perform alignment on GPU

[bmegli]

Part of #23

There was some work in librealsense for GLSL processing blocks.
There is also GLSL example.

[bmegli]

See rs2_gl_create_align

Side note:

• rs2_gl_create_yuy_decoder could be used when aligning to depth
• because we can't align to depth with YUYV
• and currently we probably use software conversion path in librealsense (needs checking)

[bmegli]

And the C++ version rs2::gl::align

[bmegli]

We clearly need development headers and link with realsense2-gl for this:

# Ubuntu 18.04
sudo apt-get install librealsense2-gl-dev

# CMake
target_link_libraries(realsense-nhve-depth-color nhve realsense2 realsense2-gl)

[bmegli]

Proof of concept implemented in 99bb1f7.

By watching htop, intel_gpu_top and nvidia-smi with and without GL acceleration I can see that:

• on my laptop with intetrated GPU + discrete NVIDIA GPU
• alignment is performed on NVIDIA
• and there is no way to control this behavior with realsense2-gl (ok for me)

[bmegli]

On the receiving side I get black screen with GL acceleration enabled (on sender).

There is some problem - maybe the data is not downloaded to CPU side for sending.

[xsol]

It seems like you didn't call rs2::init_rendering(), for me this made the difference between blank screen and output as described here IntelRealSense/librealsense#11827

[Arun-Prasad-V]

Hi @bmegli,
Please refer the PR IntelRealSense/librealsense#12556.
An example for Align-GL has been added to the development branch.

Thanks.