libmpeg2 IDCT implementation

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libmpeg2 IDCT implementation
	2007-11-30 12:54:17

Hi,   I see in pratice and in emails of the past ([mpeg2-dev] Re: mpeg2-dev Scan order - 2004-02-19, and Re: [mpeg2-dev] about the idct! - 2004-04-30) that the IDCT of two dimensions performed by idct_row and idct_col operating together (how we can see in mpeg2_idct_copy_c and mpeg2_idct_copy_add), in libmpeg2 are different of a "default" IDCT (DCT type-3). But in a experiment that I have to do, I would like to use the "default" IDCT. So I need by pass the modifications done in a block before do the IDCT or apply a function that remove this modifications. But I don't know what modifications exist, strictly speaking. I see that mpeg2_idct_init changes the order of mpeg2_scan_norm and mpeg2_scan_alt, and functions that "reconstructs" the block, like get_intra_block_B14 use this vectors. So I can see a reordering of block in here. Where are and who are the others modifications?   Another question, the values of block parameter in mpeg2_idct_copy_c and mpeg2_idct_add_c functions, after the loop that do the IDCT: " for (i = 0; i < 8; i++) ; idct_row (block + 8 * i); ; for (i = 0; i < 8; i++)   idct_col (block + i); " are the same if I use a "default" IDCT, right? Because the IDCTs "at the end" must be the same, right? Thanks! Mateus Krepsky Ludwich.
Re: libmpeg2 IDCT implementation
	2008-01-17 10:50:21

