Finding Centroids using text inserter?

Hello everyone, I wanted to ask a question about the vision.TextInserter. The following is some basic codes:

blob = vision.BlobAnalysis('CentroidOutputPort', true, 'AreaOutputPort',... 
                            false,'BoundingBoxOutputPort', false,'MinimumBlobAreaSource', 'Property',... 
                            'MinimumBlobArea', 20000);

textinCentroid = vision.TextInserter('Text', 'X:%4d, Y:%4d', ... 
                                    'LocationSource', 'Input port', ...
                                    'Color', [1 0 0], ... 
                                    'FontSize', 14);

while ~isDone(videoReader)
frame = step(videoReader);
I = finalimage; %Final result of image after process
centroid = step(blob, I);
result = step(textinCentroid, frame, centroid)

step(videoPlayer, result);
end

So the problem is that I got the following problem:

Error using vision.TextInserter/step
Not enough input arguments; expected 3 (in addition
to the object handle), got 2.

Error in ComputerVision2 (line 64)
     result = step(textinCentroid, frame,
     centroids);

So can anyone tell me what exactly am I missing here? What do I have to add to make it in such a way that my object detected have a centroid on it? Thank you.

Answer 1

[Walter Roberson]

You need the object first. You need the image second. Subtotal 2

You used %-style sequences in your string, so you need a vector of values, one per % in the string. You also need the location for the input port. Subtotal 2.

Total 4.

What I do not see documented is which order to use if you have both loc and vars. Experiment. I'm going to guess loc should be first.

[Akira Chan]

Dear Mr Walter,

Sorry but I don't quite get it. The object, you mean the object detected from the video but that is from the image itself ,no? As for the values for the % in the string, isn't it already provided by the vision.blobanalysis since I put 'true' in the CentroidOutputPort. Also, when I put the AreaOutputPort as 'true', I can obtain the value right away but is it a bit different in CentroidOutputPort? Thank you for your quick reply.

Regards, Akira

[Walter Roberson]

Setting CentroidOutputPort for the blobanalysis means that when you step() the blob analysis class object, the output of step() will be the centroid.

Setting 'LocationSource' to 'Input Port' for the TextInserter class object means that you will pass the location in as a parameter to step() instead of the class object retrieving the location from the Location property of the class object.

With those and with a String that did not include % codes, your existing code would be fine,

centroid = step(blob, I);
result = step(textinCentroid, frame, centroid);

that is you would return the value from the step() of the blob analysis class object, and you would pass that value in to the text inserter class object.

This would be the syntax for specifying where the text is to be placed.

When you use % strings for string in the text inserter, you need to pass the step() call to the inserter class object a vector containing the values to be substituted in at the point of the %. You do not use Input Port for this, that is not what it is for. Input Port is for the position.

If you want position the given text at the centroid, and you want the values of the text to be the locations of the centroid, then pass the centroid twice to the step() of the inserter class object:

result = step(textinCentroid, frame, centroid, centroid);

It turns out, by the way, that the parameters for the % format should go before the location for Input Port. See the example http://www.mathworks.com/help/vision/ref/vision.textinserter-class.html#btk0zqa-1 which uses both % and the Input Port.

[Akira Chan]

Thank you again Mr Walter, I understand it now! Cheers!