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2010.05.11 19:15 "Re: Combining multiple G4 images into a single output image", by Olivier Paquet
2010/5/11 Oliver Geisen <oliver.geisen@kreisbote.de>:
> bit shifting. I'm not really shure what it means to do such massive
> shifting operations. It really depends on the size of the images to be
It really means nothing if you put a minimal amount of work into the
implementation (ie. process more than one bit at a time). You are far
more likely to be limited by file I/O and decompression/compression.
For example, on a 2.2 GHz athlon 64 I can shift almost 1 GB/s in-place
(a little less if copying) with this basic code:
unsigned *buffer = ...
unsigned prev = 0;
for( unsigned i = 0; i < n; ++i )
{
unsigned b = buffer[i];
buffer[i] = prev | (b >> rshift);
prev = b << (32 - rshift);
}
which by the way does not work if rshift == 0 or rshift >= 32. You
will need to do a little patching on the edges of the image but it
won't matter in the overall run time. By the way, for those edges, you
can generate a mask for the leftmost (or rightmost) n bits easily
with:
unsigned rightmost = (1u << n) - 1u; /* for n 0 to 31 */
unsigned leftmost = INT_MIN >> (n - 1); /* for n 1 to 32 */
That will let you copy only some bits in a word easily. Note that all
that code assumes a 32-bit unsigned type. You should use whatever
typedef your environment provides to ensure you get that (eg. uint32_t
or similar).
Olivier