# 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 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.