sonix_compress.cpp

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#include "sonix_compress.h"

#define CLAMP(x)    ((x)<0?0:((x)>255)?255:(x))

typedef struct {
    int is_abs;
    int len;
    int val;
    int unk;
} code_table_t;


/* local storage */
static code_table_t table[256];
static int init_done = 0;

/* global variable */
int sonix_unknown = 0;

/*
    sonix_decompress_init
    =====================
        pre-calculates a locally stored table for efficient huffman-decoding.

    Each entry at index x in the table represents the codeword
    present at the MSB of byte x.

*/
void sonix_decompress_init(void)
{
    int i;
    int is_abs, val, len, unk;

    for (i = 0; i < 256; i++) {
        is_abs = 0;
        val = 0;
        len = 0;
        unk = 0;
        if ((i & 0x80) == 0) {
            /* code 0 */
            val = 0;
            len = 1;
        }
        else if ((i & 0xE0) == 0x80) {
            /* code 100 */
            val = +4;
            len = 3;
        }
        else if ((i & 0xE0) == 0xA0) {
            /* code 101 */
            val = -4;
            len = 3;
        }
        else if ((i & 0xF0) == 0xD0) {
            /* code 1101 */
            val = +11;
            len = 4;
        }
        else if ((i & 0xF0) == 0xF0) {
            /* code 1111 */
            val = -11;
            len = 4;
        }
        else if ((i & 0xF8) == 0xC8) {
            /* code 11001 */
            val = +20;
            len = 5;
        }
        else if ((i & 0xFC) == 0xC0) {
            /* code 110000 */
            val = -20;
            len = 6;
        }
        else if ((i & 0xFC) == 0xC4) {
            /* code 110001xx: unknown */
            val = 0;
            len = 8;
            unk = 1;
        }
        else if ((i & 0xF0) == 0xE0) {
            /* code 1110xxxx */
            is_abs = 1;
            val = (i & 0x0F) << 4;
            len = 8;
        }
        table[i].is_abs = is_abs;
        table[i].val = val;
        table[i].len = len;
        table[i].unk = unk;
    }

    sonix_unknown = 0;
    init_done = 1;
}


/*
    sonix_decompress
    ================
        decompresses an image encoded by a SN9C101 camera controller chip.

    IN  width
        height
        inp     pointer to compressed frame (with header already stripped)
    OUT outp    pointer to decompressed frame

    Returns 0 if the operation was successful.
    Returns <0 if operation failed.

*/
int sonix_decompress(int width, int height, unsigned char *inp, unsigned char *outp)
{
    int row, col;
    int val;
    int bitpos;
    unsigned char code;
    unsigned char *addr;

    if (!init_done) {
        /* do sonix_decompress_init first! */
        return -1;
    }

    bitpos = 0;
    for (row = 0; row < height; row++) {

        col = 0;

        /* first two pixels in first two rows are stored as raw 8-bit */
        if (row < 2) {
            addr = inp + (bitpos >> 3);
            code = (addr[0] << (bitpos & 7)) | (addr[1] >> (8 - (bitpos & 7)));
            bitpos += 8;
            *outp++ = code;

            addr = inp + (bitpos >> 3);
            code = (addr[0] << (bitpos & 7)) | (addr[1] >> (8 - (bitpos & 7)));
            bitpos += 8;
            *outp++ = code;

            col += 2;
        }

        while (col < width) {
            /* get bitcode from bitstream */
            addr = inp + (bitpos >> 3);
            code = (addr[0] << (bitpos & 7)) | (addr[1] >> (8 - (bitpos & 7)));

            /* update bit position */
            bitpos += table[code].len;

            /* update code statistics */
            sonix_unknown += table[code].unk;

            /* calculate pixel value */
            val = table[code].val;
            if (!table[code].is_abs) {
                /* value is relative to top and left pixel */
                if (col < 2) {
                    /* left column: relative to top pixel */
                    val += outp[-2*width];
                }
                else if (row < 2) {
                    /* top row: relative to left pixel */
                    val += outp[-2];
                }
                else {
                    /* main area: average of left pixel and top pixel */
                    val += (outp[-2] + outp[-2*width]) / 2;
                }
            }

            /* store pixel */
            *outp++ = CLAMP(val);
            col++;
        }
    }

    return 0;
}