SAMPLING

Free archive of digital sampling test images

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The main purpose of this dataset is providing natural images to test resampling algorithms (i.e. interpolation, zooming, enlargment and superresolution).
This dataset uniquely provides a base of 40 RBG 2400x2400 reference images with a bit-depth of 16bpp and high dynamic range (HDR).
The reference images were processed to obtain different sub-resolutions and variation with 8bpp and grayscale, for a total of more 220K images.

The test images were chosen to be square for simplicity and symmetry. The value 2400 was chosen because it is approximately the vertical resolution of a common 10 Megapixel SLR Camera, it is the vertical resolution of standard WQUXGA displays and can be divided by 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20 and 24 (reduction factors).

Since currently there are no 16bpp capable photographic cameras in the consumer market, to acquire 16bpp natural images an approach involving multiple exposures was used.

This archive contain 40 16bpp RGB "uncropped" images with a resolution of 2448x2448 pixels. A border of 24 pixels was further cropped from these images to get the reference resolution of 2400x2400 pixels, so the top left pixel in the reference image correspond to the pixel in position (25,25) of the uncropped image.

For each reference image, additional resized images were generated. The intensity value of each pixel in the resized image was obtained by averaging the values of RxR sub-arrays (where R is the reducing factor). The reduction factors were chosen to perfectly divide the 2400 width and height values of the reference images:

Reduction
factor
Resolution
[Width x Height]
Variations
(shifts)
1 2400x2400 1
2 1200x1200 9
3 800x800 25
4 600x600 49
5 480x480 81
6 400x400 121
8 300x300 225
10 240x240 361
12 200x200 529
15 160x160 841
16 150x150 961
20 120x120 1521
24 100x100 2209

The resized images were always generated starting from the uncropped image but always cosidering a sub-sampling mask of 2400x2400 pixels at the time.
For each resizing resolution, all the possible variations obtained by moving (shifting) the sub-sampling mask were saved.
The image names contain a shifting indicator, a string composed by two groups of one letter and two numbers. The letters indicates the direction of the shift (C: center, T: top, B: bottom, L: left, R: right) while the numbers indicate the number of pixels. The images aligned with the reference image, or the pixel in position (25,25) of the uncropped image, are identified by the "C00C00" shifting indicator.


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