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merge change from aturcotte

This commit is contained in:
John Cupitt 2011-06-23 10:55:02 +01:00
parent edbe0c99cc
commit 53492613bd
1 changed files with 26 additions and 25 deletions
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51
libvips/resample/nohalo.cpp
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@ -271,39 +271,40 @@ typedef struct _VipsInterpolateNohaloClass {
} VipsInterpolateNohaloClass;
/*
* MINMOD is an implementation of the minmod function which only needs
* two conditional moves.
* NOHALO_MINMOD is an implementation of the minmod function which
* only needs two "conditional moves."
* NOHALO_MINMOD(a,b,a_times_a,a_times_b) "returns"
* minmod(a,b). The macro parameter ("input") a_times_a is assumed to
* contain the square of a; a_times_b, the product of a and b.
*
* MINMOD(a,b,a_times_a,a_times_b) "returns" minmod(a,b). The
* parameter ("input") a_times_a is assumed to contain the square of
* a; the parameter a_times_b, the product of a and b.
* For uncompressed natural images in high bit depth (images for which
* the slopes a and b are unlikely to be equal to zero or be equal to
* each other), or chips with good branch prediction, the following
* version of the minmod function may work well:
*
* The version most suitable for images with flat (constant) colour
* areas, since a, which is a pixel difference, will often be 0, in
* which case both forward branches are likely:
* ( (a_times_b)>=0. ? ( (a_times_b)<(a_times_a) ? (b) : (a) ) : 0. )
*
* ( (a_times_b)>=0 ? 1 : 0 ) * ( (a_times_a)<=(a_times_b) ? (a) : (b) )
* In this version, the forward branch of the second conditional move
* is taken when |b|>|a| and when a*b<0. However, the "else" branch is
* taken when a=0 (or when a=b), which is why the above version is not
* as effective for images with regions with constant pixel values (or
* regions with pixel values which vary linearly or bilinearly) since
* we apply minmod to pairs of differences.
*
* For natural images in high bit depth (images for which the slopes a
* and b are unlikely to be equal to zero or be equal to each other),
* we recommend using
* The following version is more suitable for images with flat
* (constant) colour areas, since a, which is a pixel difference, will
* often be 0, in which case both forward branches are likely. This
* may be preferable if "branch flag look ahead" does not work so
* well.
*
* ( (a_times_b)>=0 ? 1 : 0 ) * ( (a_times_b)<(a_times_a) ? (b) : (a) )
* ( (a_times_b)>=0. ? ( (a_times_a)<=(a_times_b) ? (a) : (b) ) : 0. )
*
* instead. With this second version, the forward branch of the second
* conditional move is taken when |b|>|a| and when a*b<0. However, the
* "else" branch is taken when a=0 (or when a=b), which is why this
* second version is not recommended for images with large regions
* with constant pixel values (or even, actually, regions with nearby
* pixel values which vary bilinearly, which may arise from dirt-cheap
* demosaicing or computer graphics operations).
*
* Both of the above use a multiplication instead of a nested
* "if-then-else" because gcc does not always rewrite the latter using
* conditional moves.
* This last version appears to be slightly better than the former in
* speed tests performed on a recent multicore Intel chip, especially
* when enlarging a sharp image by a large factor, hence the choice.
*/
#define NOHALO_MINMOD(a,b,a_times_a,a_times_b) \
( (a_times_b)>=0. ? 1. : 0. ) * ( (a_times_b)<(a_times_a) ? (b) : (a) )
( ( (a_times_b)>=0. ) ? ( (a_times_a)<=(a_times_b) ? (a) : (b) ) : 0. )
#define NOHALO_ABS(x) ( ((x)>=0.) ? (x) : -(x) )
#define NOHALO_SIGN(x) ( ((x)>=0.) ? 1. : -1. )