[matplotlib-devel] updating an image with a colorbar, performance issues

[Darren D. <dar...@co...>]

I have been working on an application which is basically a dressed up version 
of embedding_in_qt4.py. I have two canvases, one is just a plot, the other an 
image with a colorbar, both of which should get updated about once a second 
as data is acquired. I noticed that for larger images, it was taking longer 
to update the canvases as time went on.

Attached is a script that demonstrates the problem (png's are also attached 
for those who dont have PyQt4). At each timeout, I check the time, generate a 
new random array, update the image's data, draw it, collect garbage, and 
check the time again. The plot in the upper window shows how long it took to 
update the image canvas as a function of iteration number.

When the image has a colorbar, the time to update the image canvas increases 
linearly, about 0.3 ms/iteration. This can really add up: after 20,000 
iterations it takes about 5-6 seconds to update the image canvas. If I 
comment out the line that creates the colorbar (line 64), the time to update 
the image canvas does not increase.

A couple other observations and benchmarks from this test:

* The intercept for the update time with a color bar was 0.17 seconds
* With the colorbar enabled but garbage collection turned off, the application 
slowly consumed memory, roughly 12KB/iteration
* Updating an image without a colorbar took 0.1 seconds per iteration, and did 
not increase
* Turning off both garbage collection and the colorbar decreased the update 
time to 0.043 seconds, with maybe 1.7 KB/iteration increase in memory usage.

I don't know if I'll get a chance to dig any further into this in the next two 
weeks. For now, I'll just disable the colorbar in my application.

Darren