How to extract a specific region in matlab ?

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sufian ahmed
sufian ahmed 2017-6-25
评论： Image Analyst 2017-6-25
I know the four coordinates of a trapezium or rectangle.Now i want to only extract this region and other things should be black. How can i do that ?
My test image
I want to do like this image:

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回答（1 个）

Image Analyst
Image Analyst 2017-6-25
Pass the coordinates into poly2mask() and use bsxfun() to mask
% Get the dimensions of the image. numberOfColorChannels should be = 3.
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Create mask
mask = poly2mask(x, y, rows, columns);
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));
  2 个评论
sufian ahmed
sufian ahmed 2017-6-25
why u take numof channels.??. as i saw there is no use here in u r code.. ?
Image Analyst
Image Analyst 2017-6-25
Because if you leave it out,
[rows, columns, numberOfColorChannels] = size(rgbImage);
columns will be the number of columns times the number of color channels. For a full explanation, see http://blogs.mathworks.com/steve/2011/03/22/too-much-information-about-the-size-function/
Alternatively you could accept the size into a single variable, which will then be a vector but then to get the rows and columns, you will have to add an index 1 or 2 respectively:
% Get the dimensions of the image. numberOfColorChannels should be = 3.
imageSize = size(rgbImage);
% Create mask
mask = poly2mask(x, y, imageSize(1), imageSize (2));
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));
I think the way I had it first is easier to understand. You could use ~ instead of numberOfColorChannels if you want:
[rows, columns, ~] = size(rgbImage);

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