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src/Source/OpenEXR/IlmImf/ImfB44Compressor.cpp  view on Meta::CPAN

//-----------------------------------------------------------------------------
//
//	class B44Compressor
//
//	This compressor is lossy for HALF channels; the compression rate
//	is fixed at 32/14 (approximately 2.28).  FLOAT and UINT channels
//	are not compressed; their data are preserved exactly.
//
//	Each HALF channel is split into blocks of 4 by 4 pixels.  An
//	uncompressed block occupies 32 bytes, which are re-interpreted
//	as sixteen 16-bit unsigned integers, t[0] ... t[15].  Compression
//	shrinks the block to 14 bytes.  The compressed 14-byte block
//	contains
//
//	 - t[0]
//
//	 - a 6-bit shift value
//
//	 - 15 densely packed 6-bit values, r[0] ... r[14], which are
//         computed by subtracting adjacent pixel values and right-
//	   shifting the differences according to the stored shift value.
//
//	   Differences between adjacent pixels are computed according
//	   to the following diagram:
//
//		 0 -------->  1 -------->  2 -------->  3
//               |     3            7           11
//               |
//               | 0
//               |
//               v 
//		 4 -------->  5 -------->  6 -------->  7
//               |     4            8           12
//               |
//               | 1
//               |
//               v
//		 8 -------->  9 --------> 10 --------> 11
//               |     5            9           13
//               |
//               | 2
//               |
//               v
//		12 --------> 13 --------> 14 --------> 15
//                     6           10           14
//
//	    Here
//
//               5 ---------> 6
//                     8
//
//	    means that r[8] is the difference between t[5] and t[6].
//
//	 - optionally, a 4-by-4 pixel block where all pixels have the
//	   same value can be treated as a special case, where the
//	   compressed block contains only 3 instead of 14 bytes:
//	   t[0], followed by an "impossible" 6-bit shift value and
//	   two padding bits.
//
//	This compressor can handle positive and negative pixel values.
//	NaNs and infinities are replaced with zeroes before compression.
//
//-----------------------------------------------------------------------------

#include "ImfB44Compressor.h"
#include "ImfHeader.h"
#include "ImfChannelList.h"
#include "ImfMisc.h"
#include "ImfCheckedArithmetic.h"
#include <ImathFun.h>
#include <ImathBox.h>
#include <Iex.h>
#include <ImfIO.h>
#include <ImfXdr.h>
#include <string.h>
#include <assert.h>
#include <algorithm>
#include "ImfNamespace.h"


OPENEXR_IMF_INTERNAL_NAMESPACE_SOURCE_ENTER


using IMATH_NAMESPACE::divp;
using IMATH_NAMESPACE::modp;
using IMATH_NAMESPACE::Box2i;
using IMATH_NAMESPACE::V2i;
using std::min;

namespace {

//
// Lookup tables for
//	y = exp (x / 8)
// and 
//	x = 8 * log (y)
//

#include "b44ExpLogTable.h"


inline void
convertFromLinear (unsigned short s[16])
{
    for (int i = 0; i < 16; ++i)
	s[i] = expTable[s[i]];
}


inline void
convertToLinear (unsigned short s[16])
{
    for (int i = 0; i < 16; ++i)
	s[i] = logTable[s[i]];
}


inline int
shiftAndRound (int x, int shift)
{
    //
    // Compute
    //
    //     y = x * pow (2, -shift),
    //
    // then round y to the nearest integer.
    // In case of a tie, where y is exactly
    // halfway between two integers, round
    // to the even one.
    //

    x <<= 1;
    int a = (1 << shift) - 1;
    shift += 1;
    int b = (x >> shift) & 1;
    return (x + a + b) >> shift;
}


int
pack (const unsigned short s[16],
      unsigned char b[14],
      bool optFlatFields,
      bool exactMax)
{
    //
    // Pack a block of 4 by 4 16-bit pixels (32 bytes) into
    // either 14 or 3 bytes.
    //

    //
    // Integers s[0] ... s[15] represent floating-point numbers
    // in what is essentially a sign-magnitude format.  Convert
    // s[0] .. s[15] into a new set of integers, t[0] ... t[15],
    // such that if t[i] is greater than t[j], the floating-point
    // number that corresponds to s[i] is always greater than
    // the floating-point number that corresponds to s[j].
    //
    // Also, replace any bit patterns that represent NaNs or
    // infinities with bit patterns that represent floating-point
    // zeroes.
    //
    //	bit pattern	floating-point		bit pattern
    //	in s[i]		value			in t[i]
    //
    //  0x7fff		NAN			0x8000
    //  0x7ffe		NAN			0x8000
    //	  ...					  ...
    //  0x7c01		NAN			0x8000
    //  0x7c00		+infinity		0x8000
    //  0x7bff		+HALF_MAX		0xfbff
    //  0x7bfe					0xfbfe
    //  0x7bfd					0xfbfd
    //	  ...					  ...
    //  0x0002		+2 * HALF_MIN		0x8002
    //  0x0001		+HALF_MIN		0x8001
    //  0x0000		+0.0			0x8000
    //  0x8000		-0.0			0x7fff
    //  0x8001		-HALF_MIN		0x7ffe
    //  0x8002		-2 * HALF_MIN		0x7ffd
    //	  ...					  ...
    //  0xfbfd					0x0f02
    //  0xfbfe					0x0401
    //  0xfbff		-HALF_MAX		0x0400
    //  0xfc00		-infinity		0x8000
    //  0xfc01		NAN			0x8000
    //	  ...					  ...
    //  0xfffe		NAN			0x8000
    //  0xffff		NAN			0x8000
    //

    unsigned short t[16];

    for (int i = 0; i < 16; ++i)
    {
	if ((s[i] & 0x7c00) == 0x7c00)
	    t[i] = 0x8000;
	else if (s[i] & 0x8000)
	    t[i] = ~s[i];
	else
	    t[i] = s[i] | 0x8000;
    }
    
    //
    // Find the maximum, tMax, of t[0] ... t[15].
    //

    unsigned short tMax = 0;

    for (int i = 0; i < 16; ++i)
	if (tMax < t[i])
	    tMax = t[i];

    //
    // Compute a set of running differences, r[0] ... r[14]:
    // Find a shift value such that after rounding off the
    // rightmost bits and shifting all differenes are between
    // -32 and +31.  Then bias the differences so that they
    // end up between 0 and 63.