[FFmpeg-cvslog] avcodec/mpegvideo_enc: fix qmat value comments

[Marton Balint]

ffmpeg | branch: master | Marton Balint <cus at passwd.hu> | Mon Jan  6 21:18:36 2025 +0100| [ed26812337b397e602e68dae5c39b20b8b599565] | committer: Marton Balint

avcodec/mpegvideo_enc: fix qmat value comments

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>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=ed26812337b397e602e68dae5c39b20b8b599565
---

 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 b72e4a8cf2..d5088fe67f 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 ||