[vorbis-dev] FLOOR_fromdB_LOOKUP

[Attila Padar]

Hello,
You know, I always worry about the precision
and the float constants... and there is a large
lookup table in the floor1.c ... and I figure out
a way to keep the code size and speed, but
to improve the precision at this lookup table.
(the difference is small, but audible)

Here is the modifications in the floor1.c:

tatic unsigned long FLOOR_fromdB_LOOKUP[256]={
0x33E4B434,0x33F390FF,0x3401B286,
0x340A2036,0x34131A1E,0x341CA95A,
0x3426D7A0,0x3431AF44,0x343D3B49,
0x34498767,0x3456A01A,0x346492AE,
0x34736D4B,0x34819F83,0x348A0BF7,
0x3493048E,0x349C9263,0x34A6BF2B,
0x34B19538,0x34BD1F8C,0x34C969DD,
0x34D680A4,0x34E4712D,0x34F3499C,
0x35018C83,0x3509F7BA,0x3512EF01,
0x351C7B70,0x3526A6BA,0x35317B31,
0x353D03D3,0x35494C56,0x35566133,
0x35644FB0,0x357325F3,0x35817985,
0x3589E381,0x3592D977,0x359C6480,
0x35A68E4C,0x35B1612C,0x35BCE81E,
0x35C92ED5,0x35D641C6,0x35E42E39,
0x35F3024F,0x3601668B,0x3609CF4B,
0x3612C3F0,0x361C4D93,0x362675E2,
0x3631472C,0x363CCC6D,0x36491157,
0x3656225E,0x36640CC6,0x3672DEB0,
0x36815393,0x3689BB17,0x3692AE6D,
0x369C36AA,0x36A65D7B,0x36B12D30,
0x36BCB0C1,0x36C8F3DE,0x36D602FB,
0x36E3EB59,0x36F2BB16,0x3701409E,
0x3709A6E7,0x371298ED,0x371C1FC4,
0x37264518,0x37311337,0x373C9518,
0x3748D669,0x3755E39C,0x3763C9F0,
0x37729781,0x37812DAC,0x378992B9,
0x3792836F,0x379C08E1,0x37A62CB9,
0x37B0F942,0x37BC7973,0x37C8B8F9,
0x37D5C442,0x37E3A88C,0x37F273F2,
0x38011ABC,0x38097E8F,0x38126DF5,
0x381BF202,0x3826145D,0x3830DF51,
0x383C5DD3,0x38489B8C,0x3855A4EC,
0x3863872D,0x38725068,0x388107CF,
0x38896A67,0x3892587F,0x389BDB26,
0x38A5FC05,0x38B0C564,0x38BC4236,
0x38C87E24,0x38D5859B,0x38E365D3,
0x38F22CE3,0x3900F4E5,0x39095643,
0x3912430B,0x391BC44D,0x3925E3B0,
0x3930AB7B,0x393C269D,0x394860C1,
0x3955664E,0x3963447E,0x39720963,
0x3980E1FE,0x39894221,0x39922D9A,
0x399BAD78,0x39A5CB5F,0x39B09195,
0x39BC0B09,0x39C84362,0x39D54706,
0x39E3232D,0x39F1E5E8,0x3A00CF1A,
0x3A092E02,0x3A12182D,0x3A1B96A6,
0x3A25B312,0x3A3077B3,0x3A3BEF79,
0x3A482607,0x3A5527C3,0x3A6301E2,
0x3A71C273,0x3A80BC38,0x3A8919E7,
0x3A9202C3,0x3A9B7FD8,0x3AA59AC8,
0x3AB05DD5,0x3ABBD3EC,0x3AC808B0,
0x3AD50884,0x3AE2E09B,0x3AF19F03,
0x3B00A959,0x3B0905CE,0x3B11ED5B,
0x3B1B690D,0x3B258281,0x3B3043FB,
0x3B3BB864,0x3B47EB5E,0x3B54E94A,
0x3B62BF5A,0x3B717B98,0x3B80967D,
0x3B88F1B8,0x3B91D7F7,0x3B9B5245,
0x3BA56A3F,0x3BB02A24,0x3BBB9CE0,
0x3BC7CE0F,0x3BD4CA14,0x3BE29E1D,
0x3BF15832,0x3C0083A4,0x3C08DDA5,
0x3C11C297,0x3C1B3B80,0x3C2551FF,
0x3C301052,0x3C3B815F,0x3C47B0C6,
0x3C54AAE3,0x3C627CE5,0x3C7134D2,
0x3C8070CD,0x3C88C995,0x3C91AD39,
0x3C9B24BF,0x3CA539C4,0x3CAFF683,
0x3CBB65E3,0x3CC79380,0x3CD48BB7,
0x3CE25BB2,0x3CF11176,0x3D005DFA,
0x3D08B588,0x3D1197DE,0x3D1B0E01,
0x3D25218B,0x3D2FDCB8,0x3D3B4A6B,
0x3D47763F,0x3D546C8F,0x3D623A83,
0x3D70EE20,0x3D804B29,0x3D88A17E,
0x3D918287,0x3D9AF747,0x3DA50957,
0x3DAFC2F0,0x3DBB2EF7,0x3DC75902,
0x3DD44D6C,0x3DE2195A,0x3DF0CACF,
0x3E00385A,0x3E088D77,0x3E116D32,
0x3E1AE08F,0x3E24F126,0x3E2FA92D,
0x3E3B1387,0x3E473BC9,0x3E542E4D,
0x3E61F836,0x3E70A783,0x3E80258F,
0x3E887973,0x3E9157E1,0x3E9AC9DC,
0x3EA4D8F8,0x3EAF8F6D,0x3EBAF81A,
0x3EC71E95,0x3ED40F33,0x3EE1D716,
0x3EF0843C,0x3F0012C6,0x3F086571,
0x3F114293,0x3F1AB32B,0x3F24C0CE,
0x3F2F75B1,0x3F3ADCB2,0x3F470165,
0x3F53F01D,0x3F61B5FB,0x3F7060FB,
0x3F800000
};

