/*
The number of entries needed to invert a lookup table should not
necessarily be the same as the original number of entries. This is
especially true of lookup tables that have a small number of entries.
For example:
Using a table like:
{0, 3104, 14263, 34802, 65535}
invert_lut will produce an inverse of:
{3, 34459, 47529, 56801, 65535}
which has an maximum error of about 9855 (pixel difference of ~38.346)
For now, we punt the decision of output size to the caller. */
static uint16_t *invert_lut(uint16_t *table, int length, size_t out_length)
{
int i;
/* for now we invert the lut by creating a lut of size out_length
* and attempting to lookup a value for each entry using lut_inverse_interp16 */
uint16_t *output = malloc(sizeof(uint16_t)*out_length);
if (!output)
return NULL;
for (i = 0; i < out_length; i++) {
double x = ((double) i * 65535.) / (double) (out_length - 1);
uint16_fract_t input = floor(x + .5);
output[i] = lut_inverse_interp16(input, table, length);
}
return output;
}
/*
The number of entries needed to invert a lookup table should not
necessarily be the same as the original number of entries. This is
especially true of lookup tables that have a small number of entries.
For example:
Using a table like:
{0, 3104, 14263, 34802, 65535}
invert_lut will produce an inverse of:
{3, 34459, 47529, 56801, 65535}
which has an maximum error of about 9855 (pixel difference of ~38.346)
For now, we punt the decision of output size to the caller. */
static uint16_t *invert_lut(uint16_t *table, int length, size_t out_length)
{
int NumZeroes;
int NumPoles;
int i;
/* for now we invert the lut by creating a lut of size out_length
* and attempting to lookup a value for each entry using lut_inverse_interp16 */
uint16_t *output = malloc(sizeof(uint16_t)*out_length);
if (!output)
return NULL;
// December/16 2015 - Compute the input curve zero and pole extents outside
// the loop and pass them to lut_inverse_interp16.
count_zeroes_and_poles(table, length, &NumZeroes, &NumPoles);
for (i = 0; i < out_length; i++) {
double x = ((double) i * 65535.) / (double) (out_length - 1);
uint16_fract_t input = floor(x + .5);
output[i] = lut_inverse_interp16(input, table, length, NumZeroes, NumPoles);
}
return output;
}
