- **C Programming**
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- - **Re: seeking bitwise operations solution**
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Re: seeking bitwise operations solutionNick Austin wrote:
> On 16 Aug 2003 22:30:10 -0700, alexr@bellnet.ca (Alex) wrote: > > >>Hi >> >>Is there any way to arrive at the following results, using only >>bitwise operators: >> >>in out >>=== === >>0 0 0 0 >>0 1 1 1 >>1 0 1 1 >>1 1 1 1 > > > What does this table represent? If you mean: > > in out > ======= ======= > a=0 b=0 a=0 b=0 > a=0 b=1 a=1 b=1 > a=1 b=0 a=1 b=1 > a=1 b=1 a=1 b=1 > > Where a and b are unsigned int then: > > a = b = a | b; Or, if a and b are the two least significant bits of an unsigned int in unsigned int out = in&1 | (in >> 1)&1; out |= (out << 1); |

Re: seeking bitwise operations solutionThanks Nick, for pointing me in the right track.
Alex LPS: I have 2-bit depth image data that I would like to mask. Data is contained in a sequence of 6200 bytes, where each byte contains the color table index values of 4 pixels (2 bits per pixel): 0 0 = table index 0 (currently associated color = white) 0 1 = table index 1 (currently associated color = light gray) 1 0 = table index 2 (currently associated color = dark gray) 1 1 = table index 3 (currently associated color = black) For example, a sequence of (white, black, dark grey, light gray) would be stored as 0 0 1 1 1 0 0 1. What I want to do is convert that sequence such that all non-white pixels will become black. For example, the sequence 0 0 1 1 1 0 0 1 would become 0 0 1 1 1 1 1 1. And I want to do that as fast as possible. Thanks for your help. Alex "E. Robert Tisdale" <E.Robert.Tisdale@jpl.nasa.gov> wrote in message news:<3F3F1EA5.1090701@jpl.nasa.gov>... > Nick Austin wrote: > > On 16 Aug 2003 22:30:10 -0700, alexr@bellnet.ca (Alex) wrote: > > > > > >>Hi > >> > >>Is there any way to arrive at the following results, using only > >>bitwise operators: > >> > >>in out > >>=== === > >>0 0 0 0 > >>0 1 1 1 > >>1 0 1 1 > >>1 1 1 1 > > > > > > What does this table represent? If you mean: > > > > in out > > ======= ======= > > a=0 b=0 a=0 b=0 > > a=0 b=1 a=1 b=1 > > a=1 b=0 a=1 b=1 > > a=1 b=1 a=1 b=1 > > > > Where a and b are unsigned int then: > > > > a = b = a | b; > > Or, if a and b are the two least significant bits > of an unsigned int in > > unsigned int out = in&1 | (in >> 1)&1; > out |= (out << 1); |

Re: seeking bitwise operations solutionalexr@bellnet.ca (Alex) writes:
> LPS: I have 2-bit depth image data that I would like to mask. Data is > contained in a sequence of 6200 bytes, where each byte contains the > color table index values of 4 pixels (2 bits per pixel): > > 0 0 = table index 0 (currently associated color = white) > 0 1 = table index 1 (currently associated color = light gray) > 1 0 = table index 2 (currently associated color = dark gray) > 1 1 = table index 3 (currently associated color = black) > > For example, a sequence of (white, black, dark grey, light gray) > would be stored as 0 0 1 1 1 0 0 1. > > What I want to do is convert that sequence such that > all non-white pixels will become black. > > For example, the sequence 0 0 1 1 1 0 0 1 would become > 0 0 1 1 1 1 1 1. > > And I want to do that as fast as possible. You might want to use a look-up table which maps each possible 8-bit sequence to the required output value. Such a table would have 256 entries, so it'd be small enough for most applications/environments. Martin |

