0.5`

to `0.5`

. Note that this differs from the IRAF, FITS
WCS, ds9, and
SourceExtractor conventions, in which the center of the bottom,
leftmost array element is `(x, y) = (1, 1)`

.

Following Python indexing, the `x`

(column) coordinate corresponds to
the second (fast) array index and the `y`

(row) coordinate corresponds
to the first (slow) index. `image[y, x]`

gives the value at pixel
coordinates `(x, y)`

.