Avisynth+

[TurboPascal7]

Note to all people having any kind of issues with mt build: first of all, try removing SetFilterMTMode("", MT_MULTI_INSTANCE) from your script. I have absolutely no idea why this was introduced in the first place since it only creates problems. Yes, this will cripple performance but no one promised you it'll be easy.

Second - setting any MT mode on script functions does nothing. For example, SetFilterMTMode("Dither_Crop16", MT_MULTI_INSTANCE) doesn't change anything. You can put any other MT mode there and it will still work they way it works by default. Only plugin functions matter.

Third - situation where one plugin works with mode 1 when used before filter X and only with mode 2 when X is not present is pretty much impossible (assuming there are no bugs in the core of course ). MT mode is a property of a filter and it doesn't depend on where it's used, only on filter implementation and input parameters (unless the filter is completely broken). I understand this is not helping much and thank you for posting scripts with this kind of problems, but most likely it means that MT mode of one of the filters was not "guessed" right. Hopefully we can get authors of some plugins who are still alive (dither for instance) to supply correct MT modes for every function in their plugins, either as a registration routine in the plugin itself, or SetFilterMtMode line. If we can't, well... we'll be guessing.

EDIT: as innocenat pointed out on IRC, it's actually possible to have filters changing their behavior if they're called before a filter with MT mode 3. All filters are getting serialized and any MT mode you're applying to them should not matter. This is an implementation detail though and you should not rely on it in any way since it might get changed without notice.

Now, to Prefetch(1)/Prefetch(0) issue. As ultim explained (no one bothered to read the full MT implementation tbh because it feels a bit (over)complicated), the number that you supply to prefetch is the number of prefetching threads. So in theory Prefetch(1) should give you two threads - one main thread and one prefetching thread. Prefetch(0) should disable any multithreading.

As usual, practice doesn't match theory very well and Prefetch(1) actually gives you one working thread (dunno if it's the main or the created one). Yes, this is a bug and it most likely will be fixed.

But if you think about it, this API design is very bad. Most users don't know and don't need to know the difference between prefetching threads and the main thread. Adding Prefetch(4) to their script, they absolutely reasonably expect it to work in 4 threads, trying to explain to them that it's actually 5 would be silly. So this bug kinda makes the API better.

In short - yes, currently Prefetch(1) and Prefetch(0) are almost equivalent. In the future one of these might error out, or this behavior will be kept (probably more reasonable).