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Bit flags
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| <pre> | <pre> | ||
| B_HMIRROR | B_HVMIRROR == 1 | 3 == 3 | B_HMIRROR | B_HVMIRROR == 1 | 3 == 3 | ||
| - | B_HMIRROR + | + | B_HMIRROR + B_HVMIRROR == 1 + 3 == 4 |
| </pre> | </pre> | ||
| In conclusion, use the [[bor]] operator when dealing with bit flags to be on the safe side. | In conclusion, use the [[bor]] operator when dealing with bit flags to be on the safe side. | ||
Revision as of 12:35, 8 March 2008
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 |
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.
