NASA / JPL-Caltech / Cornell / color mosaic by Emily Lakdawalla
'Raw' and archived Mars Exploration Rover images: what's the difference?
This blink animation compares two versions of a photo captured by Opportunity of the meteorite named Mackinac Island on sol 2037. One is "raw" -- the version made available on the Internet within hours of its return to Earth -- and the other is "archived" -- the version made available through NASA's Planetary Data System more than six months later. Before they are posted to the Internet, two operations are performed on the raw images. First, the contrast is stretched, so that the brightest few percent of pixels are set to be totally white, and the darkest few percent of pixels are set to be totally black. As a result of the contrast stretch, the highlight glints off of the metallic surface of the meteorite are lost, since brightest areas of adjacent rock are set to similar pixel values; at the same time, detail visible in the shadows of the archived image are lost in the raw version. Second, the images are converted to JPEG format, which reduces their file size substantially but also reduces the quality of the images. The JPEG conversion tends to add noise near sharp edges, resulting in a loss of detail.