TIFF images: How 16bits-RGBA image pixels are interlaced?

Question

I\'m trying to deal with 16-bits per channel RGBA TIFF images through C language, I could not find a lot of information about 16-bits images in the specifications.

I

Answer 1

the TIFFReadRGBAImage high-level interface will always read the image with a precision of 8 bit per sample.

In order to read a 16bit per channel image without loosing the precision, you could use TIFFReadScanline directly and read the correct amount of data according to SamplesPerPixel and BitsPerSample. But this would only work if the image is stored in strips (not tiles which has been introduced in TIFF 6.0) and there must be only one row in each compressed strip (if the image is compressed).

If you want to handle all kind of TIFF image wihout using TIFFReadRGBAImage then you have to detect the image format and use low-level interface such as TIFFReadEncodedStrip and TIFFReadEncodedTile.

Note that the TIFF specifications are very extensive and flexible and using those low-level interfaces to handle every possible kind of image won't be an easy task so you may be better off using a higher level library than libtiff if you can.

EDIT

What you are referring to in the comment is the first part of the TIFF 6.0 specification known as Baseline TIFF

« When TIFF was introduced, its extensibility provoked compatibility problems. The flexibility in encoding gave rise to the joke that TIFF stands for Thousands of Incompatible File Formats.[9] To avoid these problems, every TIFF reader was required to read Baseline TIFF. The Baseline TIFF does not include layers, or compression with JPEG or LZW. The Baseline TIFF is formally known as TIFF 6.0, Part 1: Baseline TIFF » from Wikipedia

A Baseline TIFF does not support bit depth higher that 8 bit, so that's why in the specification of the Baseline TIFF, the value of BitsPerSample for a grayscale image can only be 4 or 8 and for a RGB image in can only be 8 bit per channel. Higher bit depth are supported as an extension to the Baseline TIFF specification and it is not required for a TIFF reader to support them.

Tiled Images is also an extension to the Baseline specification where StripOffsets, StripByteCounts, and RowsPerStrip fields is replaced by TileWidth, TileLength, TileOffsets and TileByteCounts so you can distinguish a tiled image from a stripped image by looking at the existing fields using TIFFGetField().