Fragment shaders are where the computations happen. There are generally three coordinate spaces that show up here. These conventions help keep track of them.
This is the space the matrix (or vector) lives in. It's the one you've seen in
Linear Algebra and any time you use an Array. Indexing variables contain ints
(usually starting at zero) and describe a row or column in the matrix.
row, col
i, j, k
This is the space that pixels live in on a texture (fragment shaders work on
textures, texture is just a fancy name for image). The difference between
matrix space and this space is that pixels usually contain more than one piece
of data (four in our data textures) from a row. Indexing variables contain floats
(usually integers with a 0.5 offset) and describe data from a group of columns
in a row.
row_p, col_p
l, m, n
This is the coordinate space for accessing data from a texture. This is what
actually get's passed to WebGL texture functions (like texture2D) for
accessing data. Indexing variables contain floats in the range [0, 1]. The first
number passed to functions describes the x-axis or column (just like in math,
but opposite of most programming conventions).
col_t, row_t
x, y, z