How to use fread to read little endian data?

Question

James McGinley 2019-4-4

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https://ww2.mathworks.cn/matlabcentral/answers/454452-how-to-use-fread-to-read-little-endian-data

评论： Cris LaPierre 2019-4-4

I have a .jsf file that I am trying to read which starts off with a 16 byte common message header followed by a 240 byte message 80 specific header and then the data that I am interested in. This data changes for each row of the file. EX) row 7 = 4496 bytes, row 8 = 18704 bytes. If I want to read this data row by row, would I need to use a fseek to position myself 256 bytes into the data to start reading the 4496 bytes or the 18704 bytes and then loop it until the end of the file? The data I am reading will then be fed through image().

FileID = fopen('FileName', 'rb')
while ~feof(FileID)
    tline = fgetl(FileID)
    Start = fseek( tline, 256, 'cof')
    Read = fread(Start)
end

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Answer 1

Cris LaPierre 2019-4-4

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https://ww2.mathworks.cn/matlabcentral/answers/454452-how-to-use-fread-to-read-little-endian-data#answer_369108

编辑：Cris LaPierre 2019-4-4

在 MATLAB Online 中打开

I would seek first (outside the while loop).

Depending on your data, fgetl may not make sense (files read by bytes don't have end-of-lines). I would think just using fread would do it. You do need to know what the precision is. For example, this will read 4 bytes as a float32.

data1 = fread(fid,1,'float32');

See this post for more details.

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James McGinley 2019-4-4

Okay then fseek is the way I am going. I have to still look into the percision of the data, the message I have received from the creator of the data said : you skip over the 16 byte header and 240 byte header, and then read the binary data assuming little endian format (8 least significant bits is 1st byte of each 16-bit word, 8 most significant bits is 2nd byte of each 16-bit word). (sorry this is all new to me)

The reason I thought of putting the image(data1) inside the loop is because the data is of a sonar that is moving over time, so it should be like a video of the scanning being done. In the end I am only interested in the objects found on the sonar.