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Bit flags
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[edit] Definition
Bit flags are constants which each denotes a single unique case in one situation and can be combined to form different, unique cases. They are called bit flags, because when bits are used to denote a cases, we observe they are indeed single and unique and can be combined to form different unique combinations.
Bit flags are often used as integers:
Bit pattern | - Integer value |
0001 | - 1 |
0010 | - 2 |
0100 | - 4 |
1000 | - 8 |
These can be combined to form, for example:
Bit pattern | - Integer value |
1001 | - 9 |
0110 | - 6 |
1110 | - 14 |
0101 | - 5 |
[edit] Example
When we look at blit flags for example, we see the values:
Constant | - Value | - Description |
B_HMIRROR | - 1 | - Blit the graph horizontally mirrored. |
B_VMIRROR | - 2 | - Blit the graph vertically mirrored. |
B_TRANSLUCENT | - 4 | - Blit the graph with half transparency. |
B_ALPHA | - 8 | - Blit the graph in some way. (What does this do exactly?) |
B_ABLEND | - 16 | - Blit the graph using additive blending (nice effect for fire). |
B_SBLEND | - 32 | - Blit the graph using subtractive blending (nice effect for ghosting). |
B_NOCOLORKEY | - 128 | - Blit the transparent parts of the graph as black. |
These are all single unique cases and can be combined to form different unique cases. For example, when we want a translucent, horizontally mirrored blit operation with use of additive blending, we would do:
B_HMIRROR | B_TRANSLUCENT | B_ABLEND 1 | 4 | 16 = 21
Because the bits are unique, the addition operator can also be used. But when we consider there would be a constant called B_HVMIRROR, which has the value B_HMIRROR|B_VMIRROR (3), the addition operator can't be used all the time:
B_HMIRROR | B_HVMIRROR == 1 | 3 == 3 B_HMIRROR + B_HVMIRROR == 1 + 3 == 4
In conclusion, use the bor operator when dealing with bit flags to be on the safe side.
[edit] Usage Example
Process main() Private unsigned byte options; Begin /* Manipulate bit flags */ // To set options, use = (the bits with 1's will be targeted) options = 01000001b; // To add options, use | (the bits with 1's will be targeted) options |= 00000001b; // To remove options, use & (the bits with 1's will be targeted) options &= ~00010000b; // same: options &= 11101111h (the bits with 0's will be targeted) // To switch options, use ^ (the bits with 1's will be targeted) options ^= 10000000b; /* Manipulate bit flags using the << operator (these are equivalent to the last three) */ options|= 1<<4; // position 0 is now ON options&= ~(1<<4); // position 4 is now OFF options^= (1<<7); // position 7 is now SWITCHED /* Checking of bit flags */ // Check if a certain option is set (the bits with 1's will be checked) if((options&10000000b)==10000000b) end // Check if a certain option is not set (the bits with 1's will be checked) if((options&10000000b)==0) end // Check if multiple options are set (the bits with 1's will be checked) if((options&10000001b)==10000001b) end // Check if multiple options are not set (the bits with 1's will be checked) if((options&00000110b)==0) end // Check if at least one of multiple options is set (the bits with 1's will be checked) if((options&00000110b)!=0) end // Check if at least one of multiple options is not set (the bits with 1's will be checked) if((options&00000110b)!=00000110b) end /* Checking of bit flags using the << operator */ // Check if a certain option is set if((options&(1<<7))==(1<<7)) // position 7 is ON end // Check if a certain option is not set if((options&(1<<7))==0) // position 7 is OFF end repeat frame; until(key(_ESC)) End
Used in example: key(), Bitwise Operators