What does "underscore symbol" mean in a matlab function documentation?

Hi,

I read in this documentation:

http://www.mathworks.it/it/help/vision/ref/estimategeometrictransform.html

that for output 'status'

[___,status] = estimateGeometricTransform(matchedPoints1,matchedPoints2,transformType)

but what does "___" mean?

I tried with

[~,status] = estimateGeometricTransform(matchedPoints1,matchedPoints2,transformType)

and

[tform,status] = estimateGeometricTransform(matchedPoints1,matchedPoints2,transformType)

but i can not get out status variable and it returns an error:

Error using coder.internal.errorIf (line 9)
MATCHED_POINTS1 and MATCHED_POINTS2 do not have enough points.

Error in estimateGeometricTransform>checkRuntimeStatus (line 167)
coder.internal.errorIf(status==statusCode.NotEnoughPts, ...

Error in estimateGeometricTransform (line 149)
    checkRuntimeStatus(statusCode, status);

Answer 1

[Dima Lisin]

Hi Marco,

If you look at the description of the transformType parameter in the documentation, you will see a table listing the minimum number of matched points required for each type of transformation.

[Soso]

Thank you so much Dima.

Answer 2

[Walter Roberson]

The _ stands in for "any of the other forms listed already". So "tform" or the three-output form can both be followed by "status" as an output.

What size() are your matchedPoints1, matchedPoints2 ? The routine thinks that you are not passing in enough points (enough rows) to be able to compute the transform type you have asked for (you did not show us which one.)

[Marco]

I would like try to build a simulink model for the SURF using an Interpreted Matlab Function block but for detect a car in the viptraffic.avi video file pre-saving a crop of the car. I follow the way of example: http://www.mathworks.it/it/help/vision/examples/detect-objects-in-a-cluttered-scene-using-point-feature-matching.html but when simulation starts it return that error. So I would like make a switch-case reading 'status' variable, but it may be insufficient.

[Walter Roberson]

Okay, but what are the size() of matchedPoints1 and matchedPoints2 when the problems occur? Perhaps the problem can be predicted instead of being captured afterwards.

[Marco]

I have not simulation data now, i will check it later. However, since i used a crop file of the "scene" video as template (so a small picture) do you mean template has not enough feature points to match ?

[Marco]

matchedPoints1 and matchedPoints2 are 2x1 SURFPoints. How can i predict the error?

[Soso]

Did you solve the problem? Please share with us what was the solution? Thank you so much.

[Dima Lisin]

Hi Soso,

please see my answer below.