Hi Sean,
Can you use the position of the tile as a unique key? Then the manager
only needs to calculate each tile's position in the original image.
Each tile extracts the bits, processes and notifies the result with its
position.
Regards, Tony
On 31/01/2022 10:06 am, Murphy, Sean wrote:
I'm hitting a design issue with the way I'm using the QtConcurrent
module to do some image processing, and I'm wondering if someone can
give some pointers?
At a high level, the software needs to do some processing on every
pixel of an image. The processing can mostly be done in parallel, so
I've created the following:
1. Tile class - responsible for doing the processing on a small
subset of the original image
1. Has a constructor that takes a Position and Size. From those
parameters, the Tile knows what subset of the original image
it is going to process
2. Has a process() function which will do the work on those
assigned pixels
2. TileManager class - responsible for managing the Tile objects
1. Contains a for-loop that creates each Tile object, assigns it
a unique Position, and adds it to the QVector<Tile> vector
2. Has a processTile(Tile& t) function which calls t.process() to
tell a given Tile to begin its work
3. Calls QtConcurrent::map(tiles, processTile) to process each tile
So far this works well, but as I was timing different parts of the
codebase, I discovered that a large portion of the time is spent
allocating the QVector<Tile> vector (step 2a above) before I get to
the concurrent processing call. The reason why is obvious to me - I
need to ensure that each tile is created with a unique assignment and
as far as I can see, that need to happen in a single thread? If I
could instead pass off the Tile creation to the parallel processing
step, I might be able to improve the overall performance, but I don't
see a way around it within the QtConcurrent framework.
How can I go about creating Tile objects in parallel AND ensure that
each of them gets a unique Position assignment? I could easily move
the Tile allocation into processTile(), but if I do that, I don't see
a way make the unique position assignment since I don't see how a
given call to processTile() would know where it is in the overall
parallelization sequence to determine what Position to assign to the
Tile it creates. If I were using something like CUDA, I could use
things like blockIdx and threadIdx to do that, but as far as I can
see, those concepts don't exist (or at least aren't exposed) in
QtConcurrent.
Any thoughts?
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