Make number of output entries produced by invert_lut() a parameter
The callers of invert_lut() have been changed to pass the lut length as the
output length for now. There should be no behaviour change.
diff --git a/transform.c b/transform.c
index 784cd28..146f9e4 100644
--- a/transform.c
+++ b/transform.c
@@ -547,17 +547,30 @@
return true;
}
-static uint16_t *invert_lut(uint16_t *table, int length)
+/*
+ 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, int out_length)
{
int i;
- /* for now we invert the lut by creating a lut of the same size
+ /* 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)*length);
+ uint16_t *output = malloc(sizeof(uint16_t)*out_length);
if (!output)
return NULL;
- for (i = 0; i < length; i++) {
- double x = ((double) i * 65535.) / (double) (length - 1);
+ 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);
}
@@ -1351,7 +1364,7 @@
} else if (trc->count == 1) {
compute_precache_pow(output, 1./u8Fixed8Number_to_float(trc->data[0]));
} else {
- uint16_t *inverted = invert_lut(trc->data, trc->count);
+ uint16_t *inverted = invert_lut(trc->data, trc->count, trc->count);
if (!inverted)
return false;
compute_precache_lut(output, inverted, trc->count);
@@ -1441,7 +1454,7 @@
*output_gamma_lut = build_pow_table(gamma, 4096);
*output_gamma_lut_length = 4096;
} else {
- *output_gamma_lut = invert_lut(trc->data, trc->count);
+ *output_gamma_lut = invert_lut(trc->data, trc->count, trc->count);
*output_gamma_lut_length = trc->count;
}
