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bboxcrop

Crop bounding boxes

Description

example

bboxB = bboxcrop(bboxA,window) crops bounding boxes from a set of input bounding boxes, bboxA, located in the cropping area, window.

[bboxB,indices] = bboxcrop(bboxA,window) additionally returns a vector of indices that indicate which bounding boxes in bboxA are within the cropping window, window.

[___] = bboxcrop(___,'OverlapThreshold',threshold) additionally sets an overlap threshold.

Examples

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Read an image.

I = imread('peppers.png');

Define bounding boxes and labels.

bboxA = [
    410 230 100 90
    186 78  80  60
    ]
bboxA = 2×4

   410   230   100    90
   186    78    80    60

labelsA = [
    "garlic"
    "onion"
    ];

Create a center cropping window.

targetSize = [256 256];
win = centerCropWindow2d(size(I),targetSize);

Center crop the image.

[r,c] = deal(win.YLimits(1):win.YLimits(2),win.XLimits(1):win.XLimits(2));
J = I(r,c,:);

Center crop boxes and labels. Boxes outside the cropping window are removed.

[bboxB,indices] = bboxcrop(bboxA,win);
labelsB = labelsA(indices);

Display the results.

figure
I = insertObjectAnnotation(I,'Rectangle',bboxA,labelsA);
J = insertObjectAnnotation(J,'Rectangle',bboxB,labelsB);
imshowpair(I,J,'montage')

Input Arguments

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Bounding boxes, specified as an M-by-4, M-by-5, or M-by-9 nonsparse numeric matrix. M is the number of bounding boxes. Each row of the matrix defines a bounding box as either an axis-aligned rectangle, a rotate rectangle, or a cuboid. This table describes the format for each bounding box.

Bounding BoxDescription
Axis-aligned rectangle

Defined in pixel coordinates as an M-by-4 numeric matrix with rows of the form [x y w h], where:

  • M is the number of axis-aligned rectangles.

  • x and y specify the upper-left corner of the rectangle.

  • w specifies the width of the rectangle, which is its length along the x-axis.

  • h specifies the height of the rectangle, which is its length along the y-axis.

Rotated rectangle

Defined in spatial coordinates as an M-by-5 numeric matrix with rows of the form [xctr yctr xlen ylen yaw], where:

  • M is the number of rotated rectangles.

  • xctr and yctr specify the center of the rectangle.

  • xlen specifies the width of the rectangle, which is its length along the x-axis before rotation.

  • ylen specifies the height of the rectangle, which is its length along the y-axis before rotation.

  • yaw specifies the rotation angle in degrees. The rotation is clockwise-positive around the center of the bounding box.

Square rectangle rotated by -30 degrees.

Cuboid

Defined in spatial coordinates as an M-by-9 numeric matrix with rows of the form [xctr yctr zctr xlen ylen zlen xrot yrot zrot], where:

  • M is the number of cuboids.

  • xctr, yctr, and zctr specify the center of the cuboid.

  • xlen, ylen, and zlen specify the length of the cuboid along the x-axis, y-axis, and z-axis, respectively, before rotation.

  • xrot, yrot, and zrot specify the rotation angles of the cuboid around the x-axis, y-axis, and z-axis, respectively. The xrot, yrot, and zrot rotation angles are in degrees about the cuboid center. Each rotation is clockwise-positive with respect to the positive direction of the associated spatial axis. The function computes rotation matrices assuming ZYX order Euler angles [xrot yrot zrot].

The figure shows how these values determine the position of a cuboid.

Crop window, specified as a four-element vector or an images.spatialref.Cuboid object.

When you specify bboxA as a rectangular input, the cropping window must be a four-element vector in the format [x,y,width,height].

When you specify bboxA as a cuboid, the cropping window must be an images.spatialref.Cuboid object.

Overlap threshold, specified positive scalar less than or equal to 1. The amount of overlap between transformed boxes and the area defined by the output view is defined as:

area(bbox intersect bounding rectangle) / area(bbox)

  • bounding rectangle — Defined by the input spatial reference object, ref.

  • bbox — Result of transforming boxes in bboxA.

If the computed overlap value is greater than the value of the threshold property, then the transformed boxes are clipped to the bounding rectangle border. Otherwise, the boxes are discarded. Lowering the threshold can result in parts of the object getting discarded.

Output Arguments

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Warped bounding boxes, returned as an M2-by-N matrix of M2 bounding boxes. The number of bounding boxes returned is less than the number of bounding boxes in the input. Each row, M2, of the matrix defines one bounding box of the same type as the input bboxA.

Indices, returned as a vector of integers. The indices indicate which bounding boxes in the input, bboxA, are within the cropping window.

Introduced in R2019b