Hello,     I finally discovered the answer to my previous question: "what modifications libmpeg2 does in a block before it enter in the IDCT functions?" So, the 64 values of a block are multiplied by 16 and reordered. And the IDCT functions of libmpeg2 know this and its output are the same of a "default" IDCT (DCT type-3), of course. I would like to thank Dmitriy Startsev that tell me about the scaled by 16. And the reordering I discovered by myself. And only after this I saw a email of Matthew Fullerton that answering a question of Uros damnjanovic (Re: [mpeg2-dev] Extracting DCT coefficients for each frame - coefficient order) and brought the inverse reorder that I was looking for... ;p ;   Below are the mapping to reorder libmpeg2 IDCT input to the "default" IDCT input, and the inverse reorder mapping ("default" IDCT input to libmpeg2 IDCT input). Libmpeg2 IDCT input --> "Default" IDCT input / "Default" IDCT input --> Libmpeg2 IDCT input 0 --> 0       /     0 --> 0 1 --> 2       /     1 --> 4 2 --> 4       /     2 --> 1 3 --> 6       /     3 --> 5 4 --> 1       /     4 --> 2 5 --> 3       /     5 --> 6 6 --> 5       /     6 --> 3   7 --> 7       /     7 --> 7   8 --> 16       /     8 --> 32   9 --> 18       /     9 --> 36   10 --> 20     /     10 --> 33 11 --> 22     /     11 --> 37 12 --> 17     /     12 --> 34 13 --> 19     /     13 --> 38 14 --> 21     /     14 --> 35 15 --> 23     /     15 --> 39 16 --> 32     /     16 --> 8 17 --> 34     /     17 --> 12 18 --> 36     /     18 --> 9 19 --> 38     /     19 --> 13 20 --> 33     /     20 --> 10 21 --> 35     /     21 --> 14 22 --> 37     /     22 --> 11 23 --> 39     /     23 --> 15 24 --> 48     /     24 --> 40 25 --> 50     /     25 --> 44 26 --> 52     /     26 --> 41 27 --> 54     /     27 --> 45 28 --> 49     /     28 --> 42 29 --> 51     /     29 --> 46 30 --> 53     /     30 --> 43 31 --> 55     /     31 --> 47 32 --> 8       /     32 --> 16 33 --> 10     /     33 --> 20 34 --> 12     /     34 --> 17 35 --> 14     /     35 --> 21 36 --> 9       /     36 --> 18 37 --> 11     /     37 --> 22 38 --> 13     /     38 --> 19 39 --> 15     /     39 --> 23 40 --> 24     /     40 --> 48 41 --> 26     /     41 --> 52 42 --> 28     /     42 --> 49 43 --> 30     /     43 --> 53 44 --> 25     /     44 --> 50 45 --> 27     /     45 --> 54 46 --> 29     /     46 --> 51 47 --> 31     /     47 --> 55 48 --> 40     /     48 --> 24 49 --> 42     /     49 --> 28 50 --> 44     /     50 --> 25 51 --> 46     /     51 --> 29 52 --> 41     /     52 --> 26 53 --> 43     /     53 --> 30 54 --> 45     /     54 --> 27 55 --> 47     /     55 --> 31 56 --> 56     /     56 --> 56 57 --> 58     /     57 --> 60 58 --> 60     /     58 --> 57 59 --> 62     /     59 --> 61 60 --> 57     /     60 --> 58 61 --> 59     /     61 --> 62 62 --> 61     /     62 --> 59 63 --> 63     /     63 --> 63 ; A function that maps a libmpeg2 IDCT input to the "default" IDCT input can be: // Hard-coded way: void libmpeg2ToNormal(short * block) {   short aux[64];     aux[0] = block[0]; ; aux[1] = block[4]; ; aux[2] = block[1]; ; aux[3] = block[5]; ; aux[4] = block[2];   aux[5] = block[6]; ; aux[6] = block[3]; ; aux[7] = block[7]; ; aux[8] = block[32];   aux[9] = block[36];   aux[10] = block[33];   aux[11] = block[37];   aux[12] = block[34];   aux[13] = block[38];   aux[14] = block[35];   aux[15] = block[39];   aux[16] = block[8]; ; aux[17] = block[12];   aux[18] = block[9]; ; aux[19] = block[13];   aux[20] = block[10];   aux[21] = block[14];   aux[22] = block[11];   aux[23] = block[15];   aux[24] = block[40];   aux[25] = block[44];   aux[26] = block[41];   aux[27] = block[45];   aux[28] = block[42];   aux[29] = block[46];   aux[30] = block[43];   aux[31] = block[47];   aux[32] = block[16];   aux[33] = block[20];   aux[34] = block[17];   aux[35] = block[21];   aux[36] = block[18];   aux[37] = block[22];   aux[38] = block[19];   aux[39] = block[23];   aux[40] = block[48];   aux[41] = block[52];   aux[42] = block[49];   aux[43] = block[53];   aux[44] = block[50];   aux[45] = block[54];   aux[46] = block[51];   aux[47] = block[55];   aux[48] = block[24];   aux[49] = block[28];   aux[50] = block[25];   aux[51] = block[20];   aux[52] = block[26];   aux[53] = block[30];   aux[54] = block[27];   aux[55] = block[31];   aux[56] = block[56];   aux[57] = block[60];   aux[58] = block[57];   aux[59] = block[61];   aux[60] = block[58];   aux[61] = block[62];   aux[62] = block[59];   aux[63] = block[63];     int i;   for (i = 0; i < 64; ++ i) {     block[i] = aux[i] / 16; ; } } 2007/11/30, Mateus Krepsky Ludwich < mkludwichgmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">mkludwichgmail.com>: Hi,   I see in pratice and in emails of the past ([mpeg2-dev] Re: mpeg2-dev Scan order - 2004-02-19, and Re: [mpeg2-dev] about the idct! - 2004-04-30) that the IDCT of two dimensions performed by idct_row and idct_col operating together (how we can see in mpeg2_idct_copy_c and mpeg2_idct_copy_add), in libmpeg2 are different of a "default" IDCT (DCT type-3). But in a experiment that I have to do, I would like to use the "default" IDCT. So I need by pass the modifications done in a block before do the IDCT or apply a function that remove this modifications. But I don't know what modifications exist, strictly speaking. I see that mpeg2_idct_init changes the order of mpeg2_scan_norm and mpeg2_scan_alt, and functions that "reconstructs" the block, like get_intra_block_B14 use this vectors. So I can see a reordering of block in here. Where are and who are the others modifications?   Another question, the values of block parameter in mpeg2_idct_copy_c and mpeg2_idct_add_c functions, after the loop that do the IDCT: " for (i = 0; i < 8; i++) ; idct_row (block + 8 * i); ; for (i = 0; i < 8; i++)   idct_col (block + i); " are the same if I use a "default" IDCT, right? Because the IDCTs "at the end" must be the same, right? Thanks! Mateus Krepsky Ludwich.

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