thresh_17.c

dsp6713开发板的许多例程.对入门特别有用

void clear_below_thresh(unsigned char *restrict image, int count, 
                        unsigned char threshold)
{
    int i;
    unsigned t3_t2_t1_t0;               /* Threshold (replicated)   */
    unsigned p7_p6_p5_p4, p3_p2_p1_p0;  /* Pixels                   */
    unsigned c7_c6_c5_c4, c3_c2_c1_c0;  /* Comparison results       */
    unsigned x7_x6_x5_x4, x3_x2_x1_x0;  /* Expanded masks           */

    /* Replicate the threshold value four times in a single word */
    temp        = _pack2(threshold, threshold); 
    t3_t2_t1_t0 = _packl4(temp, temp);

    for (i = 0; i < count; i += 8)
    {
        /* Load 8 pixels from input image (one double-word).        */
        p7_p6_p5_p4 = _hi(*(double*) &image[i]);
        p3_p2_p1_p0 = _lo(*(double*) &image[i]);

        /* Compare each of the pixels to the threshold.             */
        c7_c6_c5_c4 = _cmpgtu4(p7_p6_p5_p4, t3_t2_t1_t0);
        c3_c2_c1_c0 = _cmpgtu4(p3_p2_p1_p0, t3_t2_t1_t0);

        /* Expand the comparison results to generate a bitmask.     */
        x7_x6_x5_x4 = _xpnd4(c7_c6_c5_c4);
        x3_x2_x1_x0 = _xpnd4(c3_c2_c1_c0);
        
        /* Apply mask to the pixels.  Pixels that were less than or */
        /* equal to the threshold will be forced to 0 because the   */
        /* corresponding mask bits will be all 0s. The pixels that  */
        /* were greater will not be modified, because their mask    */
        /* bits will be all 1s.                                     */
        p7_p6_p5_p4 = p7_p6_p5_p4 & x7_x6_x5_x4;
        p3_p2_p1_p0 = p3_p2_p1_p0 & x3_x2_x1_x0;

        /* Store the thresholded pixels back to the image.          */
        *(double*) &image[i] = _itod(p7_p6_p5_p4, p3_p2_p1_p0);
    }
}