Conventions

In the Julia Astro ecosystem, images follow the following conventions.

Axes

For simple 2D images, the first axis is the horizontal axis and the second axis is the vertical axis. So images are indexed by img[xi, yi].

The origin is at the bottom left of the image, so img[1,1] refers to the bottom left corner as does img[begin,begin]. img[end,end] is the top right corner, img[begin,end] is the top left, etc.

Note that this is transposed and flipped from how Julia prints arrays at the REPL.

Pixels

This library considers the exact location of img[1,1] to be the center of the pixel in the bottom left corner. This means that plot limits should have the 1 tick slightly away from the left/bottom spines of the image. The default plot limits for implot are -0.5 to end+0.5 along both axes.

There is a known bug with the Plots.jl GR backend that leads ticks to be slightly offset. PyPlot and Plotly backends show the correct tick locations.