How to extract a specific region in matlab ?

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:

Answer 1

[Image Analyst]

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));

[sufian ahmed]

why u take numof channels.??. as i saw there is no use here in u r code.. ?

[Image Analyst]

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);