On Thu, May 27, 2010 at 4:01 PM, Frieder Ferlemann
<frieder.ferlemann@xxxxxx> wrote:
> Hi,
>
> Am 28.05.2010 00:04, schrieb Conn Clark:
>> On Thu, May 27, 2010 at 8:51 AM, Brian Paul <brianp@xxxxxxxxxx> wrote:
>>
>> This code could be written with a faster algorithm requiring just 13 operations
>>
>> + pixel_number |= ((x >> 0) & 1) << 0; // pn[0] = x[0]
>> + pixel_number |= ((y >> 0) & 1) << 1; // pn[1] = y[0]
>> + pixel_number |= ((x >> 1) & 1) << 2; // pn[2] = x[1]
>> + pixel_number |= ((y >> 1) & 1) << 3; // pn[3] = y[1]
>> + pixel_number |= ((x >> 2) & 1) << 4; // pn[4] = x[2]
>> + pixel_number |= ((y >> 2) & 1) << 5; // pn[5] = y[2]
>>
>
>
>> /* suitable for all 16 bit or greater processors that can do an
>> unsigned 16 bit or greater multiply */
>> /* tested and verified */
>>
>> pixel_number = ((((x & 0x07) * 0x1111 & 0x8421) * 0x1249 >> 9) & 0x55 ) |
>> ((((y & 0x07) * 0x1111 & 0x8421) * 0x1249
>>>> 8) & 0xAA );
>>
>> Note if it is known that x and y are less than or equal to 7 it can be
>> done in 11 operations.
>
> Cool. How does it compare to:
>
> const unsigned char /*int*/ spread_bits[8] = {
> 0x00, /* 0b000 to 0b00000 */
> 0x01, /* 0b001 to 0b00001 */
> 0x04, /* 0b010 to 0b00100 */
> 0x05, /* 0b011 to 0b00101 */
> 0x10, /* 0b100 to 0b10000 */
> 0x11, /* 0b101 to 0b10001 */
> 0x14, /* 0b110 to 0b10100 */
> 0x15, /* 0b111 to 0b10101 */
> };
>
> pixel_number |= spread_bits[x & 0x07];
> pixel_number |= spread_bits[y & 0x07] << 1;
>
>
> Greetings,
> Frieder
>
Look up tables have some hidden penalties but I think it might be a
win. Looks like we may have to benchmark the solutions against one
another to really know which is best in real life.
Conn
--
Conn O. Clark
Observation: In formal computer science advances are made
by standing on the shoulders of giants. Linux has proved
that if there are enough of you, you can advance just as
far by stepping on each others toes.
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