2007.07.14 12:39 "Re: [Tiff] Re: Bit shifting and rotating of TIFF images", by Oliver Geisen

Richard,

    Concerning rotating TIFF images and compositing them onto a
background image. Check out the CVS release and find the new utility
tiffcrop in the utils directory. I am working on an update to the
currently available version but the code to rotate images should work
for you. Note that rotations of 90 or 270 degrees may change the size of
the buffer due to the handling of the padding bytes at the end of each
scanline.

I found your code, compiled it, and it worked well, good work!

But allow me some questions/conclusions about that: What i want to do is, image rotation by 180 degrees only. The whole point in processing images with libtiff is that one have only a limited part of the image inside a memoryblock to work with. This is an advantage because some images (our own also) will get very big.

Special care must be taken if imagedata is bilevel (1 bit per pixel) but converting all the data into an 8 bit pixel, process it and reconvert it to 1 bit pixels seems to be clumsy.

When only concerning bilevel images with 180 degree rotation, how could this be done?

Here is my approach:
(for striped images)

• open input tiff, determine number of strips and stripsize (in bytes)
• create output tiff with almost the same IFD
• create strip buffer for output tiff (equal size of input tiff)
• loop through each strip of input tiff, from last to first

• determine number of bytes per scanline (= StripSize/RowsPerStrip)
• loop though each byte in scanline, from last to first

• read byte value (= 8 Pixels)
• "rotate" the value by using a lookup-table (each value from 0-255 has an corresponding value with bits swapped)
• write byte into the corresponing line of the output buffer

     - write rotated strip to output tiff
   - close tiff

(for tiled images)
   ? No clue?

I look around to solve the bit-swapping problem and the best solution i found was a lookup table like:

idx   1 = 1000 0000
idx   2 = 1100 0000
...
idx 254 = 0000 0001

To get the bit-swapped result just use the current pixel-data as an index into this table.

   cswap = lookup[c];

One BIG problem is that if the width of an image (scanline) is not a multiple of 8, there is a remaining number of bits on the right side. If one rotate it by 180 deg. the remaining bits are on the left side now. So just swapping the byte value is not enough. Assume an image which is 10 pixels in width, this would be represented by 2 bytes. Now swap those bytes and the bits inside to achieve rotation. The image has virtually shiftet it's starting (left) position by 6 pixels (bits).

So after byte and bit swapping there must be also be a bit shifting by the amount of remaining bits (1..7).

This is a challenge, even if perfomance does not matter ;-)

Oliver Geisen
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