[FFmpeg-devel] [PATCH] avutil/eval: add function to track variable use

[Gyan]

On 05-11-2019 04:35 am, Michael Niedermayer wrote:
> On Sun, Nov 03, 2019 at 11:14:25AM +0530, Gyan wrote:
>> Helps better identification of expr eval failures.
>>
>> Gyan
>>   eval.c    |   21 +++++++++++++++++++++
>>   eval.h    |   10 ++++++++++
>>   version.h |    4 ++--
>>   3 files changed, 33 insertions(+), 2 deletions(-)
>> 3dd142baa0144fd324eb9da8a9932cfd7ab2cd98  0001-avutil-eval-add-function-to-track-variable-use.patch
>>  From 19bce329464676f071707b99575f80e5abe1cd4c Mon Sep 17 00:00:00 2001
>> From: Gyan Doshi <ffmpeg at gyani.pro>
>> Date: Sat, 2 Nov 2019 20:16:42 +0530
>> Subject: [PATCH] avutil/eval: add function to track variable use
>>
>> Helps avoid multiple evals of cross-referenced expressions
>> and catch the use of non-applicable variables with respect
>> to eval or special mode in filters
> Maybe you should provide more details of the use case of this, maybe
> with an example. Because it seems not completely obvious

An example of a cross-referenced expression would be 'y=x+rand(10)'.  
Normally in filters, X would be evaluated first, then Y, then X again. 
The 2nd eval of X will overwrite the first and can generate a different 
value. With this func, we can avoid the 2nd eval.

I'm in the process of modifying the scale filter to allow 'n', 't', 
'pos' variables in frame eval mode (it already supports dynamic output 
based on incoming frame changes) and I would like to catch their 
presence in init mode, since av_expr_eval will fail. Now, it can also 
fail due to circularly referenced expressions e.g. 'x=y+10' and 'y=x+10' 
and there's no way to detect whether it's this case or if inapplicable 
variables have been used.

Regards,
Gyan