When does it make sense to describe an image in the post text?

Under certain circumstances, it makes sense to have an image description in the post text, of course in addition to the image description in the alt-text. This mostly applies to places in the Fediverse that aren't vanilla Mastodon, i.e. that have post character limits way higher than the 1,500-character limit for alt-texts imposed by Mastodon, Misskey and their respective forks on the whole Fediverse.

For example, the image may contain so much text that image description and text transcript don't fit into 1,500 characters. In this case, the text transcript may go into the post text. In this case, the alt-text should mention that there is a text transcript in the post text. If the transcript must be embedded in an image description, e.g. to describe where the text is and/or what it looks like if this is of importance, then this image description must go into the post text, too.

Another example is when the image description requires one or several explanations so that everyone can understand it, regardless of prior knowledge. Explanations must never go into the alt-text because there are people who cannot access alt-text, and so these explanations, along with the surrounding image description, must go into the post text. In addition, there must still of course be an image description in the alt-text, along with a notice that there is another image description with explanations in the post text.

Some image may even require visual descriptions so detailed that they exceed 1,500 characters. In this case, the full, long description would go into the post text, and a shortened description with a notice that there is a longer image description in the post text would go into the alt-text.

In fact, Veronica Lewis of Veronica With Four Eyes recommends to always describe all images twice, once in the alt-text "under" the image, once in the post text.¹ This is because those like her who are visually impaired enough to not be able to see the image clearly or read the text at its normal size, but who can read the text when its enlarged, and who don't need screen readers, have an image description to read without having to access the alt-text.