Tom Whitfield

I start by splitting the formats into two groups, because most of the confusion comes from treating them alike. Lossless formats keep every pixel. Converting between lossless formats is safe in principle, though there is a catch: an image with an alpha channel cannot be represented in a format without one, and transparency silently becomes a background colour. That is the one conversion to check before you run it on a batch. Lossy formats discard information on purpose. Converting a JPG to WebP re-encodes an already-encoded image, and the result carries the artefacts of both encodings. There is no reason to do that unless you need the format, and if you have the original you should convert from it every time. Colour profile is the other half. An image with an embedded profile displayed on a system without one will have its colours shifted, and the shift is far more visible than the compression artefact people expected to see. Converting without handling the profile is the most common way an otherwise fine conversion looks wrong. I cover the format decisions by use: a photograph for display, a logo with transparency, an image with text in it where artefacts are unacceptable, and an image destined for print where colour space changes everything. Batch conversion is where mistakes get expensive. I write about previewing a single file and reading the output size before running the directory. One more caution about alpha. Converting a transparent image to a format without transparency produces a background colour chosen for you, usually black, and on a dark page that reads as a black rectangle around the subject rather than as an error.