Suppose a dynamic texture source like
ofVideoGrabber, persistent pixels for storing the contents of the texture, and an instance of
ofVideoGrabber grabber; ofPixels pixels; ofxAsyncTransferReader reader;
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
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
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
ofTexture texture; ofxAsyncTransferWriter writer;
const auto pixels = writer.bind<unsigned char>(texture); updatePixels(pixels); writer.unbind();
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.
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.
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.
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.