The Independent JPEG Group's JPEG software v5a
diff --git a/jccolor.c b/jccolor.c
index 4ab3d7e..6707911 100644
--- a/jccolor.c
+++ b/jccolor.c
@@ -32,9 +32,14 @@
* normalized to the range 0..MAXJSAMPLE rather than -0.5 .. 0.5.
* The conversion equations to be implemented are therefore
* Y = 0.29900 * R + 0.58700 * G + 0.11400 * B
- * Cb = -0.16874 * R - 0.33126 * G + 0.50000 * B + MAXJSAMPLE/2
- * Cr = 0.50000 * R - 0.41869 * G - 0.08131 * B + MAXJSAMPLE/2
+ * Cb = -0.16874 * R - 0.33126 * G + 0.50000 * B + CENTERJSAMPLE
+ * Cr = 0.50000 * R - 0.41869 * G - 0.08131 * B + CENTERJSAMPLE
* (These numbers are derived from TIFF 6.0 section 21, dated 3-June-92.)
+ * Note: older versions of the IJG code used a zero offset of MAXJSAMPLE/2,
+ * rather than CENTERJSAMPLE, for Cb and Cr. This gave equal positive and
+ * negative swings for Cb/Cr, but meant that grayscale values (Cb=Cr=0)
+ * were not represented exactly. Now we sacrifice exact representation of
+ * maximum red and maximum blue in order to get exact grayscales.
*
* To avoid floating-point arithmetic, we represent the fractional constants
* as integers scaled up by 2^16 (about 4 digits precision); we have to divide
@@ -46,11 +51,12 @@
* for 12-bit samples it is still acceptable. It's not very reasonable for
* 16-bit samples, but if you want lossless storage you shouldn't be changing
* colorspace anyway.
- * The MAXJSAMPLE/2 offsets and the rounding fudge-factor of 0.5 are included
+ * The CENTERJSAMPLE offsets and the rounding fudge-factor of 0.5 are included
* in the tables to save adding them separately in the inner loop.
*/
#define SCALEBITS 16 /* speediest right-shift on some machines */
+#define CBCR_OFFSET ((INT32) CENTERJSAMPLE << SCALEBITS)
#define ONE_HALF ((INT32) 1 << (SCALEBITS-1))
#define FIX(x) ((INT32) ((x) * (1L<<SCALEBITS) + 0.5))
@@ -94,9 +100,13 @@
rgb_ycc_tab[i+B_Y_OFF] = FIX(0.11400) * i + ONE_HALF;
rgb_ycc_tab[i+R_CB_OFF] = (-FIX(0.16874)) * i;
rgb_ycc_tab[i+G_CB_OFF] = (-FIX(0.33126)) * i;
- rgb_ycc_tab[i+B_CB_OFF] = FIX(0.50000) * i + ONE_HALF*(MAXJSAMPLE+1);
+ /* We use a rounding fudge-factor of 0.5-epsilon for Cb and Cr.
+ * This ensures that the maximum output will round to MAXJSAMPLE
+ * not MAXJSAMPLE+1, and thus that we don't have to range-limit.
+ */
+ rgb_ycc_tab[i+B_CB_OFF] = FIX(0.50000) * i + CBCR_OFFSET + ONE_HALF-1;
/* B=>Cb and R=>Cr tables are the same
- rgb_ycc_tab[i+R_CR_OFF] = FIX(0.50000) * i + ONE_HALF*(MAXJSAMPLE+1);
+ rgb_ycc_tab[i+R_CR_OFF] = FIX(0.50000) * i + CBCR_OFFSET + ONE_HALF-1;
*/
rgb_ycc_tab[i+G_CR_OFF] = (-FIX(0.41869)) * i;
rgb_ycc_tab[i+B_CR_OFF] = (-FIX(0.08131)) * i;
