map8.six doesn't display

[roblourens]

Testing #183563

This worked with snake.six but something is wrong with this image and nothing is displayed when I cat it https://github.com/saitoha/libsixel/blob/master/images/map8.six

[Tyriar]

@jerch any ideas why map8.six wouldn't work in xterm-addon-image? It looks fairly simple

[jerch]

It seems the image is smaller than 1 row in height - since the addon only supports vt340 cursor mode, this line will get overwritten by the next shell prompt right after image output. To work around this, just append a NL, e.g. by appending and empty echo command:

$> curl -s https://raw.githubusercontent.com/saitoha/libsixel/master/images/map8.six && echo

(To see whats actually going on - you can also replace the echo with a sleep 5...)

[Tyriar]

Oh, is this something that other terminals do that we don't then? Could we force the empty line for small images?

[jerch]

Oh, is this something that other terminals do that we don't then?

Well yes and no. Its related to the question, where the text cursor should be at after a sixel sequence. There are like 5 different ideas about that floating around, which all have flaws for different reasons. Initially I had like 4 of them implemented, but after a longer discussion with @j4james and @hackerb9 I came to the conclusion, that only the vt340 cursor mode makes sense and avoids those flaws. (To put it in other words - the other emulators are broken in this regard)

Could we force the empty line for small images?

Nope this would not avoid that "issue", as it happens for every last row of an image, that does not form a full row height. It is basically up to app side to anticipate, whether a newline should be written or not (e.g. by grabbing terminal metrics & compare with image metrics).

Without going too much into details - in vt340 cursor mode the text cursor sticks to the row where the bottom-most sixel was printed, at the first sixel containing cell (does not move to the right, as sixel is "unbound" to the right side).

[j4james]

It's worth noting that XTerm updated their sixel cursor positioning algorithm to more closely match the VT340 behavior, so that map8.six test would also now be overwritten in Xterm, unless followed with an echo. And I know Contour also handles that case correctly.

It's been a long time since I tested any other terminals, and I suspect most are still wrong, but I'm hopeful that they'll all eventually start to converge on the correct behavior now that XTerm has gone that route (it's not perfect yet, but it's definitely improving).

[Tyriar]

Thanks for the info, sounds like it's by design then 👌

[hackerb9]

Last I check XTerm worked properly for map8.six.

Note that if you have a short prompt (PS1='$ ') you should actually see the image if you cat it to the screen. It's just that your prompt usually overwrites it.

Here's a screenshot from my VT340:

[Not visible in this image: The flashing text cursor after the first cat was inverting the olive color to a surprising purple.]