[FFmpeg-devel] GSoC project (JPEG 2000)

[rukhsana afroz]

On Wed, Apr 6, 2011 at 6:35 PM, rukhsana afroz <rukhsana.afroz at gmail.com>wrote:

>
>
> On Wed, Apr 6, 2011 at 9:24 AM, Michael Niedermayer <michaelni at gmx.at>wrote:
>
>>  I suspect the If statement is there as the implementation only supports
>> cblk_style==0
>> the others simply are not implemented
>>
>> Do you think you can implement them?
>>
>> Thanks Michael for your reply. I read the specification once. I believe, I
> will be able to implement existing missing features of decoder. Later on, I
> can work on encoder once I know the JPEG 2000 coding standard thoroughly. I
> will submit my SoC application tonight, or tomorrow morning.
>
> Thanks
> Ruby
>
>
I have found what to do for particularly this missing feature. It looks
like, parameters have been defined for all values of cblk_style in j2k.h
file.

// Codeblock coding styles
#define J2K_CBLK_BYPASS    0x01 // Selective arithmetic coding bypass
#define J2K_CBLK_RESET     0x02 // Reset context probabilities
#define J2K_CBLK_TERMALL   0x04 // Terminate after each coding pass
#define J2K_CBLK_VSC       0x08 // Vertical stripe causal context formation
#define J2K_CBLK_PREDTERM  0x10 // Predictable termination
#define J2K_CBLK_SEGSYM    0x20 // Segmentation symbols present

Specification also says, these are the at most number of code block styles.
These cblk_style is used while decoding the a particular code block (which
is resided inside one band, one component-tile contains a number of bands,
one tile is subdivided again into a number of component-tiles, entire
codestream consists of a number of tiles). From the code, i see only the
code block style J2K_CBLK_SEGSYM has been implemented. When code block style
is zero, no action has been taken and it is default. Logic for this
implementation is in the function decode_cblk and then from there main logic
is  in decode_clnpass function.

static int decode_cblk(J2kDecoderContext *s, J2kCodingStyle *codsty,
J2kT1Context *t1, J2kCblk *cblk,
                       int width, int height, int bandpos)
{
    int passno = cblk->npasses, pass_t = 2, bpno = cblk->nonzerobits - 1, y;

    for (y = 0; y < height+2; y++)
        memset(t1->flags[y], 0, (width+2)*sizeof(int));

    for (y = 0; y < height; y++)
        memset(t1->data[y], 0, width*sizeof(int));

    ff_mqc_initdec(&t1->mqc, cblk->data);
    cblk->data[cblk->length] = 0xff;
    cblk->data[cblk->length+1] = 0xff;

    while(passno--){
        switch(pass_t){
            case 0: decode_sigpass(t1, width, height, bpno+1, bandpos);
                    break;
            case 1: decode_refpass(t1, width, height, bpno+1);
                    break;
           * case 2: decode_clnpass(s, t1, width, height, bpno+1, bandpos,
                                   codsty->cblk_style & J2K_CBLK_SEGSYM);*
                    break;
        }

        pass_t++;
        if (pass_t == 3){
            bpno--;
            pass_t = 0;
        }
    }
    return 0;
}

static void decode_clnpass(J2kDecoderContext *s, J2kT1Context *t1, int
width, int height,
                           int bpno, int bandno, int seg_symbols)
{
    int mask = 3 << (bpno - 1), y0, x, y, runlen, dec;

    for (y0 = 0; y0 < height; y0 += 4) {
        for (x = 0; x < width; x++){
            if (y0 + 3 < height && !(
            (t1->flags[y0+1][x+1] & (J2K_T1_SIG_NB | J2K_T1_VIS |
J2K_T1_SIG)) ||
            (t1->flags[y0+2][x+1] & (J2K_T1_SIG_NB | J2K_T1_VIS |
J2K_T1_SIG)) ||
            (t1->flags[y0+3][x+1] & (J2K_T1_SIG_NB | J2K_T1_VIS |
J2K_T1_SIG)) ||
            (t1->flags[y0+4][x+1] & (J2K_T1_SIG_NB | J2K_T1_VIS |
J2K_T1_SIG)))){
                if (!ff_mqc_decode(&t1->mqc, t1->mqc.cx_states + MQC_CX_RL))
                    continue;
                runlen = ff_mqc_decode(&t1->mqc, t1->mqc.cx_states +
MQC_CX_UNI);
                runlen = (runlen << 1) | ff_mqc_decode(&t1->mqc,
t1->mqc.cx_states + MQC_CX_UNI);
                dec = 1;
            } else{
                runlen = 0;
                dec = 0;
            }

            for (y = y0 + runlen; y < y0 + 4 && y < height; y++){
                if (!dec){
                    if (!(t1->flags[y+1][x+1] & (J2K_T1_SIG | J2K_T1_VIS)))
                        dec = ff_mqc_decode(&t1->mqc, t1->mqc.cx_states +
ff_j2k_getnbctxno(t1->flags[y+1][x+1], bandno));
                }
                if (dec){
                    int xorbit, ctxno =
ff_j2k_getsgnctxno(t1->flags[y+1][x+1], &xorbit);
                    t1->data[y][x] = (ff_mqc_decode(&t1->mqc,
t1->mqc.cx_states + ctxno) ^ xorbit) ? -mask : mask;
                    ff_j2k_set_significant(t1, x, y, t1->data[y][x] < 0);
                }
                dec = 0;
                t1->flags[y+1][x+1] &= ~J2K_T1_VIS;
            }
        }
    }
 *   if (seg_symbols) {
        int val;
        val = ff_mqc_decode(&t1->mqc, t1->mqc.cx_states + MQC_CX_UNI);
        val = (val << 1) + ff_mqc_decode(&t1->mqc, t1->mqc.cx_states +
MQC_CX_UNI);
        val = (val << 1) + ff_mqc_decode(&t1->mqc, t1->mqc.cx_states +
MQC_CX_UNI);
        val = (val << 1) + ff_mqc_decode(&t1->mqc, t1->mqc.cx_states +
MQC_CX_UNI);
        if (val != 0xa) {
            av_log(s->avctx, AV_LOG_ERROR,"Segmentation symbol value
incorrect\n");
        }*
    }
}


I am working on the modification to take into account all code block styles.
Please let me know if you have any suggestion. I truly appreciate all of
your helps.

Thanks

-- 
Rukhsana Ruby
Phd Student
Department of Electrical & Computer Engineering
The University of British Columbia
============================