Warper

Apply same geometric transformation to many images efficiently

Description

A Warper object applies an affine2d or projective2d geometric transformation to images with a specific size.

Creation

Syntax

w = images.geotrans.Warper(tform,inputSize)

w = images.geotrans.Warper(tform,inputRef)

w = images.geotrans.Warper(tform,inputRef,outputRef)

w = images.geotrans.Warper(sourceX,sourceY)

w = images.geotrans.Warper(___,Name,Value)

Description

w = images.geotrans.Warper(tform,inputSize) creates an image warper from the geometric transformation object tform and sets the InputSize property.

w = images.geotrans.Warper(tform,inputRef) specifies the coordinate system of the input images, inputRef.

w = images.geotrans.Warper(tform,inputRef,outputRef) specifies the coordinate system of the output image, outputRef. This syntax can be used to improve performance by limiting the application of the geometric transformation to a specific output region of interest.

w = images.geotrans.Warper(sourceX,sourceY) specifies the input image coordinates, sourceX and sourceY, required to perform the geometric transformation.

w = images.geotrans.Warper(___,Name,Value) sets the Interpolation and FillValue properties using one or more name-value pair arguments. Enclose each property name in single quotes.

For example, warper = images.geotrans.Warper(tform,size(im),'FillValue',1) specifies a fill value of 1 for pixels outside the original image.

Input Arguments

tform — Geometric transformation

affine2d object | projective2d object

Geometric transformation, specified as an affine2d or projective2d geometric transformation object.

inputRef — Referencing object associated with input image

imref2d object

Referencing object associated with the input image, specified as an imref2d spatial referencing object.

outputRef — Referencing object associated with output image

imref2d object

Referencing object associated with the output image, specified as an imref2d spatial referencing object.

sourceX, sourceY — Input image coordinates

2-D matrix

Input image coordinates, specified as a 2-D matrix the same size as the required output image. Each (x, y) index in sourceX and sourceY specifies the location in the input image for the corresponding output pixel.

Data Types: single

Properties

InputSize — Size of the input images

2-element vector of positive integers | 3-element vector of positive integers

Size of the input images, specified as a 2- or 3-element vector of positive integers.

OutputSize — Size of the first two dimensions of the output image

2-element vector of positive integers

Size of the first two dimensions of the output image, specified as a 2-element vector of positive integers.

Interpolation — Interpolation method

'linear' (default) | 'nearest' | 'cubic'

Interpolation method, specified as 'linear', 'nearest', or 'cubic'.

Data Types: char | string

FillValue — Value used for output pixels outside the input image boundaries

0 (default) | numeric scalar

Value used for output pixels outside the input image boundaries, specified as a numeric scalar. Warper casts the fill value to the data type of the input image.

Object Functions

warp	Apply geometric transformation

Examples

Apply Shear to Multiple Images

Pick a set of images of the same size. The example uses a set of images that show cells.

imds = imageDatastore(fullfile(matlabroot,'toolbox','images','imdata','AT*'));

Create a geometric transform to rotate each image by 45 degrees and to shrink each image.

tform = affine2d([ 0.5*cos(pi/4) sin(pi/4)     0;
                  -sin(pi/4)     0.5*cos(pi/4) 0;
                   0             0             1]);

Create a Warper object, specifying the geometric transformation object, tform, and the size of the input images.

im = readimage(imds,1);
warper = images.geotrans.Warper(tform,size(im));

Determine the number of images to be processed and preallocate the output array.

numFiles = numel(imds.Files);
imr = zeros([warper.OutputSize 1 numFiles],'like',im);

Apply the geometric transformation to each of the input images by calling the warp function of the Warper object.

for ind = 1:numFiles
    im = read(imds);
    imr(:,:,1,ind) = warp(warper,im);
end

Visualize the output images. (Turn off the warning message about the images being scaled for display.)

warning('off','images:initSize:adjustingMag')
montage(imr);

Tips

• If the input images are RGB images or 3-D grayscale images of size m-by-n-by-p, then warp applies the transformation to each color channel or plane p independently.

Algorithms

Warper is optimized to apply the same geometric transformation across a batch of same size images. Warper achieves this optimization by splitting the warping process into two steps: computation of the transformed coordinates (done once) and interpolation on the image (done for each image). Compared to imwarp, this approach speeds up the whole process significantly for small to medium-sized images, with diminishing returns for larger images.