test.lua

-- Exhibits differences between torch's image.scale and opencv's resize
-- function.

local cv = require 'cv' -- https://github.com/VisionLabs/torch-opencv
require 'cv.imgcodecs'
require 'cv.imgproc'
require 'image'

local path = './foobar.png'
-- RGB, shape channels * height * width
local torch_image = image.load(path, 3, 'byte')
-- BGR, shape height * width * channels
local opencv_image = cv.imread({path, cv.IMREAD_COLOR}):byte()

-- Ensure the two images are equal. This should, and does, pass.
assert(torch.all(torch.eq(opencv_image[{{}, {}, 1}], torch_image[3])) and
       torch.all(torch.eq(opencv_image[{{}, {}, 2}], torch_image[2])) and
       torch.all(torch.eq(opencv_image[{{}, {}, 3}], torch_image[1])))

torch_image = image.scale(torch_image, 256, 256)
opencv_image = cv.resize({opencv_image, {256, 256}})

if not (torch.all(torch.eq(opencv_image[{{}, {}, 1}], torch_image[3])) and
        torch.all(torch.eq(opencv_image[{{}, {}, 2}], torch_image[2])) and
        torch.alltorch.eq(opencv_image[{{}, {}, 3}], torch_image[1]))
    then
    -- This prints, although it shouldn't.
    print('The two images are not equal!')
end

-- Save torch and opencv images.
image.save('torch-resized.png', torch_image)
-- Convert opencv to be RGB, channels*width*height shape, and save it.
local opencv_clone = opencv_image:clone():permute(3, 1, 2) -- h*w*c -> c*h*w
opencv_clone[1] = opencv_image[{{}, {}, 3}]
opencv_clone[3] = opencv_image[{{}, {}, 1}]
image.save('opencv-resized.png', opencv_clone)
print('See images at torch-resized.png and opencv-resized.png')

The differences are easy to spot if you open the two images in two tabs and flip between them.

Original image (rgb16-2x1.png):

Torch resized (torch-resized.png):

OpenCV resized (opencv-resized.png):