Opencv – how to draw a rectangle by specifying its 4 corners

image processingobject-detectionopencvopencv4android

I am using OpenCV4Android version 2.4.11. I am reading frames from the camera and I detect any rectangular shapes in the frame. Then I try to draw a semi-transparent rectangle around a detected object.

what i want to do is, to draw a semi-transparent rectangale given the four corners of the detected object. But, in openCV you can draw a rectangle by spcifying only two points of it "the topLeft and BottomRight".

please let me know how to draw a rectangle by spcifying the the four corners of it NOT ONLY by spcifying the topLeft and BottomRight corners.

the below posted image is to show you my attempts and to show you that what i want is to draw a rectangle around the four detected corners "red, green, blue, white"

image:

Best Answer

Same idea as @Saransh, but compiles for me:

auto x1 = std::min(tlx, std::min(trx, std::min(brx, blx))); // top-left pt. is the leftmost of the 4 points
auto x2 = std::max(tlx, std::max(trx, std::max(brx, blx))); // bottom-right pt. is the rightmost of the 4 points
auto y1 = std::min(tly, std::min(try, std::min(bry, bly))); //top-left pt. is the uppermost of the 4 points
auto y2 = std::max(tly, std::max(try, std::max(bry, bly))); //bottom-right pt. is the lowermost of the 4 points

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OpenCV Object Detection – Center Point

There's already an example of how to do rectangle detection in OpenCV (look in samples/squares.c), and it's quite simple, actually.

Here's the rough algorithm they use:

0. rectangles <- {}
1. image <- load image
2. for every channel:
2.1  image_canny <- apply canny edge detector to this channel
2.2  for threshold in bunch_of_increasing_thresholds:
2.2.1   image_thresholds[threshold] <- apply threshold to this channel
2.3  for each contour found in {image_canny} U image_thresholds:
2.3.1   Approximate contour with polygons
2.3.2   if the approximation has four corners and the angles are close to 90 degrees.
2.3.2.1    rectangles <- rectangles U {contour}

Not an exact transliteration of what they are doing, but it should help you.

How to find Waldo with Mathematica

I've found Waldo!

waldo had been found

How I've done it

First, I'm filtering out all colours that aren't red

waldo = Import["http://www.findwaldo.com/fankit/graphics/IntlManOfLiterature/Scenes/DepartmentStore.jpg"];
red = Fold[ImageSubtract, #[[1]], Rest[#]] &@ColorSeparate[waldo];

Next, I'm calculating the correlation of this image with a simple black and white pattern to find the red and white transitions in the shirt.

corr = ImageCorrelate[red, 
   Image@Join[ConstantArray[1, {2, 4}], ConstantArray[0, {2, 4}]], 
   NormalizedSquaredEuclideanDistance];

I use Binarize to pick out the pixels in the image with a sufficiently high correlation and draw white circle around them to emphasize them using Dilation

pos = Dilation[ColorNegate[Binarize[corr, .12]], DiskMatrix[30]];

I had to play around a little with the level. If the level is too high, too many false positives are picked out.

Finally I'm combining this result with the original image to get the result above

found = ImageMultiply[waldo, ImageAdd[ColorConvert[pos, "GrayLevel"], .5]]

Best Answer

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OpenCV Object Detection – Center Point

How to find Waldo with Mathematica

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