(From Steven Fuller. --ryan.)

"Hi, Here's a patch that fixes problems (attenuation on unsigned samples would cause the samples to fall towards 0 when it should instead fall towards 128/32768) when doing panning on 8-bit sounds and 16-bit unsigned samples."
libsdl-org · Dec 27, 2002 · ea81ade · ea81ade
1 parent 2105242
commit ea81ade
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Showing 2 changed files with 40 additions and 18 deletions.
diff --git a/effect_position.c b/effect_position.c
@@ -104,8 +104,11 @@ static void _Eff_position_u8(int chan, void *stream, int len, void *udata)
     }
 
     for (i = 0; i < len; i += sizeof (Uint8) * 2) {
-        *(ptr++) = (Uint8)((((float) *ptr) * args->left_f) * args->distance_f);
-        *(ptr++) = (Uint8)((((float) *ptr) * args->right_f) * args->distance_f);
+        /* must adjust the sample so that 0 is the center */
+        *(ptr++) = (Uint8) ((Sint8) ((((float) (Sint8) (*ptr - 128)) 
+            * args->left_f) * args->distance_f) + 128);
+        *(ptr++) = (Uint8) ((Sint8) ((((float) (Sint8) (*ptr - 128)) 
+            * args->right_f) * args->distance_f) + 128);
     }
 }
 
@@ -237,15 +240,24 @@ static void _Eff_position_u16lsb(int chan, void *stream, int len, void *udata)
 
     for (i = 0; i < len; i += sizeof (Uint16) * 2) {
 #if (SDL_BYTEORDER == SDL_BIG_ENDIAN)
-        Uint16 swapl = (Uint16) ((((float) SDL_Swap16(*(ptr))) *
-                                    args->left_f) * args->distance_f);
-        Uint16 swapr = (Uint16) ((((float) SDL_Swap16(*(ptr+1))) *
-                                    args->right_f) * args->distance_f);
+        Sint16 sampl = (Sint16) (SDL_Swap16(*(ptr+0)) - 32768);
+        Sint16 sampr = (Sint16) (SDL_Swap16(*(ptr+1)) - 32768);
+
+        Uint16 swapl = (Uint16) ((Sint16) (((float) sampl * args->left_f)
+                                    * args->distance_f) + 32768);
+        Uint16 swapr = (Uint16) ((Sint16) (((float) sampr * args->right_f)
+                                    * args->distance_f) + 32768);
+
         *(ptr++) = (Uint16) SDL_Swap16(swapl);
         *(ptr++) = (Uint16) SDL_Swap16(swapr);
 #else
-        *(ptr++) = (Uint16) ((((float) *ptr)*args->left_f)*args->distance_f);
-        *(ptr++) = (Uint16) ((((float) *ptr)*args->right_f)*args->distance_f);
+        Sint16 sampl = (Sint16) (*(ptr+0) - 32768);
+        Sint16 sampr = (Sint16) (*(ptr+1) - 32768);
+
+        *(ptr++) = (Uint16) ((Sint16) (((float) sampl * args->left_f) 
+                                    * args->distance_f) + 32768);
+        *(ptr++) = (Uint16) ((Sint16) (((float) sampr * args->right_f) 
+                                    * args->distance_f) + 32768);
 #endif
     }
 }
@@ -281,15 +293,24 @@ static void _Eff_position_u16msb(int chan, void *stream, int len, void *udata)
 
     for (i = 0; i < len; i += sizeof (Sint16) * 2) {
 #if (SDL_BYTEORDER == SDL_LIL_ENDIAN)
-        Uint16 swapl = (Uint16) ((((float) SDL_Swap16(*(ptr))) *
-                                    args->left_f) * args->distance_f);
-        Uint16 swapr = (Uint16) ((((float) SDL_Swap16(*(ptr+1))) *
-                                    args->right_f) * args->distance_f);
+        Sint16 sampl = (Sint16) (SDL_Swap16(*(ptr+0)) - 32768);
+        Sint16 sampr = (Sint16) (SDL_Swap16(*(ptr+1)) - 32768);
+
+        Uint16 swapl = (Uint16) ((Sint16) (((float) sampl * args->left_f)
+                                    * args->distance_f) + 32768);
+        Uint16 swapr = (Uint16) ((Sint16) (((float) sampr * args->right_f)
+                                    * args->distance_f) + 32768);
+
         *(ptr++) = (Uint16) SDL_Swap16(swapl);
         *(ptr++) = (Uint16) SDL_Swap16(swapr);
 #else
-        *(ptr++) = (Uint16) ((((float) *ptr)*args->left_f)*args->distance_f);
-        *(ptr++) = (Uint16) ((((float) *ptr)*args->right_f)*args->distance_f);
+        Sint16 sampl = (Sint16) (*(ptr+0) - 32768);
+        Sint16 sampr = (Sint16) (*(ptr+1) - 32768);
+
+        *(ptr++) = (Uint16) ((Sint16) (((float) sampl * args->left_f) 
+                                    * args->distance_f) + 32768);
+        *(ptr++) = (Uint16) ((Sint16) (((float) sampr * args->right_f) 
+                                    * args->distance_f) + 32768);
 #endif
     }
 }

diff --git a/effects_internal.c b/effects_internal.c
@@ -67,8 +67,9 @@ void *_Eff_build_volume_table_u8(void)
         if (rc) {
             _Eff_volume_table = (void *) rc;
             for (volume = 0; volume < 256; volume++) {
-                for (sample = 0; sample < 256; sample ++) {
-                    *rc = (Uint8)(((float) sample) * ((float) volume / 255.0));
+                for (sample = -128; sample < 128; sample ++) {
+                    *rc = (Uint8)(((float) sample) * ((float) volume / 255.0)) 
+                        + 128;
                     rc++;
                 }
             }
@@ -83,7 +84,7 @@ void *_Eff_build_volume_table_u8(void)
  *
  * Each column of the table is a possible sample, while each row of the
  *  table is a volume. Volume is a Uint8, where 0 is silence and 255 is full
- *  volume. So _Eff_volume_table[128][mysample] would be the value of
+ *  volume. So _Eff_volume_table[128][mysample+128] would be the value of
  *  mysample, at half volume.
  */
 void *_Eff_build_volume_table_s8(void)
@@ -97,7 +98,7 @@ void *_Eff_build_volume_table_s8(void)
         if (rc) {
             _Eff_volume_table = (void *) rc;
             for (volume = 0; volume < 256; volume++) {
-                for (sample = 0; sample < 256; sample ++) {
+                for (sample = -128; sample < 128; sample ++) {
                     *rc = (Sint8)(((float) sample) * ((float) volume / 255.0));
                     rc++;
                 }