im_global_balance, im_global_balancef - perform global mosaic balancing
on an image
im_global_balance( IMAGE *in, IMAGE *out, double gamma )
im_global_balance_float( IMAGE *in, IMAGE *out, double gamma )
These functions takes an image assembled with the mosaicing functions
(im_*merge*(), im_*mosaic*()), take it apart, and reassemble it,
globally optimising the image balance. This is useful for assembling
image mosaics from sources where the exposure is uncontrolled and may
vary from tile to tile --- such as video, or photographic sources.
The function finds a set of factors, one for each of the input images,
and scales each image by its factor before reassembling. The factors
are chosen so as to minimise the average grey-level difference between
neighboring images at their overlaps. Trivial overlaps (where the
width and height of the overlap are both less than 20 pixels) are
The gamma parameter is the gamma of the image input system. It is used
during brightness adjustment. Set to 1.0 to disable gamma, to 1.6 for a
typical IR vidicon camera, or 2.3 for a typical video camera.
It relies on information left by the mosaicing functions in ".desc"
files. If the ".desc" file of the input image has been corrupted, or is
strangely complicated, or if any of the original input images have been
moved or deleted, the function can fail.
The function will fail for mosaics larger than about 7 by 7 frames,
since it will run out of file descriptors (UNIX sets a limit of 256 per
process). To balance larger mosaics, just assemble them in 7x7
sections, balancing and saving each part in turn, before loading,
assembling and balancing the final image. The function can also fail
if there are significant mosaicing errors.
im_global_balancef() works as im_global_balance(), but outputs a float
rather than a uchar image. This lets you adjust the range of the image
manually, if the automatically-found scales are causing burn-out.
All functions return 0 on success and -1 on error.
Birkbeck College and the National Gallery, 1991 - 1995.
13 May 1991 IM_GLOBAL_BALANCE(3)