{"id":364,"date":"2011-03-08T23:29:46","date_gmt":"2011-03-09T04:29:46","guid":{"rendered":"https:\/\/blogs.mathworks.com\/steve\/2011\/03\/08\/tips-for-reading-a-camera-raw-file-into-matlab\/"},"modified":"2019-10-29T16:31:36","modified_gmt":"2019-10-29T20:31:36","slug":"tips-for-reading-a-camera-raw-file-into-matlab","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/steve\/2011\/03\/08\/tips-for-reading-a-camera-raw-file-into-matlab\/","title":{"rendered":"Tips for reading a camera raw file into MATLAB"},"content":{"rendered":"
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An academic colleague asked me recently how to read the sensor data (in Bayer<\/a> or color filter array form) from a Nikon raw camera file (an NEF file) into MATLAB. We figured out a way to do it, and I\r\n thought I'd pass it along.\r\n <\/p>\r\n

I should caution you, though, that there are multiple steps involved, some of which are \"advanced maneuvers.\"<\/p>\r\n

First, I suggested that we try using the free Adobe DNG Converter<\/a> program to convert the NEF file to a DNG (Digital Negative) file. After some trial-and-error we found that it's necessary\r\n to tell DNG Converter to do the conversion \"uncompressed.\" Here are some screenshots of the relevant dialogs.\r\n <\/p>\r\n

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<\/p>\r\n

Under the covers, a DNG file is a very specialized kind of TIFF file. However, we can't read the raw sensor data using imread<\/tt>. If you call imread<\/tt> on the DNG file, it will just give you back the thumbnail image. Instead, we have to make use of the MATLAB Tiff<\/tt> class<\/a>. This class offers low-level access to the \"guts\" of a TIFF file. To use it successfully, it helps to have some familiarity\r\n with how TIFF files work.\r\n <\/p>\r\n

