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84 lines
2.6 KiB
Plaintext
84 lines
2.6 KiB
Plaintext
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Smart Dirty Rectangle Management
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--------------------------------
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pig_dirty() contains an algorithm that tries to find
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the the best dirtyrect candidates for merging. While
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searching, it looks out for perfect or sufficiently
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good candidates.
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(Perfect candidate:)
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The merged rectangle is of the same size
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as the largest of the two input rectangles:
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Amerged <= MAX(A1, A2)
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We don't actually test for this, but rather for...
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Instant Pick candidate:
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Not Perfect, but good enough to be treated
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as such, considering the cost of going on
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searching for a better candidate:
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Amerged 100 / MAX(A1, A2) - 100 <= PIG_INSTANT_MERGE
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(That is, the area of the merged rect must be
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no more than PIG_INSTANT_MERGE % bigger than
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the area of the larger of the two input rects.)
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Note that this is also about how likely it is
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that thereis* a better candidate. Assuming
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that PIG_INSTANT_MERGE is set to a sensible
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value, it is not very likely at all. There
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would have to be another dirtyrect nearby, and
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the chance of that being a better match is
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rather low, since that would most likely have
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caused it to be merged with the tested dirtyrect
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long before our new rect came in.
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(Good candidate:)
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The area of the merged rectangle is smaller
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than the total area of the two input rectangles:
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(Amerged - A1 - A2) < 0
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We don't actually test for this, but rather for...
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Acceptable candidate:
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The area of the merged rectangle is larger
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than the total of the two input rectangles, but
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since there is some per-rectangle overhead,
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merging is still a win:
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(Amerged - A1 - A2) <= PIG_WORST_MERGE
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The default setting assumes that the cost of a
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rectangle is in the range of 300 pixels. One
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should probably benchmark a few different systems
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to see if that's reasonable.
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Unacceptable candidate:
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The area of the merged rectangle is larger than
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the total of the input rectangles to the degree
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that merging is a definite loss:
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(Amerged - A1 - A2) > PIG_WORST_MERGE
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The algorithm instantly returns Perfect candidates as
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solutions. If there are only Good and Acceptable
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candidates, the best one (lowest number of wasted
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pixels) is the solution.
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If there are only Unacceptable candidates, there is no
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sensible merger solution, so pig_dirty() will try to add
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a new dirtyrect.
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If that fails (table full), the best candidate is used,
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even though it would have been Unacceptable under normal
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circumstances.
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TODO: Thereare* more alternatives than just "merge" or
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TODO: "don't merge"! For example, it might pay off to
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TODO: detect overlapping and clip dirtyrects to avoid
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TODO: it, when merging is not a viable option.
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