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76 lines
4.3 KiB
Text
76 lines
4.3 KiB
Text
Optional properties in caps:
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During the 0.8 series of GStreamer, we regularly felt the need to add
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properties to caps, thereby sometimes breaking specific caps negotiation
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cases or annoying developers. This document outlines problems it could
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lead to and tries to explain in which cases properties can be added and
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when they can't. It also explains when properties can be optional (both
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temporarily and permanently) and when they cannot.
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--
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There's two cases where optional properties could be added:
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1) to fix an issue that makes any case fail
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2) to fix an issue that makes some cases fail
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Case 1 can be compared to not providing extra_data in caps for WMA. The caps
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are defined, but it will never work because you cannot decode WMA audio
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without the sequence header. In this case, adding the property breaks
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caps compatibility, but it is still allowed because there is no regression
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and it fixes a bug. No optional property should be added here, it should be
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made a required property directly. Another example here is channel-positions
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for channels>2.
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Case 2 is more complex. There's various subcases:
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a] not providing this property means ANY (or don't care, or unknown, each
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of which is another way of saying ANY)
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b] not providing this property means a specific value
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.] adding this property will lead to unwanted behaviour
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.] adding this property will not lead to unwanted behaviour
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An example case for 2a is the buffer-frames property in float audio or the
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frames property in MPEG audio. Buffer-frames is 0 (which should be removed)
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means ANY. The reason that it should not be zero is because connecting an
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element with buffer-frames=SOME_VALUE should be allowed to connect to any
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element out there that has no buffer-frames requirement. The opposite is
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true reversely: an element with no buffer-frames property should never be
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allowed to connect to any element requesting a specific buffer-frames value.
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For MPEG audio, it is TRUE likewise. Mathematically, buffer-frames=0 does not
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exist. It implies ANY. Similarly, framed=FALSE cannot exist, because it
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implies framed={FALSE,TRUE} (in words: an element cannot require non-framed
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MPEG audio, because framed MPEG audio is a subset of non-framed MPEG audio
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and thus valid input). In all those cases for 2a, optional properties are
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fine. Subtraction will not work, but as explained, values are subsets of
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another value and thus subtraction is irrelevant (because the mathematical
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value of the subtraction has no real value).
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This same principle is true for rate/channels on (for example) MPEG audio.
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Our caps negotiation already allows for all of this, and optional properties
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are already being used for this.
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2b is complex, since subtraction actually has a value here, and addition
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of such properties may lead to regressions or crashes. Let's give another
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two examples: stride (for raw video; not providing this value implies 4-byte
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aligned video) and pixel-aspect-ratio (default value being 1/1). Adding p-a-r
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was in this case an example of 2bII, whereas stride is 2bI. The reason for
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this is simple: adding pixel-aspect-ratio to some element and not to others
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could lead to misunderstanding size. However, this is not a regression,
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because not adding it altogether would lead to the same misunderstanding.
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In both cases, the result would be wrongly sized video. Therefore, there
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is no regression and there is a bugfix, so this is fine. Obviously, the
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optional property is in this case very specifically a temporary solution.
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As soon as we can, this property should become obligatory.
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Stride is different, because elements have implicit associated behaviour
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based on the previous beahviour. This hebaviour could break if some elements
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do implement stride and others still don't. Therefore, adding an optional
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property as a temporary hack is, in this case, not a good idea and should
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be disallowed. A proper fix should be done in the same timeframe as 2bII. In
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this case, optional properties should not be added. Another example of this
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case was adding channel-positions to audio caps with channels=1,2, which
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was rejected for the same reason: it would break a perfectly-working set
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of rules in a stable series.
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--
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Obviously, with all of the above, people will start fighting about which
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group their specific properties change belongs to. General consensus is
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the only way to get around that problem. Long live politics.
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