MATLAB: “Invalid graphics handle” error when using imrect() or imellipse()

graphicsImage Acquisition Toolboximage processingImage Processing ToolboximrectMATLAB

I'm trying to apply a mask onto an image for processing, but the call:

h = imrect(output,[2 3 4 5]); %xmin ymin width height

aways gives me the errors:

Error using roiParseInputs (line 75) HPARENT must be a valid graphics handle.

Error in imrect>imrectAPI (line 182) [commonArgs,specificArgs] = roiParseInputs(0,2,varargin,mfilename,{'DrawAPI'});

Error in imrect (line 82) [h_group,draw_api] = imrectAPI(varargin{:});

I have no clue how to resolve this error.

\removed code\

Best Answer

The first step is always to check your inputs.

help imrect

You'll see that the first input to this function is an axis handle. Try this (or better yet, the actual handle to your axes).

h = imrect(gca, output,[2 3 4 5]);

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MATLAB: Multiple ROI in 2 images

Hi Meera,

The issue is caused since 'imshowpair' function creates only one axis. The same functionality can be achieved using 'subplot' to create a figure with two axis. When working with multiple axes objects, 'imrect' should be called using the required axis handle. To avoid the execution 'jump', you can use 'wait' function. It will pause the execution and allow you to modify the ROI after initially dragging the rectangle. Once satisfied with the dimensions of ROI, you can resume the execution by double-clicking inside the selected region.

Here is the modified custom code:

%%Load a video and draw ROI in the video/image frames
%%Input video file which needs to be stabilized.
filename = 'shaky_car.avi';
%Load the video using a video reader object.
videoFReader  = vision.VideoFileReader(filename, ...
    'ImageColorSpace', 'Intensity',...
    'VideoOutputDataType', 'double');
%Create a video player object to play the video file
videoPlayer = vision.VideoPlayer;
while ~isDone(videoFReader)
    frame = videoFReader();
    videoPlayer(frame);
    pause(0.01);
end
imgA = step(videoFReader);
imgB = step(videoFReader);
%%Code modified from this point
% imshowpair(imgA,imgB,'montage');
ax1 = subplot(1,2,1);  %axishandle to first image is stored in ax1
imshow(imgA);
ax2 = subplot(1,2,2);  %axishandle to second image is stored in ax2
imshow(imgB);
%imrect used with axis handle, wait function allows modification of ROI
objectRegionA = round(wait(imrect(ax1))); % result assigned to objectRegionA
objectRegionB = round(wait(imrect(ax2))); % result assigned to objectRegionB
objectImageA = insertShape(imgA,'Rectangle',objectRegionA,'Color','red'); % objectRegionA is used

objectImageB = insertShape(imgB,'Rectangle',objectRegionB,'Color','red'); % objectRegionB is used

figure;
imshowpair(objectImageA, objectImageB ,'montage');
title('Red box shows object region');
%Detect interest points in the object region.
pointsA = detectMinEigenFeatures(imgA,'ROI',objectRegionA); % objectRegionA is used
pointsB = detectMinEigenFeatures(imgB,'ROI',objectRegionB); % objectRegionB is used
%%Code modified till this point
%Display the detected points.
pointImageA = insertMarker(imgA,pointsA.Location,'+','Color','white');
pointImageB = insertMarker(imgB,pointsB.Location,'+','Color','white');
figure;
imshowpair(pointImageA,pointImageB,'montage');
title('Detected interest points');
release(videoFReader);
release(videoPlayer);

Note: For selecting multiple ROI in the same image, you can use a 'for' loop or any other iterative method.

Hope this helps.

Best Answer

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