Re: [Openexr-devel] Writing Deep tiled images: help !

[Peter Hillman]

In general with OpenEXR, you cannot write to a file from different 
threads unless they have a shared framebuffer. Calling "setFrameBuffer" 
from one thread while another is writing may cause a crash.

Even if you do have a shared framebuffer, the threads lock to serialise 
the writes: only one "writePixels" or "writeTile" request goes through 
at once.
If you don't use the IlmThread library to run your threads it may be 
wise to implement your own lock mechanism around writePixel and 
writeTile calls.


There are two ways of writing tiled data: if you use 
xTileCoords/yTileCoords, then pixel(0,0) is the bottom left corner of 
each tile, implying the image buffer pointed to by the slice is the same 
size as the tile.
if you don't do that, then pixel(0,0) is the bottom left corner of the 
displayWindow, implying the image buffer pointed to be the slice is the 
size of the dataWindow, accounting for offset data windows appropriately.

In your case, it would seem like you want the second option: preallocate 
your dataWindow sized array of pointers-to-samples, and call 
setFrameBuffer before any rendering begins.
Then each thread sets the pointers to the samples for the tile it is 
going to write and fills in the corresponding part of the samplecount 
array before calling writeTile.

The pointers need not be valid when setFrameBuffer is called. When you 
call writeTile, only the pointers and sampleCount data for the pixels 
being written need to be valid, and need not remain valid afterwards. So 
yes, you can delete data for the actual sample data of a tile after 
calling writeTile, though you'll need to keep the sampleCount array and 
the pointers-to-samples array for each channel.

Note that if you are writing tiles in anything other than RANDOM_Y 
order, then whenever your threads deliver tiles out of order OpenEXR 
will have to buffer data so it can write the tiles sequentially.
I suspect this may well be the case from multithreaded write 
applications. With deep data this could take a significant amount of 
memory.
You may wish to enforce RANDOM_Y order to allow the library to write 
tiles to disk as soon as you call writeTile. If your read pattern is 
generally also random, rather than in 'scanline order', there'll be 
little performance hit.

You might also consider having one worker thread managing the writing of 
tile data and marshalling into the format required for the DeepSlices, 
and have other threads compute data and signal the worker thread when 
they have tiles ready for processing.

Yes, you can use an Array2D<float*> for your data points (dataZ and 
dataA in the example). Each value is a pointer to the first sample of 
the channel data for each pixel.
Subsequent samples are found using the offset provided. That means you 
can store pixel data in separate or interleaved channels.



On 11/04/14 02:16, Michel Lerenard wrote:
> Hi,
>
> I'm trying to write deep data into an EXR file and am having trouble 
> understanding how the API works.
> I'd like to be able to write tiles from different threads. The 
> application I'm working on (Isotropix Clarisse) runs several threads, 
> each one render a part of an image.
> I'd like to write deep data when the current tile is completely computed.
>
> I'm using the "Reading And Writing Image Files" documentation, and the 
> section about Deep Tile File writing on page 26 is confusing to me.
>
> From what I understand, I need a DeepFrameBuffer to write an image. It 
> seems it should be the same for all threads, as a DeepTiledOutputFile 
> can only use one. Is it thread safe ? Can I use the same FrameBuffer 
> from all threads ?
> I was puzzled by the type of dataZ and data0, reading the next example 
> (reading a deep tiled file) I understand that the arrays are only 
> storing pointers to deep data for a particular pixel, and that they 
> could as well be Array2D<float*>. Is that correct ?
>
> If it is, this means that each one of my threads should set pointers 
> in the framebuffer DeepSlice for each pixel of the tile it's 
> computing, then call writeTile. After that can I delete the data and 
> reset the pointers in the DeepSlice ?
>
>
> Thanks,
>
> Michel
>
>
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