Question

I am working on a C++ code. Not very sure what the code below is trying to do. Would like someone’s help on it.

int pval = t_gamma[depth[i]];
        int lb = pval & 0xff;
        switch (pval>>8) {
            case 0:
                depth_mid[3*i+0] = 255;
                depth_mid[3*i+1] = 255-lb;
                depth_mid[3*i+2] = 255-lb;
                break;
            case 1:
                depth_mid[3*i+0] = 255;
                depth_mid[3*i+1] = lb;
                depth_mid[3*i+2] = 0;
                break;
            case 2:
                depth_mid[3*i+0] = 255-lb;
                depth_mid[3*i+1] = 255;
                depth_mid[3*i+2] = 0;
                break;
            case 3:
                depth_mid[3*i+0] = 0;
                depth_mid[3*i+1] = 255;
                depth_mid[3*i+2] = lb;
                break;
            case 4:
                depth_mid[3*i+0] = 0;
                depth_mid[3*i+1] = 255-lb;
                depth_mid[3*i+2] = 255;
                break;
            case 5:
                depth_mid[3*i+0] = 0;
                depth_mid[3*i+1] = 0;
                depth_mid[3*i+2] = 255-lb;
                break;
            default:
                depth_mid[3*i+0] = 0;
                depth_mid[3*i+1] = 0;
                depth_mid[3*i+2] = 0;
                break;
        }

Answer 1

0xff means “the hexadecimal number ff” – in other words, the integer 255, which has the binary representation 00000000000000000000000011111111 (when using 32-bit integers). The & operator performs a bitwise AND operation. a & b will give you an integer with a bit pattern that has a 0 in all positions where b has a 0, while in all positions where b has a 1, the corresponding bit value from a is used (this also goes the other way around). For example, the bitwise AND of 10110111 and 00001101 is 00000101.

In this case, pval & 0xff will give you the rightmost 8 bits from pval.

As for what the code does, I believe it tries to transform color values from a special format to standard RGB: the color values seem to contain eleven bits, where three of them indicate a color range (for example, 000 seems to indicate the range “white to blue”, while 001 indicates “red to yellow”), and the rightmost eight bits indicate a color in this range.