[Octave-bug-tracker] [bug #56796] [octave forge] (image) imtransform changes the contrast of image when using bilinear interpolation

[Hartmut]

Follow-up Comment #8, bug #56796 (project octave):

@Avinoam: Here are the source files for my short tests leading to my comment
#6:

1. the image file "raw_image.jpg". This file is made with the Gimp image
manipulation software by cutting out the upper left corner of the OP and
removing the colored "a" label.

2. The following Octave code:

clear, close all

im = imread('raw_image.jpg');
imshow(im)

in_p = [0, 0; 1, 0; 0, 1];
factor = 2.4      % you can use also different values here, this one is
random
out_p = factor.*in_p;

T = cp2tform(in_p, out_p, 'affine');  %% uses interp2 from core Octave for
interpolation

im2 = imtransform(im, T);
figure
subplot(2,2,1)
imshow(im)
subplot(2,2,2)
imshow(im2)

im3 = imtransform(im, T, 'bicubic');
subplot(2,2,4)
imshow(im3)
                             % -> results with image 2.12.0 release
mean_orig = mean(im(:))      % -> 224.8
mean_bilinear = mean(im2(:)) % -> 229.9 = too bright
mean_bicubic = mean(im3(:))  % -> 224.6


This could point to a bug on core Octave's interp2 function. Or it could just
mean nothing at all, because nobody claims that interpolation preserves mean
values (or am I wrong with this assumption?)

(file #51911)
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Additional Item Attachment:

File name: raw_image.jpg                  Size:36 KB
    <https://file.savannah.gnu.org/file/raw_image.jpg?file_id=51911>



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