Help with reading a binary array of intensity values from .img file?

I downloaded a .img file from the opportunity rover. I want to read the intensity values associated with each pixel. Currently I'm writing to a vector but I'm pretty sure I'm getting the wrong values since I'm getting a range of 0-65000. The file should just be a binary array (I removed the ascii detachable string).

There's 16 bits per pixel and I know the height and width of the image. The intensity values should be from 0-1. Does anybody have any ideas? I'm using the function 'fread' right now.

Currently, I'm doing this.

if true
  fileID = fopen("filenamepath")
  [A, count] = fread(fileID, [height, width], 'double')
end

I'm getting less data than I thought because using double is 8 bytes but each pixel is 2 bytes....thoughts?

Answer 1

[Walter Roberson]

16 bit floating point is possible but it is not common, and I think it unlikely that is what you are seeing.

https://en.wikipedia.org/wiki/Half-precision_floating-point_format is the standard and if that is what is in the file then you could use https://www.mathworks.com/matlabcentral/fileexchange/23173-ieee-754r-half-precision-floating-point-converter to convert it.

More likely is that you should use

    [temp, count] = fread(fileID, [height, width], '*uint16');
    im2double(temp);

or possibly

    [temp, count] = fread(fileID, [width, height], '*uint16') .';
    im2double(temp);

[Sgt. Wiggles]

This definitely got me some values between 0-1. I don't know if it should be half precision floating point, like you said.

The image isn't correct though. Even before conversion to double precision, I plotted the image and it's wrong so I must be misinterpreting how the data is set up. I'll keep working on it!

[Walter Roberson]

Could you post a URL for the data? What I find at http://mars.nasa.gov/mer/gallery/all/opportunity.html is all jpeg images.