Help patch cleanups and harmonization

[sebescudie]

Opening this PR so we can discuss things about help patches cleaning before merging.

In 340ff31, i reworked patches from the General category. I'd like to show some information on why HoldLatestCopy is used in HowTo Apply operations asynchronously instead of a plain HoldLatest. Guess it has to do with the fact that we're dealing with images here, but could you shed some light on this @ravazquez ?

Thanks!

[sebescudie]

In 4f4844a, I drastically simplified the HowTo Count pixel that are not black. It was bloated with too many nodes, which made the very goal of the patch unclear.
Took the AND (Filter) node out and brought it to a new patch with OR (Filter) and XOR (Filter) : HowTo Apply boolean operations on images, which basically shows what HowTo Count pixel [...] did, with OR and XOR added.

Happy to discuss!

[ravazquez]

Opening this PR so we can discuss things about help patches cleaning before merging.

In 340ff31, i reworked patches from the General category. I'd like to show some information on why HoldLatestCopy is used in HowTo Apply operations asynchronously instead of a plain HoldLatest. Guess it has to do with the fact that we're dealing with images here, but could you shed some light on this @ravazquez ?

Thanks!

In the case of OpenCV we try to copy the image data or allocate new memory as little as possible for efficiency, this means that we reuse the same image reference (or the same memory) over and over which is fine for most cases. However there are cases in which you need to further do stuff with the image (modify it), or cases in which the image is being written into from another thread while you are reading it, both of which would give you memaccess and corrupted data problems either upstream or downstream.

In general, to avoid reading into memory which may be being written into or which may not be ready, you can use HoldLatestCopy which will just copy the data over and hold the copy, allowing any further processing on the original image to be of no concern to you, this of course comes with a cost.

As a rule of thumb always try using HoldLatest unless it gives you issues, in which case you should use HoldLatestCopy.

@azeno can probably explain this in much more detail.

[sebescudie]

@ravazquez Thanks, that's clear to me and surely enough for some in-patch explanations.