Intraprocess publishing is only significantly faster if you're publishing a shared_ptr to the message, otherwise it still has to serialize/deserialize as usual.

So instead of keeping an image as part of the class, filling that image, and then publishing it, you need to allocate the image each time you publish:
sensor_msgs::ImagePtr image(new sensor_msgs::Image);
sensor_msgs::fillImage(*image, "bgr8", height, width, 3 * width, data);
pub_.publish(image);

It doesn't look like this is explained anywhere on the wiki other than the cturtle release notes: http://www.ros.org/wiki/ROS/ChangeList/1.2/roscpp_changes#Publish.2BAC8-Subscribe_Changes, I'll remedy that tomorrow.

Josh

On Fri, Sep 17, 2010 at 1:01 AM, Cartwright, Joel J <J.J.Cartwright@hw.ac.uk> wrote:

Hi all,

I've been running some rough performance tests on transferring images
using nodelets, and have found the overhead to be higher than I
expected. For reference, I'm running ROS cturtle on Ubuntu 10.04 64 bit
(stock kernel 2.6.32-24-generic) on a 2 GHz Intel Core 2 Duo laptop. CPU
use monitored approximately using htop.

Exchanging one 1920x1280 bgr8 image at 10 fps incurs ~30% CPU use for
the nodelet manager. As expected, this is better than when the nodelets
are run standalone (then ~25% CPU each), but I wanted to check if I'm
doing something wrong that might affect nodelet performance. I've been
using raw Image publishing rather than image_transport, to keep the test
as simple as possible.

The image 'source' is just an uninitialised unsigned char array of the
correct size. As a comparison, I created a node that merely copies the
source array a number of times, and found that 5 memcpy operations
produce about the same CPU load as the best nodelet case, around 30%.
Of course CPU caching may improve the speed of those 5 consecutive
memcpy operations, so I'm not saying "ROS is copying the data 5 times".

I've attached the relevant source files. The question is: Is this the
best we can do at present for image transfer, and if not, are there
modifications on the horizon that will improve performance?

Joel
--
Research Assistant
Ocean Systems Laboratory
Heriot-Watt University, UK


Heriot-Watt University is a Scottish charity registered under charity number SC000278.

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