tatic void render_line(int x0,int x1,int y0,int y1,float *d){
  int dy=y1-y0;
  int adx=x1-x0;
  int ady=abs(dy);
  int base=dy/adx;
  int sy=(dy<0?base-1:base+1);
  int x=x0;
  int y=y0;
  int err=0;

  ady-=abs(base*adx);

  d[x]*=*((float *)&FLOOR_fromdB_LOOKUP[y]);
  while(++x<x1){
    err=err+ady;
    if(err>=adx){
      err-=adx;
      y+=sy;
    }else{
      y+=base;
    }
    d[x]*=*((float *)&FLOOR_fromdB_LOOKUP[y]);
  }
}

tatic void render_line0(int x0,int x1,int y0,int y1,float *d){
  int dy=y1-y0;
  int adx=x1-x0;
  int ady=abs(dy);
  int base=dy/adx;
  int sy=(dy<0?base-1:base+1);
  int x=x0;
  int y=y0;
  int err=0;

  ady-=abs(base*adx);

  d[x]=*((float *)&FLOOR_fromdB_LOOKUP[y]);
  while(++x<x1){
    err=err+ady;
    if(err>=adx){
      err-=adx;
      y+=sy;
    }else{
      y+=base;
    }
    d[x]=*((float *)&FLOOR_fromdB_LOOKUP[y]);
  }
}

I made the lookup table with this small program.
(I hope this is a correct calculating):

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

void main(void)
{
 int i;
 float lookup;
 unsigned long *p=(unsigned long *)&lookup;
 FILE *fp=fopen("lookup.txt","wt");
 const double d=log(2.0f)/log10(2.0f)/20.0f;

 for(i=0;i<256;i++){
  double a=(double)(255-i)/256.0f * -140.0f;
  lookup=(float)exp(a*d);

  if(!(i%3))
   fprintf(fp,"\n");
  fprintf(fp,"0x%8.8X,",*p);
 }
 fclose(fp);
}

So, this is just an idea.
I hope it's useful.

regards
Attila Padar

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