The existing documentation on this seemed a little sparse, so I figured I’d post up an example based on what I figured out for my project.

I have a 1024×768 sensor_msgs/Image coming out of image rectification on the topic /overhead_cam/image_rect, with camera info on the predictably-named topic /overhead_cam/camera_info. What I want is to crop off the top 120 px of that image so that it’s the right aspect ratio to get scaled to 1680×1050 without distortion (technically, scaling is a distortion, but it’s the only distortion I want). Then I want to scale it up to 1680×1050.

<!-- Video cropping --> <node pkg="nodelet" type="nodelet" args="standalone image_proc/crop_decimate" name="crop_img"> <param name="x_offset" type="int" value="0" /> <param name="y_offset" type="int" value="120" /> <param name="width" type="int" value="1024" /> <param name="height" type="int" value="684" /> <!-- remap input topics --> <remap from="camera/image_raw" to="overhead_cam/image_rect_color"/> <remap from="camera/image_info" to="overhead_cam/camera_info"/> <!-- remap output topics --> <remap from="camera_out/image_raw" to="camera_crop/image_rect_color"/> <remap from="camera_out/image_info" to="camera_crop/camera_info"/> </node> <!-- Video resizing --> <node pkg="nodelet" type="nodelet" args="standalone image_proc/resize" name="resize_img"> <!-- remap input topics --> <remap from="image" to="camera_crop/image_rect_color"/> <remap from="camera_info" to="camera_crop/camera_info"/> <!-- remap output topics --> <remap from="resize_image/image" to="camera_resize/image_rect_color"/> <remap from="resize_image/camera_info" to="camera_resize/camera_info"/> </node> <!-- Dynamic reconfigure the resizing nodelet --> <node name="$(anon dynparam)" pkg="dynamic_reconfigure" type="dynparam" args="set_from_parameters resize_img"> <param name="use_scale" type="int" value="0" /> <param name="width" type="int" value="1680" /> <param name="height" type="int" value="1050" /> </node>

The first bit does the cropping, running the decimate nodelet as a standalone node and doing the cropping. It can also decimate (shrink images by deleting rows/cols), but I’m not doing that.

The second bit starts up the video resizing nodelet as standalone, and the third bit fires off a dynamic reconfigure signal to set the image size. If you set use_scale to true, it will scale an image by some percentage, which isn’t quite what I wanted.

I imagine it’s possible to fold these into image rectification as nodelets (image rectification is itself a debayer nodelet and two rectify nodelets, one for the mono image and one for the color image), but I didn’t look into that because this worked for me.