[Audacity-devel] "Stem plot" option for viewing samples

[Steve t. F. <ste...@gm...>]

Moving this discussion from GitHub to the devel@ list.

GitHub ref. https://github.com/audacity/audacity/pull/186
Forum ref. http://forum.audacityteam.org/viewtopic.php?f=28&t=94931

Brian Armstrong issued a pull request for a proposed new feature:
When zoomed in to sample level, display samples as a "stem plot".

Here is a screenshot of what it looks like:



This style of visually representing PCM samples is widely used in other
software (particularly 'scientific' DSP type software), including MatLab,
and is arguably more 'truthful' than joining the dots with straight lines
as we do currently.

I'd like to see this as an option in Audacity, subject to a couple of
caveats:

1) I think it should be a non-default 'option', so there would need to be a
way to switch between the current "join the dots" and "stem plot" styles.

It has also been suggested that in the future we could have another 'style'
using sinc interpolation / oversampling to join the dots with a smooth
curve. I'm very much in favour of this option also (to the extent that, if
it works well enough, it could become the default).

2) Ideally, Brian would like stem plots to be used from a relatively low
zoom level, as the horizontal distance between samples becomes greater than
pixel size, but this can cause unpleasant aliasing:



Brian has proposed two methods to avoid aliasing:

a) Restrict zoom levels to factors that give an exact number of pixels per
sample period.

b) Use sub-pixels rendering or anti-aliasing.

Option (a) has a significant down-side, which is that "zoom to fit" fails
if the number of samples selected is not an integer factor of the track
width in samples.

Example: if the track width is 1000 pixels, and 600 samples are selected,
then to fit the selection, we would need sample spacing of 1.67 pixels per
sample, but that will either cause aliasing, or we would zoom to the 600
selected + 400 not selected pixels.In my opinion, breaking "zoom to fit" in
this way is not an acceptable option.

Unfortunately Brian also hit a problem with option (b), which he believes
"may require a fairly substantial amount of work" to get this working
cross-platform.

On the other hand, at zoom levels where we switch from just a line to
"dots", aliasing is hardly, or not at all noticeable, and the Stem Plot
style works nicely.


In order to move the Stem Plot proposal forward, I would like to propose
that we implement Stem Plot as an option, for zoom ranges where aliasing is
not a problem. Extending the feature for Stem Plot at lower zoom levels
could be added later if / when sub-pixels rendering can be made to work
cross-platform.
​
I've created a branch based on Brian's original stem plot and a preference
setting to select this new style. As the Tracks menu was getting rather
long, I've also moved the Tracks Behaviors preferences into a sub-page.

I also fixed a minor bug where stems could overflow from the lower (right)
channel into the upper (left) channel when zooming vertically on the
negative half of the waveform of a stereo track.

A bit of tweaking may be required to enable building on Windows, but it
works for me on Linux (Ubuntu) and Mac (Sierra).
https://github.com/SteveDaulton/audacity/tree/stem-plot

Steve