ofxAsyncTransfer

Addon for asynchronous pixel transfers using Pixel Buffer Objects for techniques like streaming texture uploads and asynchronous read-back

Usage

Asynchronous Reading

Suppose a dynamic texture source like ofVideoGrabber, persistent pixels for storing the contents of the texture, and an instance of ofxAsyncTransferReader:

ofVideoGrabber grabber;
ofPixels pixels;
ofxAsyncTransferReader reader;

In update() or other functions periodically called:

grabber.update();
reader.copyToPixels(grabber.getTexture(), pixels);

When the frame size of reader is more than 1, copyToPixels() asynchronously transfers the contents of a texture to a front pixel buffer in its circular buffer, and copies the contents of a back pixel buffer to pixels, so that tranfering GPU memory to CPU one does not block execution of copying pixels. This will give a boost in data transfer throughput and frame rate with the cost of some amount of CPU usage, GPU memory, and importantly, latency.

copyToPixels() is just a shorthand function, which is equivalent to the code below except an automatic allocation of pixels:

grabber.update();
const auto bound = reader.bind<unsigned char>(grabber.getTexture());
std::copy(std::begin(bound), std::end(bound), pixels.getData());
reader.unbind();

Or using a shorthand function:

grabber.update();
const auto bound = reader.bindAsPixels(grabber.getTexture());
std::copy(std::begin(bound), std::end(bound), pixels.getData());
reader.unbind();

Reading from GPU Targets

TBD

ofTexture texture;
ofxAsyncTransferReader reader;

const auto pixels = reader.bind<unsigned char>(texture);
processPixels(pixels);
reader.unbind();

reader.copyToPixels<unsigned char>(texture, pixels);

Writing to GPU Targets

TBD

ofTexture texture;
ofxAsyncTransferWriter writer;

const auto pixels = writer.bind<unsigned char>(texture);
updatePixels(pixels);
writer.unbind();

Implementation Details

Constness of Pixels

There is a workaround in ofxAsyncTransferPixels_<T> for avoiding the violation of constness of its data, since ofPixels_<T> does not accept const types in its template argument.

When a ofxAsyncTransferPixels_ is specialized with const unsigned char, the returned type of getPixels() will be const ofPixels_<unsigned char> so that it retains constness for most of the time except in cases where const_cast is used for it.

Temporary Returned Value and Implicit Conversion

ofxAsyncTransferPixels_<T> is implicitly convertible to ofPixels_<T>. Note that the returned type of bind() is a temporary object and that of getPixels() is a Lvalue reference of a member variable of Pixels_<T>. The following code does not work because the returned value will be immediately destructed.

const ofPixels& pixels = reader.bindAsPixels(target);
// Do something to the pixels

Instead of the code above, you should do like:

const auto bound = reader.bindAsPixels(target);
const ofPixels& pixels = bound;
// Do something to the pixels

so that the returned value remains alive.

Shorthand Functions

Not really shorthands with respect to the number of characters, but for those who are not familiar with template meta programming, those will be easier to handle.

Shorthand	Full
bindAsPixels()	bind<unsigned char>()
bindAsShortPixels()	bind<unsigned short>()
bindAsFloatPixels()	bind<float>()

References

• Official documentation on Pixel Buffer Object
• Descriptions about async tranfers

License

The MIT License

Copyright (C) 2016 Shota Matsuda

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.