bboxwarp
Apply geometric transformation to bounding boxes
Syntax
Description
bboxB = bboxwarp(
transforms bounding boxes in bboxA
,tform
,ref
)bboxA
according to the geometric
transformation defined by tform
. Bounding boxes can be axisaligned
rectangles, rotated rectangles, or cuboids. The spatial reference object,
ref
, defines the output view into which the boxes are
transformed.
[___] = bboxwarp(___,'OverlapThreshold',
additionally sets an overlap threshold.threshold
)
Examples
Transform Images and Corresponding Bounding Boxes
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" ];
Define a transform to horizontally flip and translate the image.
tform = affine2d([1 0 0; 0 1 0; 50 50 1]);
Create an output view for imwarp
.
rout = affineOutputView(size(I),tform);
Warp the image.
J = imwarp(I,tform,'OutputView',rout);
Warp the boxes.
[bboxB,indices] = bboxwarp(bboxA,tform,rout); labelsB = labelsA(indices);
Display the results.
annotatedI = insertObjectAnnotation(I,'Rectangle',bboxA,labelsA); annotatedJ = insertObjectAnnotation(J,'Rectangle',bboxB,labelsB); figure montage({annotatedI, annotatedJ})
Input Arguments
bboxA
— Bounding boxes
Mby4 matrix  Mby5 matrix  Mby9 matrix  nonsparse numeric
Bounding boxes, specified as an Mby4, Mby5, or Mby9 nonsparse numeric matrix of M bounding boxes. Each row, M, of the matrix defines a bounding box as either an axisaligned rectangle, a rotate rectangle, or a cuboid. The table below describes the format of the bounding boxes.
Bounding Box  Description 

Axisaligned rectangle 
Defined in pixel coordinates as an Mby4 numeric matrix with rows of the form [x y w h], where:

Rotated rectangle 
Defined in spatial coordinates as an Mby5 numeric matrix with rows of the form [xctr yctr xlen ylen yaw], where:

Cuboid 
Defined in spatial coordinates as an Mby9 numeric matrix with rows of the form [xctr yctr zctr xlen ylen zlen xrot yrot zrot], where:
The figure shows how these values determine the position of a cuboid. 
tform
— Geometric transformation
affine2d
object (default)
Geometric transformation, specified in an affine2d
object. bboxwarp function supports only scale, rotation, and
translation affine transformations.
ref
— Spatial reference
imref2d
object
Spatial reference, specified as an imref2d
object returned by the imwarp
or affineOutputView
function. The object
defines the output view to transform boxes. Boxes that are transformed completely
outside of the output view defined by ref
are
discarded.
[J,rout] = imwarp(I,tform); [bboxB,indices] = bboxwarp(bboxA,tform,rout);
rout = affineOutputView(size(I),tform) J = imwarp(I,tform,'OutputView',rout); [bboxB,indices] = bboxwarp(bboxA,tform,rout);
threshold
— Overlap threshold
1 (default)  positive scalar less than or equal to 1
Overlap threshold, specified as a 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)
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
bboxB
— Warped bounding boxes
M2byN matrix  nonsparse numeric
Warped bounding boxes, returned as an M2byN
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
.
See Also
imcrop
 bboxcrop
 bboxresize
 imwarp
 bboxerase
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