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I have a really limited knowledge of photography, so, bearing that in mind, here is my problem.

I downloaded an IMG file with a separate label file from NASA PDS because I wanted to visualise the original image without the loss of quality of JPEG. So I used IMG to PNG to convert the raw file to PNG. But when I opened it and zoomed-in I noticed that there is no visible difference from the JPEG version. When I zoom, the level of detail is the same and pixels become confused (noise). Is this due to the limits of the original raw data or am I doing something wrong?

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  • \$\begingroup\$ i forgot to say that the IMG file was taken from curiosity mastcam and it was 4,5 mb \$\endgroup\$
    – Gaetano
    Commented Nov 24, 2015 at 11:51
  • \$\begingroup\$ 1 person thought this question was well researched. CAN THAT PERSON EXPLAIN THE QUESTION TO ME? Also for Nasa stuff we use FITS format, not PNG. fits.gsfc.nasa.gov/fits_libraries.html \$\endgroup\$
    – Alec Teal
    Commented Nov 27, 2015 at 0:14

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For photographic images and when a not too high level of compression is used, the loss of quality in the JPEG format is negligible and invisible. You'll pretty much only be able to notice it by directly comparing individual pixels around sharp edges or in very smooth color gradients.

This is why JPEG is so popular. If it always resulted in noticeable loss of quality, people would not be using it so much.

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  • \$\begingroup\$ Thanks Michael...and what about JPEG artifacts? from what i understand RAW or PNG should not show artifacts that compression can create... \$\endgroup\$
    – Gaetano
    Commented Nov 24, 2015 at 16:39
  • \$\begingroup\$ @Gaetano What Michael is saying that at low levels of compression, these artifacts are minor and hard or impossible to identify by eye. See this and this for some more background. \$\endgroup\$
    – mattdm
    Commented Nov 24, 2015 at 20:17
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    \$\begingroup\$ To summarise, JPEG allows you to set varying levels of quality when compressing. When using a high enough quality level JPEG should be visually indistinguishable from lossless compression like PNG. Another way of saying that is that at high enough quality levels, JPEG artifacts should not be visible to the human eye. \$\endgroup\$
    – thomasrutter
    Commented Nov 24, 2015 at 23:34
  • \$\begingroup\$ @thomasrutter: it also depends on the image content. Even at the maximum quality level, you will see JPEG artifacts e.g. in the text of a screenshot of this webpage. But in photos (which are naturally "noisy" and tend not to have sharp edges) they disappear. \$\endgroup\$
    – Michael Borgwardt
    Commented Nov 25, 2016 at 11:32
  • \$\begingroup\$ @MichaelBorgwardt At a high enough quality level, you shouldn't be able to see JPEG artifacts in any content, even text. Here is a JPEG of this text. The artifacts will be there, but should not be visible without zooming to the individual pixel level and/or measuring with tools. \$\endgroup\$
    – thomasrutter
    Commented Nov 29, 2016 at 0:36
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For the following answer: when referring to a file that contains the contents of a disk image file we will use "IMG". When referring to a file that contains a picture in an actual visual image format, we will use "image".

If the information in your question is taken at face value

It appears to be mainly due to the limited resolution of the original image file contained in the IMG file, or to the resolution your application used when it converted the IMG file to an image file, be it .png or .jpeg. 4.5MB is not very large for a raw image file. It is quite possible the image file contained in the IMG file was already in a compressed format such as jpeg.

IMG is not a visual image file format per se. It is a format for creating an IMAGE (or copy) of a digital storage drive.

If your question is really about converting files downloaded from NASA's Planetary Data System

The 4.5MB file you downloaded appears to have been the "IMG2PNG" application used to convert image files from NASA's Planetary Data Systems network to .png files. That is the size of the file of the convertor written by Bjorn Jonsson and popularly available on the internet.

If the image file you downloaded was also 4.5MB, then it was not a very high resolution file to begin with. It is likely that the .jpeg version released by NASA was minimally compressed and contained just as much visual information that can be perceived by human vision as the original file you downloaded from PDS.

Also note that when used in the context of imagery from a NASA probe sent to investigate other bodies in the Solar System, referring to a "raw" image does not mean the same thing as when we refer to a "raw" image from a typical digital camera. It just means that NASA has released an image to the public without it first being highly processed and scientifically calibrated to correct for things such as optical distortion of the camera system on the probe, color variations, etc. Most "raw" images released to the public by NASA that are of pictures produced by planetary probes are released in the .jpeg format. Please see this link for more.

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