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.
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