CODEWITHSUNDEEP
opencvcropmatroi
shjnlee
shjnlee

Reputation: 243

Crop half of an image in OpenCV

How can I crop an image and only keep the bottom half of it?

I tried:

Mat cropped frame = frame(Rect(frame.cols/2, 0, frame.cols, frame.rows/2));

but it gives me an error.

I also tried:

double min, max;
Point min_loc, max_loc;
minMaxLoc(frame, &min, &max, &min_loc, &max_loc);
int x = min_loc.x + (max_loc.x - min_loc.x) / 2;
Mat croppedframe = = frame(Rect(x, min_loc.y, frame.size().width, frame.size().height / 2));

but it doesn't work as well.

Upvotes: 5

Views: 19683

Answers (3)

Osi
Osi

Reputation: 448

To quickly copy paste:

image = YOURIMAGEHERE #note: image needs to be in the opencv format
height, width, channels = image.shape
croppedImage = image[int(height/2):height, 0:width] #this line crops

Explanation:

In OpenCV to select a part of an image,you can simply select the start and end pixels from the image. The meaning is:

image[yMin:yMax, xMin:xMax]

In human speak: yMin = top | yMax = bottom | xMin = left | xMax = right |

" : " means from the value on the left of the : to the value on the right

To keep the bottom half we simply do [int(yMax/2):yMax, xMin:xMax] which means from half the image to to the bottom. x is 0 to the max width.

Keep in mind that OpenCV starts from the top left of an image and increasing the Y value means downwards.

To get the width and height of an image you can do image.shape which gives 3 values:

yMax,xMax, amount of channels of which you probably won't use the channels. To get just the height and width you can also do:

height, width = image.shape[0:2]

This is also known as getting the Region of Interest or ROI

Upvotes: 4

rajeevr22
rajeevr22

Reputation: 128

Here's a the python version for any beginners out there.

def crop_bottom_half(image):
    cropped_img = image[image.shape[0]/2:image.shape[0]]
    return cropped_img

Upvotes: 11

Olivier
Olivier

Reputation: 921

The Rect function arguments are Rect(x, y, width, height). In OpenCV, the data are organized with the first pixel being in the upper left corner, so your rect should be: 

Mat croppedFrame = frame(Rect(0, frame.rows/2, frame.cols, frame.rows/2));

Upvotes: 6

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