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Re: Images with captions and alt

for

From: Jukka K. Korpela
Date: Aug 10, 2005 6:00AM

On Wed, 10 Aug 2005, Penny Roberts wrote:

> If an image has a caption which describes the image and the
> alt would be very much the same wording should I use an empty alt instead?

A caption should identify the image. An alt text should present the
information content of the image, or its relevant part. These are very
different jobs. In practice we might resort to label-like alt texts since
we are unable to write real alternative text, and _then_ the problem may
arise.

> For example: an image has a caption (which comes straight after the image
> in the code) reads "Figure 6: The catalogue search results screen" and the
> alt tag is "Catalogue screen showing search results". Wouldn't that be
> redundant to anyone reading the screen with images turned off and just
> plain annoying and time wasting to anyone hearing the screen?

It would be annoying indeed. But does it help the user to hear or see the
text even once? After all, neither the caption nor the alt text presents
the information content of the image; they just refer to it.

This is a tough question, and it depends on the nature and purpose of the
image. For example, sometimes a purely decorative image has a caption
(as text, not as embedded into the image). It is not quite sufficient to
use alt="", since the user will see or hear the caption and wonder what it
is about. In this special case, even alt="(decorative image)" would make
some sense. For an analysis of some types of cases, see
http://www.cs.tut.fi/~jkorpela/html/alt.html#content

An adequate textual replacement for some catalogue screen showing search
results would consist of text that explains, in sufficient detail as
required by the purpose of presenting the material, what the search
results consist of and how they are presented. This could consist of the
transcript of the same search using a text-only user interface.

It would generally be impractical to present such a large amount of text
in an alt attribute. Some other strategy would be needed. In this case,
I might use alt="(figure 6)" for the image and add text like
"There is alternative example of <a href="...">search dialogue
in text-only mode</a>, illustrating the same thing as Figure 6
but in a different user interface." Well, it can probably be made shorter.
The point is: write the alternate content into a document of its own
(perhaps a plain text file, obtained using a suitable tool for
getting an interaction log), and link to it with words.

--
Jukka "Yucca" Korpela, http://www.cs.tut.fi/~jkorpela/