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This moves the tap updating to a function shared by rendering and
context create... so we can have a valid externally-driven tap
value before rendering a single frame if possible. (this is
important for not having spurious frames/flickers in rkt
sequences)
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This really needs SIMD to fly on-cpu, but this improves things
some.
Using `--module=mixer,style=fade,a_module=roto,b_module=roto\
--video=mem,size=1366x768 --defaults --go` to test:
Before FPS: 92-95 floating mostly around 94-95
After FPS: 107-111 floating mostly around 108-109
so +14.8% FPS
(2c/4t i7 X230)
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It's problematic/racy to always be dereferencing the tap pointer
to access it's current value.
Running checkers:fill_module=mixer on a 2c/4t (n_cpus=4) machine
was quite effective at crashing in mixer::render_fragment()
due to dereferencing a NULL snapshot fragment.
The "tween" T value is being used to indicate when interpolation
of the snapshots is necessary. But repeatedly re-reading the T
value via the tap pointer would race with the driving tap. In
the case of checkers, which is a threaded module, the mixer
contexts are all rendering in parallel at the same path so
they're sharing a single pipe for their T taps.
This situation of sets of contexts sharing a single path,
resulting in their taps all landing on a single pipe, is still
largely up in the air and might be actively prevented in the
future. But until then, it can be made far less crashy and
problematic by just being more careful about dereferencing the
tap to access its latest value just once at the prepare frame
stage, storing it in the local T variable in the context. Then
all the render_fragment() accesses can at least find a stable
value in the context from prepare-to-render, so at least the
snapshots are there when they should be according to the T value
etc.
Something like this will probably need to be done regardless of
what happens with the context sets sharing the same path. Since
when a given tap isn't the driver, it still has to take care to
just grab the updated value once for the frame... The tap API
can't really automagically do that single update of the local
variable when passenger though, since it's been deliberately kept
devoid of all the type-specific accessor junk (ptr/elems etc are
all void **/void* in the tap api side). Hence why this commit is
just ad-hoc updating the local variable in the else branch; it's
best positioned to do so directly. But there still needs to be
more consideration for thread-safety.
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This is not optimized at all and tends to hurt the FPS
significantly. This is one of those things that would hugely
benefit from SIMD, but even without SIMD it could be done better.
I just slapped together something obvious for now, as I'd like to
focus more on the rkt side but need a better fader for scene
transitions than style=flicker.
Also changed {a,b}_module= preferred values to blank,compose
so you see something happen if you just run --defaults.
Otherwise, the compose,compose would just fade between two
identical compositions invisibly.
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Originally I was thinking a variadic inputs= like compose::layers
would be desirable, with the T value's integer serving as an
index into the inputs, and the fraction between the integers as
the mixing T.
But I changed my mind and am instead constraining mixer
explicitly to two distinctly named modules; a_module= and
b_module= with the T value 0-1 mapping to a-b.
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I thought the build was already using -Wall but that seems to not
be the case, maybe got lost somewhere along the line or messed up
in configure.ac
After forcing a build with -Wall -Werror, these showed up.
Fixed up in the obvious way, nothing too scary.
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this is very early/unfinished, hence experimental flag
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