Post by island on Mar 9, 2016 3:11:58 GMT -5
The link fotoforensics.com/tutorial-ela.php
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Tutorial: Error Level Analysis
Error Level Analysis (ELA) permits identifying areas within an image that are at different compression levels. With JPEG images, the entire picture should be at roughly the same level. If a section of the image is at a significantly different error level, then it likely indicates a digital modification.
What To Look For
ELA highlights differences in the JPEG compression rate. Regions with uniform coloring, like a solid blue sky or a white wall, will likely have a lower ELA result (darker color) than high-contrast edges. The things to look for:
Edges Similar edges should have similar brightness in the ELA result. All high-contrast edges should look similar to each other, and all low-contrast edges should look similar. With an original photo, low-contrast edges should be almost as bright as high-contrast edges.
Textures Similar textures should have similar coloring under ELA. Areas with more surface detail, such as a close-up of a basketball, will likely have a higher ELA result than a smooth surface.
Surfaces Regardless of the actual color of the surface, all flat surfaces should have about the same coloring under ELA.
Look around the picture and identify the different high-contrast edges, low-contrast edges, surfaces, and textures. Compare those areas with the ELA results. If there are significant differences, then it identifies suspicious areas that may have been digitally altered.
Resaving a JPEG removes high-frequencies and results in less differences between high-contrast edges, textures, and surfaces. A very low quality JPEG will appear very dark.
Scaling a picture smaller can boost high-contrast edges, making them brighter under ELA. Similarly, saving a JPEG with an Adobe product will automatically sharpen high-contrast edges and textures, making them appear much brighter than low-texture surfaces.
Applying ELA
JPEG images use a lossy compression system. Each re-encoding (resave) of the image adds more quality loss to the image. Specifically, the JPEG algorithm operates on an 8x8 pixel grid. Each 8x8 square is compressed independently. If the image is completely unmodified, then all 8x8 squares should have similar error potentials. If the image is unmodified and resaved, then every square should degrade at approximately the same rate.
ELA saves the image at a specified JPEG quality level. This resave introduces a known amount of error across the entire image. The resaved image is then compared against the original image.
If an image is modified, then every 8x8 square that was touched by the modification should be at a higher error potential than the rest of the image. Modified areas will appear with a higher potential error level.
JPEG Encoding Blocks
JPEG stores colors using the YUV color space. 'Y' is the luminance, or gray-scale intensity of the image, 'U' and 'V' are the chrominance-blue and chrominance-red color portions. For displaying, the JPEG decoder converts the image from YUV to RGB.
JPEG always encodes luminance with an 8x8 grid. However, chrominance may be encoded using 8x8, 8x16, 16x8, or 16x16. The chrominance subsampling is a JPEG encoding option. Depending on the selected chrominance subsampling, each 8x8, 8x16, 16x8, 16x16 grid will be independently encoded.
The link has Image examples to learn from
Evaluating ELA
With ELA, every grid that is not optimized for the quality level will show grid squares that change during a resave. For example, digital cameras do not optimize images for the specified camera quality level (high, medium, low, etc.). Original pictures from digital cameras should have a high degree of change during any resave (high ELA values). Each subsequent resave will lower the error level potential, yielding a darker ELA result. With enough resaves, the grid square will eventually reach its minimum error level, where it will not change anymore.
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