how to crop multiple objects from an image in matlab

Question

This is the code of object detection from a video.

I want to crop objects from this video frame by frame.

videoSource = vision.VideoFileReader('viptraffic.avi','ImageColorSpace','Intensity'...
'VideoOutputDataType','uint8');

detector = vision.ForegroundDetector(...
   'NumTrainingFrames', 5, ... 
   'InitialVariance', 30*30);

 blob = vision.BlobAnalysis(...
   'CentroidOutputPort', false, 'AreaOutputPort', false, ...
   'BoundingBoxOutputPort', true, ...
   'MinimumBlobAreaSource', 'Property', 'MinimumBlobArea', 250);

 shapeInserter = vision.ShapeInserter('BorderColor','White');

videoPlayer = vision.VideoPlayer();

while ~isDone(videoSource)

 frame  = step(videoSource);
 fgMask = step(detector, frame);
 bbox   = step(blob, fgMask);
 out    = step(shapeInserter, frame, bbox); 
 step(videoPlayer, out); 
end
release(videoPlayer);  
release(videoSource);

when I want to crop bbox from frame it always give me error "invalid input arguments"

if I write this this command.

 frame(bbox(1):bbox(1)+bbox(3), bbox(2):bbox(2)+bbox(4), :);

"Index exceeds matrix dimensions" error came. please help me how to crop objects from image

Answer 1

Try

frame(bbox(2):bbox(2)+bbox(4), bbox(1):bbox(1)+bbox(3), :);

The values in bbox are in format [ x y w h ] while the indices into frame should be in row-column order: you need to change the order of x and y to row-column.

Answer 2

You need to handle cases when no boxes have been detected (bbox is empty) and when more than one box is detected (bbox is an M-by-2 matrix).

So you should have a loop:

for i = 1:size(bbox, 1)
  croppedImage = frame(bbox(i, 2):bbox(i, 2)+bbox(i, 4), bbox(i, 1):bbox(i, 1)+bbox(i, 3), :);
  % do something wiht croppedImage
end

Alternatively, you can use the imcrop function:

croppedImage = imcrop(bbox(i, :));

Just be aware that imcrop will return an array 1 pixel smaller in x and y, than the other approach.