questions on 0xff

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:
              开发者_Go百科  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;
        }

---

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.

---

int lb = pval & 0xff;

This operation performs a bitwise AND operation on pval with 0xff.

Because 0xff is represented as 1's in the lower 8 bits of the integer, it effectively masks pval so it leaves only the value in the last 8 bits.

So, in other words it just ignores all the rest of the integer value, and does a simple modulo 256 calculation. But usually it's just for retrieving the last bits.

---

pval is an int, but the lowest 8 bits have a special meaning, and the rest of the bits have another

           3               1
           1               6        8        0
          +--------+--------+--------+--------+
pval =    |aaaaaaaa|aaaaaaaa|aaaaaaaa|bbbbbbbb|
          +--------+--------+--------+--------+

The code 'int lb = pval & 0xff' returns just the value of the 'b' bits.

pval >> 8 returns the value of the 'a' bits.

As to what the code is doing, it seems to me that it's converting some data to a colour map so it can be displayed. There's not enough information to be really sure though.