What is PNG?

What it can hold

PNG is an image format, and the important consequence is that it is a one-way trip: PNG holds a lossless picture of a table, in which the values exist only as pixels once it is written. Keep whatever you converted from, because reading the table back out of PNG is a much harder job than writing it.

It keeps

• A line break inside a cell, which survives rather than ending the row.
• Presentation - weight, alignment, colour and widths - alongside the values themselves.

It cannot express

• Types. Every value is text. A column of digits is indistinguishable from a column of numbers until something downstream decides to read it as one, which is where a leading zero usually disappears.
• A null. An absent value and an empty one look the same, and nothing in the file says which was meant.
• Nesting. Cells are flat, so structured values have to be flattened into extra columns or written into one cell as text.
• A partial read. The document has to be complete to mean anything, so a truncated file yields nothing and a large one has to be held in memory whole.