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Transform

The Guile-CV procedures and methods to transform images.

Procedures

im-rgba->rgb

im-rgba->gray

im-rgb->gray

im-resize

im-resize-channel

im-rotate

im-rotate-channel

im-flip

im-flip-channel

im-invert

im-invert-channel

im-transpose

im-transpose-channel

im-complement

im-clip

im-clip-channel

im-crop

im-crop-channel

im-crop-size

im-padd

im-padd-channel

im-padd-size

im-local-minima

im-local-minima-channel

im-local-maxima

im-local-maxima-channel

im-fft

im-fft-channel

im-fft-inverse

im-fft-inverse-channel

im-fcc

im-fcc-channel

im-fncc

im-fncc-channel

Procedure: im-rgba->rgb image [#:bg '(0.0 0.0 0.0)] ¶

Procedure: im-rgba->gray image [#:bg '(0.0 0.0 0.0)] ¶

Procedure: im-rgb->gray image ¶

Returns a new RGB or GRAY image.

In the RGBA case, image channels are first normalized. The new RGB channels are obtained by applying the following pseudo code algorithm:

R = (((1 - Source.A) * BG.R) + (Source.A * Source.R)) * 255.0
G = (((1 - Source.A) * BG.G) + (Source.A * Source.G)) * 255.0
B = (((1 - Source.A) * BG.B) + (Source.A * Source.B)) * 255.0

Procedure: im-resize image new-width new-height [#:i-mode 'bilinear] ¶

Procedure: im-resize-channel channel width height new-width new-height [#:i-mode 'bilinear] ¶

Returns a new image or chanbnel resized to new-width, new-height.

The interpolation mode #:i-mode, can be one of:

none

bilinear

biquadratic

bicubic

? (fixme)

Procedure: im-rotate image angle [#:i-mode 'bilinear] ¶

Procedure: im-rotate-channel channel width height angle [#:i-mode 'bilinear] ¶

Returns a new image or channel rotated by angle.

The angle is in degrees: +/-[0.0 360.0].

It is neccessary, for rotations other than multiples of 90°, to recalculate the target coordinates, since after the rotation, they might be floats. The ’next neighbor’ interpolation possible modes, #:i-mode, are:

bilinear

biquadratic

bicubic

? (fixme)

Procedure: im-flip image plane ¶

Procedure: im-flip-channel channel width height plane ¶

Returns a new image or channel flipped according to the selected plane.

Valid flipping plane values are:

hori horizontal

vert vertical

both

Procedure: im-invert image ¶

Procedure: im-invert-channel channel width height ¶

Returns a new inversed image or channel.

Calculating the inverse of an image or a channel consist of applying the exponent function, expt, to all pixel values, raising them to the power of -1.

Procedure: im-transpose image ¶

Procedure: im-transpose-channel channel width height ¶

Returns a new tranposed image or channel.

Transposing an image or a channel consist of flipping it over its main diagonal. In the transposed result, switched in size, row values are the original column values and column values are the original row values.

Procedure: im-complement image ¶

Returns a new image.

This procedure computes the mathematical complement of image, which for Guile-CV means that for each pixel of each channel, the new value is (- 255.0 pixel-value).

Procedure: im-clip image [#:lower 0.0] [#:upper 255.0] ¶

Procedure: im-clip-channel channel width height [#:lower 0.0] [#:upper 255.0] ¶

Returns a new clipped image or channel.

Clipping an image or a channel consist of replacing all pixel values below lower by the lower value and all pixel values above upper by the upper value.

Procedure: im-crop image left top right bottom ¶

Procedure: im-crop-channel channel width height left top right bottom [#:new-w #f] [#:new-h #f] ¶

Returns a new image, resulting of the crop of image at left, top, right and bottom.

Procedure: im-crop-size width height left top right bottom ¶

Returns a list, (new-width new-height).

Given the original image width and height, this procedure checks that left, top, right and bottom are valid and return a list, (new-width new-height), otherwise, it raises an error.

Procedure: im-padd image left top right bottom [#:color '(0.0 0.0 0.0)] ¶

Procedure: im-padd-channel channel width height left top right bottom [#:new-w #f] [#:new-h #f] [#:value 0.0] ¶

Returns a new image or channel, respectively padding image or channel by left, top, right and bottom pixels initialized respectively to color or value. Note that when im-padd is called upon a GRAY image, color is reduced to its corresponding gray value:

(/ (reduce + 0 color) 3)

Procedure: im-padd-size width height left top right bottom ¶

Returns a list, (new-width new-height).

Given the original image width and height, this procedure checks that left, top, right and bottom are >= 0 and return a list, (new-width new-height), otherwise, it raises an error.

Procedure: im-local-minima image [#:threshold +float-max+] ¶

Procedure: im-local-maxima image [#:threshold (- +float-max+)] ¶

Procedure: im-local-minima-channel channel width height [#:threshold +float-max+] ¶

Procedure: im-local-maxima-channel channel width height [#:threshold (- +float-max+)] ¶

All local mnima and maxima related procedures also accept the following additional optional keyword arguments: [#:con 8] [#:marker 1.0] [#:borders? #f] [#:plateaus? #f] [#:epsilon 1.0e-4]

Returns a new image or channel.

Finds the local minima or maxima in image or channel. Local minima or maxima are defined as ‘points’ that are not on the borders (unless #:borders? is #t), and whose values are lower or higher, respectively, then the values of all direct neighbors. In the result image or channel, these points are marked using the #:marker value (all other pixels values will be set to 0).

By default, the algorithm uses 8-connectivity to define a neighborhood, which can be changed passing the optional keyword argument #:con, which can be either 4 or 8.

The #:threshold optional keyword argument can be used to discard minima and maxima whose (original pixel) value is not below or above the threshold, respectively. Both default values depend on +float-max+, which is defined (and so is +float-min+) using the corresponding limit value as given by the C float.h header.

The #:plateaus? optional keyword argument can be used to allow regions of ‘constant’ (original pixel) value whose neighbors are all higher (minima) or lower (maxima) than the value of the region. Tow pixel values are considered part of the same region (representing the same ‘constant’ value) if the absolute value of their difference is <= to #:epsilon.

Notes:

• If you want to know how many minima or maxima were found, use im-reduce upon the result;
• If you are interested by the original minima or maxima pixel values, Use im-times between the original image and the result.

Procedure: im-fft image ¶

Procedure: im-fft-channel channel width height ¶

Returns two images or channels.

Computes and returns the Fast Fourier Transform²¹ of the image or channel. It returns two values, images or channels: the first contains the real part and the second the imaginary part of the transformation.

Procedure: im-fft-inverse real imaginary ¶

Procedure: im-fft-inverse-channel real-channel imaginary-channel width height ¶

Returns two images or channels.

Computes and returns the inverse Fast Fourier Transform given its real and imaginary or real-channel and imaginary-channel parts. It returns two values, images or channels: the first contains the real part and the second the imaginary part of the inverse transformation.

Procedure: im-fcc image mask ¶

Procedure: im-fncc image mask ¶

Procedure: im-fcc-channel i-chan m-chan width height m-width m-height ¶

Procedure: im-fncc-channel i-chan m-chan width height m-width m-height ¶

Returns an image or a channel.

Computes and returns the Fast Cross Correlation or Fast Normalized Cross Correlation between image and a (usually smaller) mask²², using the Fast Fourier Transform²³.