But first let's poke at a sample DNG file using imfinfo<\/tt>.\r\n <\/p>

info = imfinfo('books.dng'<\/span>)<\/pre>
\r\ninfo = \r\n\r\n                     Filename: '\\\\mathworks\\home\\eddins\\files\\I\\ipblog_material\\2011\\books.dng'\r\n                  FileModDate: '08-Mar-2011 12:01:20'\r\n                     FileSize: 25079002\r\n                       Format: 'tif'\r\n                FormatVersion: []\r\n                        Width: 256\r\n                       Height: 170\r\n                     BitDepth: 24\r\n                    ColorType: 'truecolor'\r\n              FormatSignature: [73 73 42 0]\r\n                    ByteOrder: 'little-endian'\r\n               NewSubFileType: 1\r\n                BitsPerSample: [8 8 8]\r\n                  Compression: 'Uncompressed'\r\n    PhotometricInterpretation: 'RGB'\r\n                 StripOffsets: 133234\r\n              SamplesPerPixel: 3\r\n                 RowsPerStrip: 170\r\n              StripByteCounts: 130560\r\n                  XResolution: []\r\n                  YResolution: []\r\n               ResolutionUnit: 'None'\r\n                     Colormap: []\r\n          PlanarConfiguration: 'Chunky'\r\n                    TileWidth: []\r\n                   TileLength: []\r\n                  TileOffsets: []\r\n               TileByteCounts: []\r\n                  Orientation: 1\r\n                    FillOrder: 1\r\n             GrayResponseUnit: 0.0100\r\n               MaxSampleValue: [255 255 255]\r\n               MinSampleValue: 0\r\n                 Thresholding: 1\r\n                       Offset: 8\r\n                         Make: 'NIKON CORPORATION '\r\n                        Model: 'NIKON D90 '\r\n                     Software: 'Ver.1.00 '\r\n                     DateTime: '2011:03:06 14:45:58 '\r\n                      SubIFDs: {2x1 cell}\r\n                          XMP: [1x7309 char]\r\n                DigitalCamera: [1x1 struct]\r\n                   DNGVersion: [4x1 double]\r\n           DNGBackwardVersion: [4x1 double]\r\n            UniqueCameraModel: 'Nikon D90 '\r\n                 ColorMatrix1: [9x1 double]\r\n                 ColorMatrix2: [9x1 double]\r\n                AnalogBalance: [3x1 double]\r\n                AsShotNeutral: [3x1 double]\r\n             BaselineExposure: 0.2500\r\n                BaselineNoise: 1\r\n            BaselineSharpness: 1\r\n           LinearResponseUnit: 1\r\n           CameraSerialNumber: '3276464 '\r\n                     LensInfo: [4x1 double]\r\n                  ShadowScale: 1\r\n               DNGPrivateData: [121588x1 double]\r\n       CalibrationIlluminant1: 17\r\n       CalibrationIlluminant2: 21\r\n           AliasLayerMetadata: [16x1 double]\r\n          OriginalRawFileName: 'books.NEF '\r\n                  UnknownTags: [15x1 struct]\r\n\r\n<\/pre>
You can see that imfinfo<\/tt> thinks that this file is a 256-by-170 truecolor image. But as I mentioned above, that's just a thumbnail image. The data\r\n      we want is hiding in something called a \"SubIFD.\"\r\n   <\/p>
info.SubIFDs{1}<\/pre>
\r\nans = \r\n\r\n                     Filename: '\\\\mathworks\\home\\eddins\\files\\I\\ipblog_material\\2011\\books.dng'\r\n                  FileModDate: '08-Mar-2011 12:01:20'\r\n                     FileSize: 25079002\r\n                       Format: 'tif'\r\n                FormatVersion: []\r\n                        Width: 4310\r\n                       Height: 2868\r\n                     BitDepth: 16\r\n                    ColorType: 'CFA'\r\n              FormatSignature: [73 73 42 0]\r\n                    ByteOrder: 'little-endian'\r\n               NewSubFileType: 0\r\n                BitsPerSample: 16\r\n                  Compression: 'Uncompressed'\r\n    PhotometricInterpretation: 'CFA'\r\n                 StripOffsets: 356842\r\n              SamplesPerPixel: 1\r\n                 RowsPerStrip: 2868\r\n              StripByteCounts: 24722160\r\n                  XResolution: []\r\n                  YResolution: []\r\n               ResolutionUnit: 'None'\r\n                     Colormap: []\r\n          PlanarConfiguration: 'Chunky'\r\n                    TileWidth: []\r\n                   TileLength: []\r\n                  TileOffsets: []\r\n               TileByteCounts: []\r\n                  Orientation: 1\r\n                    FillOrder: 1\r\n             GrayResponseUnit: 0.0100\r\n               MaxSampleValue: 65535\r\n               MinSampleValue: 0\r\n                 Thresholding: 1\r\n                       Offset: 130252\r\n                CFAPlaneColor: [3x1 double]\r\n                    CFALayout: 1\r\n           LinearizationTable: [772x1 double]\r\n          BlackLevelRepeatDim: [2x1 double]\r\n                   BlackLevel: 0\r\n                   WhiteLevel: 3767\r\n                 DefaultScale: [2x1 double]\r\n            DefaultCropOrigin: [2x1 double]\r\n              DefaultCropSize: [2x1 double]\r\n              BayerGreenSplit: 0\r\n            AntiAliasStrength: 1\r\n             BestQualityScale: 1\r\n                   ActiveArea: [4x1 double]\r\n                  UnknownTags: [2x1 struct]\r\n\r\n<\/pre>
That's the good stuff! It's a 16-bit-per-sample 4310-by-2868 color filter array (CFA). Here's how to read it. I'm just going\r\n      to give the steps without explanation and refer you to the documentation for the Tiff<\/tt> class<\/a> for more information.\r\n   <\/p>
warning off<\/span> MATLAB:tifflib:TIFFReadDirectory:libraryWarning<\/span>\r\nt = Tiff('books.dng'<\/span>,'r'<\/span>);\r\noffsets = getTag(t,'SubIFD'<\/span>);\r\nsetSubDirectory(t,offsets(1));\r\ncfa = read(t);\r\nclose(t);<\/pre>
Here's a screen shot of imtool<\/tt> showing the resulting color filter array at 400% magnification. You can clearly see the Bayer array pattern.\r\n   <\/p>\r\n   
 <\/p>\r\n   
We wondered why the maximum sensor value was 768. After some investigation, we found that Nikon appears to be storing a nonlinear\r\n      quantization off the original 12-bit values. The linearization curve can be found in the DNG file as follows:\r\n   <\/p>
curve = info.SubIFDs{1}.LinearizationTable;\r\nplot(curve)<\/pre> 
So if you were going to do calculations based on this raw data, you'd probably want to linearize it back to the original 12-bit\r\n      range first using this curve.\r\n   <\/p>\r\n   
Have you used the MATLAB Tiff<\/tt> class to do advanced maneuvers with TIFF files? Let us know what you've done by posting a comment here.\r\n   <\/p>