Pillow/src/libImaging/ColorLUT.c

#include "Imaging.h"
#include <math.h>


/* 8 bits for result. Table can overflow [0, 1.0] range,
   so we need extra bits for overflow and negative values.
   NOTE: This value should be the same as in _imaging/_prepare_lut_table() */
#define PRECISION_BITS (16 - 8 - 2)
#define PRECISION_ROUNDING (1<<(PRECISION_BITS-1))

/* 8 — scales are multiplied on byte.
   6 — max index in the table
       (max size is 65, but index 64 is not reachable) */
#define SCALE_BITS (32 - 8 - 6)
#define SCALE_MASK ((1<<SCALE_BITS) - 1)

#define SHIFT_BITS (16 - 1)


static inline UINT8 clip8(int in)
{
    return clip8_lookups[(in + PRECISION_ROUNDING) >> PRECISION_BITS];
}

static inline void
interpolate3(INT16 out[3], const INT16 a[3], const INT16 b[3], INT16 shift)
{
    out[0] = (a[0] * ((1<<SHIFT_BITS)-shift) + b[0] * shift) >> SHIFT_BITS;
    out[1] = (a[1] * ((1<<SHIFT_BITS)-shift) + b[1] * shift) >> SHIFT_BITS;
    out[2] = (a[2] * ((1<<SHIFT_BITS)-shift) + b[2] * shift) >> SHIFT_BITS;
}

static inline void
interpolate4(INT16 out[4], const INT16 a[4], const INT16 b[4], INT16 shift)
{
    out[0] = (a[0] * ((1<<SHIFT_BITS)-shift) + b[0] * shift) >> SHIFT_BITS;
    out[1] = (a[1] * ((1<<SHIFT_BITS)-shift) + b[1] * shift) >> SHIFT_BITS;
    out[2] = (a[2] * ((1<<SHIFT_BITS)-shift) + b[2] * shift) >> SHIFT_BITS;
    out[3] = (a[3] * ((1<<SHIFT_BITS)-shift) + b[3] * shift) >> SHIFT_BITS;
}

static inline int
table_index3D(int index1D, int index2D, int index3D,
               int size1D, int size1D_2D)
{
    return index1D + index2D * size1D + index3D * size1D_2D;
}


/*
 Transforms colors of imIn using provided 3D look-up table
 and puts the result in imOut. Returns imOut on sucess or 0 on error.
 
 imOut, imIn — images, should be the same size and may be the same image.
    Should have 3 or 4 channels.
 table_channels — number of channels in the look-up table, 3 or 4.
    Should be less or equal than number of channels in imOut image;
 size1D, size_2D and size3D — dimensions of provided table;
 table — flatten table,
    array with table_channels × size1D × size2D × size3D elements,
    where channels are changed first, then 1D, then​ 2D, then 3D.
    Each element is signed 16-bit int where 0 is lowest output value
    and 255 << PRECISION_BITS (16320) is highest value.
*/ 
Imaging
ImagingColorLUT3D_linear(Imaging imOut, Imaging imIn, int table_channels,
                         int size1D, int size2D, int size3D,
                         INT16* table)
{
    /* This float to int conversion doesn't have rounding
       error compensation (+0.5) for two reasons:
       1. As we don't hit the highest value,
          we can use one extra bit for precision.
       2. For every pixel, we interpolate 8 elements from the table:
          current and +1 for every dimension and their combinations.
          If we hit the upper cells from the table,
          +1 cells will be outside of the table.
          With this compensation we never hit the upper cells
          but this also doesn't introduce any noticeable difference. */
    UINT32 scale1D = (size1D - 1) / 255.0 * (1<<SCALE_BITS);
    UINT32 scale2D = (size2D - 1) / 255.0 * (1<<SCALE_BITS);
    UINT32 scale3D = (size3D - 1) / 255.0 * (1<<SCALE_BITS);
    int size1D_2D = size1D * size2D;
    int x, y;

    if (table_channels < 3 || table_channels > 4) {
        PyErr_SetString(PyExc_ValueError, "table_channels could be 3 or 4");
        return NULL;
    }

    if (imIn->type != IMAGING_TYPE_UINT8 ||
        imOut->type != IMAGING_TYPE_UINT8 ||
        imIn->bands < 3 ||
        imOut->bands < table_channels
    ) {
        return (Imaging) ImagingError_ModeError();
    }

    /* In case we have one extra band in imOut and don't have in imIn.*/
    if (imOut->bands > table_channels && imOut->bands > imIn->bands) {
        return (Imaging) ImagingError_ModeError();
    }

    for (y = 0; y < imOut->ysize; y++) {
        UINT8* rowIn = (UINT8 *)imIn->image[y];
        UINT32* rowOut = (UINT32 *)imOut->image[y];
        for (x = 0; x < imOut->xsize; x++) {
            UINT32 index1D = rowIn[x*4 + 0] * scale1D;
            UINT32 index2D = rowIn[x*4 + 1] * scale2D;
            UINT32 index3D = rowIn[x*4 + 2] * scale3D;
            INT16 shift1D = (SCALE_MASK & index1D) >> (SCALE_BITS - SHIFT_BITS);
            INT16 shift2D = (SCALE_MASK & index2D) >> (SCALE_BITS - SHIFT_BITS);
            INT16 shift3D = (SCALE_MASK & index3D) >> (SCALE_BITS - SHIFT_BITS);
            int idx = table_channels * table_index3D(
                index1D >> SCALE_BITS, index2D >> SCALE_BITS,
                index3D >> SCALE_BITS, size1D, size1D_2D);
            INT16 result[4], left[4], right[4];
            INT16 leftleft[4], leftright[4], rightleft[4], rightright[4];

            if (table_channels == 3) {
                interpolate3(leftleft, &table[idx + 0], &table[idx + 3], shift1D);
                interpolate3(leftright, &table[idx + size1D*3],
                             &table[idx + size1D*3 + 3], shift1D);
                interpolate3(left, leftleft, leftright, shift2D);

                interpolate3(rightleft, &table[idx + size1D_2D*3],
                             &table[idx + size1D_2D*3 + 3], shift1D);
                interpolate3(rightright, &table[idx + size1D_2D*3 + size1D*3],
                             &table[idx + size1D_2D*3 + size1D*3 + 3], shift1D);
                interpolate3(right, rightleft, rightright, shift2D);

                interpolate3(result, left, right, shift3D);

                rowOut[x] = MAKE_UINT32(
                        clip8(result[0]), clip8(result[1]),
                        clip8(result[2]), rowIn[x*4 + 3]);
            }

            if (table_channels == 4) {
                interpolate4(leftleft, &table[idx + 0], &table[idx + 4], shift1D);
                interpolate4(leftright, &table[idx + size1D*4],
                             &table[idx + size1D*4 + 4], shift1D);
                interpolate4(left, leftleft, leftright, shift2D);

                interpolate4(rightleft, &table[idx + size1D_2D*4],
                             &table[idx + size1D_2D*4 + 4], shift1D);
                interpolate4(rightright, &table[idx + size1D_2D*4 + size1D*4],
                             &table[idx + size1D_2D*4 + size1D*4 + 4], shift1D);
                interpolate4(right, rightleft, rightright, shift2D);

                interpolate4(result, left, right, shift3D);

                rowOut[x] = MAKE_UINT32(
                        clip8(result[0]), clip8(result[1]),
                        clip8(result[2]), clip8(result[3]));
            }
        }
    }

    return imOut;
}