Using vision.Geo​metricTran​sformer on a webcam video stream

Hello,

I'm working on a stereo vision project using two webcams. I take two still images and rectify them using the standard method as described by the help documentation.

The final step in the rectification processes uses the vision.GeometricTransformer to transform and crop the two images as described by their transformation matrices.

I'd like to use this same function on the live video stream, so that the user may aim in 3D. However, I get the error:

Error using vision.GeometricTransformer/step Inputs must be either numeric or logical variables or fi objects.

My code is as follows:

function GetImage

%Load camera settings and transformation matrices 
camcal = load('files/camcal.mat');
recmat = load('files/recmat.mat');

%Activate camera
vidL = videoinput('winvideo', camcal.selectLv, 'YUY2_1920x1080',...
    'ReturnedColorSpace', 'rgb');

%Make transformer
htrans = vision.GeometricTransformer(...
    'TransformMatrixSource', 'Input port', ...
    'OutputImagePositionSource', 'Property', ...
    'OutputImagePosition', recmat.bbox);

%Perform transformation
vidLt = step(htrans, vidL, recmat.grayl);

%Show result
preview(vidLt);

I produced the same using Simulink, using all the same matrix values, and it works fine. Which makes me think it should be able to work in Matlab.

Any ideas?

Thanks.