Re: seeking bitwise operations solutionOn 17 Aug 2003 14:21:58 -0700, alexr@bellnet.ca (Alex) wrote:
>Thanks Nick, for pointing me in the right track. > >Alex > >LPS: I have 2-bit depth image data that I would like to mask. Data is >contained in a sequence of 6200 bytes, where each byte contains the >color table index values of 4 pixels (2 bits per pixel): > >0 0 = table index 0 (currently associated color = white) >0 1 = table index 1 (currently associated color = light gray) >1 0 = table index 2 (currently associated color = dark gray) >1 1 = table index 3 (currently associated color = black) > >For example, a sequence of (white, black, dark grey, light gray) >would be stored as 0 0 1 1 1 0 0 1. > >What I want to do is convert that sequence such that >all non-white pixels will become black. > >For example, the sequence 0 0 1 1 1 0 0 1 would become >0 0 1 1 1 1 1 1. Well that's easy. Convert all four pixels as one operation: newcolor = ( (oldcolor<<1) & 0xAA ) | oldcolor | ( (oldcolor>>1) & 0x55 ); >And I want to do that as fast as possible. So make it a table look-up: newcolor = conversion[oldcolor]; Of course you need to populate the table first. Create a function that does this for all 256 values and call that function during initialisation. Alternatively make the populate function into a separate program and paste all 256 result values into your program as an initialiser: const unsigned char conversion[] = { 0x00, 0x03, 0x03, 0x03, 0x0c, 0x0f, 0x0f, 0x0f, 0x0c, 0x0f, 0x0f, 0x0f, 0x0c, 0x0f, 0x0f, 0x0f, 0x30, 0x33, 0x33, 0x33, 0x3c, 0x3f, 0x3f, 0x3f, 0x3c, 0x3f, 0x3f, 0x3f, 0x3c, 0x3f, 0x3f, 0x3f, /* etc... */ } Nick. |

Re: seeking bitwise operations solutionThanks to everyone for their help. Problem solved thanks to you.
Extra side effect: a lesson in modesty was obtained. Coming soon: more easy questions ! :) Alex Nick Austin <nickDIGITONE@nildram.co.uk> wrote in message news:<sfuvjvkju384lfq6dqf6e16035c7hd78fq@4ax.com>. .. > On 17 Aug 2003 14:21:58 -0700, alexr@bellnet.ca (Alex) wrote: > > >Thanks Nick, for pointing me in the right track. > > > >Alex > > > >LPS: I have 2-bit depth image data that I would like to mask. Data is > >contained in a sequence of 6200 bytes, where each byte contains the > >color table index values of 4 pixels (2 bits per pixel): > > > >0 0 = table index 0 (currently associated color = white) > >0 1 = table index 1 (currently associated color = light gray) > >1 0 = table index 2 (currently associated color = dark gray) > >1 1 = table index 3 (currently associated color = black) > > > >For example, a sequence of (white, black, dark grey, light gray) > >would be stored as 0 0 1 1 1 0 0 1. > > > >What I want to do is convert that sequence such that > >all non-white pixels will become black. > > > >For example, the sequence 0 0 1 1 1 0 0 1 would become > >0 0 1 1 1 1 1 1. > > Well that's easy. Convert all four pixels as one operation: > > newcolor = ( (oldcolor<<1) & 0xAA ) | oldcolor | > ( (oldcolor>>1) & 0x55 ); > > >And I want to do that as fast as possible. > > So make it a table look-up: > > newcolor = conversion[oldcolor]; > > Of course you need to populate the table first. Create a function > that does this for all 256 values and call that function during > initialisation. > > Alternatively make the populate function into a separate program > and paste all 256 result values into your program as an initialiser: > > const unsigned char conversion[] = > { > 0x00, 0x03, 0x03, 0x03, 0x0c, 0x0f, 0x0f, 0x0f, > 0x0c, 0x0f, 0x0f, 0x0f, 0x0c, 0x0f, 0x0f, 0x0f, > 0x30, 0x33, 0x33, 0x33, 0x3c, 0x3f, 0x3f, 0x3f, > 0x3c, 0x3f, 0x3f, 0x3f, 0x3c, 0x3f, 0x3f, 0x3f, > /* etc... */ > } > > Nick. |

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