[FFmpeg-devel] [PATCH 2/3] avcodec/mpegvideo_enc: fix qmat value comments

[Marton Balint]

The comments supposed to track the possible value of the qmat and qmat16
matrices, but they were not updated properly in the long history of the
mpegvideo encoder. Also they wrongly assumed the usage of built-in quantizer
matrices and linear quantization.

Signed-off-by: Marton Balint <cus at passwd.hu>
---
 libavcodec/mpegvideo_enc.c | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/libavcodec/mpegvideo_enc.c b/libavcodec/mpegvideo_enc.c
index c5f20c2d85..7001d5a566 100644
--- a/libavcodec/mpegvideo_enc.c
+++ b/libavcodec/mpegvideo_enc.c
@@ -133,11 +133,11 @@ void ff_convert_matrix(MpegEncContext *s, int (*qmat)[64],
             for (i = 0; i < 64; i++) {
                 const int j = s->idsp.idct_permutation[i];
                 int64_t den = (int64_t) qscale2 * quant_matrix[j];
-                /* 16 <= qscale * quant_matrix[i] <= 7905
-                 * Assume x = ff_aanscales[i] * qscale * quant_matrix[i]
-                 *             19952 <=              x  <= 249205026
-                 * (1 << 36) / 19952 >= (1 << 36) / (x) >= (1 << 36) / 249205026
-                 *           3444240 >= (1 << 36) / (x) >= 275 */
+                /* 1 * 1 <= qscale2 * quant_matrix[j] <= 112 * 255
+                 * Assume x = qscale2 * quant_matrix[j]
+                 *                 1 <=              x  <= 28560
+                 *     (1 << 22) / 1 >= (1 << 22) / (x) >= (1 << 22) / 28560
+                 *           4194304 >= (1 << 22) / (x) >= 146 */
 
                 qmat[qscale][i] = (int)((UINT64_C(2) << QMAT_SHIFT) / den);
             }
@@ -145,11 +145,11 @@ void ff_convert_matrix(MpegEncContext *s, int (*qmat)[64],
             for (i = 0; i < 64; i++) {
                 const int j = s->idsp.idct_permutation[i];
                 int64_t den = ff_aanscales[i] * (int64_t) qscale2 * quant_matrix[j];
-                /* 16 <= qscale * quant_matrix[i] <= 7905
-                 * Assume x = ff_aanscales[i] * qscale * quant_matrix[i]
-                 *             19952 <=              x  <= 249205026
-                 * (1 << 36) / 19952 >= (1 << 36) / (x) >= (1 << 36) / 249205026
-                 *           3444240 >= (1 << 36) / (x) >= 275 */
+                /* 1247 * 1 * 1 <= ff_aanscales[i] * qscale2 * quant_matrix[j] <= 31521 * 112 * 255
+                 * Assume x = ff_aanscales[i] * qscale2 * quant_matrix[j]
+                 *              1247 <=              x  <= 900239760
+                 *  (1 << 36) / 1247 >= (1 << 36) / (x) >= (1 << 36) / 900239760
+                 *          55107840 >= (1 << 36) / (x) >= 76 */
 
                 qmat[qscale][i] = (int)((UINT64_C(2) << (QMAT_SHIFT + 14)) / den);
             }
@@ -157,14 +157,17 @@ void ff_convert_matrix(MpegEncContext *s, int (*qmat)[64],
             for (i = 0; i < 64; i++) {
                 const int j = s->idsp.idct_permutation[i];
                 int64_t den = (int64_t) qscale2 * quant_matrix[j];
-                /* We can safely suppose that 16 <= quant_matrix[i] <= 255
-                 * Assume x = qscale * quant_matrix[i]
-                 * So             16 <=              x  <= 7905
-                 * so (1 << 19) / 16 >= (1 << 19) / (x) >= (1 << 19) / 7905
-                 * so          32768 >= (1 << 19) / (x) >= 67 */
+                /* 1 * 1 <= qscale2 * quant_matrix[j] <= 112 * 255
+                 * Assume x = qscale2 * quant_matrix[j]
+                 *                 1 <=              x  <= 28560
+                 *     (1 << 22) / 1 >= (1 << 22) / (x) >= (1 << 22) / 28560
+                 *           4194304 >= (1 << 22) / (x) >= 146
+                 *
+                 *                 1 <=              x  <= 28560
+                 *     (1 << 17) / 1 >= (1 << 17) / (x) >= (1 << 17) / 28560
+                 *            131072 >= (1 << 17) / (x) >= 4 */
+
                 qmat[qscale][i] = (int)((UINT64_C(2) << QMAT_SHIFT) / den);
-                //qmat  [qscale][i] = (1 << QMAT_SHIFT_MMX) /
-                //                    (qscale * quant_matrix[i]);
                 qmat16[qscale][0][i] = (2 << QMAT_SHIFT_MMX) / den;
 
                 if (qmat16[qscale][0][i] == 0 ||
-- 
2.43.0