In this first exercise you will learn:
- How to create a simple image processing kernel that performs a grayscaling.
- How to linearize the work-item id in row-major and column major ordering and how this affects global memory access coalescing.
- How to vectorize kernel code using SYCL vector types.
An image can be grayscaled using the following algorithm:
Y = (R * 0.229) + (G * 0.587) + (B * 0.114)
Where RGA are the red, green and blue channels of an RGBA four channel image format.
For the purposes of this exercise the STB image loading and writing library has been made available and the source for this exercise already contains the appropriate API calls to get you started.
1.) Write a SYCL kernel function for performing a grayscaling
The source for this example provides a stub which loads and write an image using the STB image library. You will need to fill in the SYCL code for performing a grayscaling algorithm on the image using what you have learnt in the previous exercises.
The source also contains a call to a benchmarking utility that will print the time taken to execute the SYCL code, the SYCL code should go inside the lambda that is passed to the benchmark function.
It's recommended that you use a 2-dimensional range for parallel_for when working with images.
Note you will have to update the path to an image. There is an image in the repository but feel free to use any image you choose. Though it's recommend that you use a png image whose dimensions are multiples of 2 (for example 512x512) and has four channels (RGBA).
2.) Evaluate global memory access
Now that you have a working grayscaling kernel you should evaluate whether the global memory access patterns in your kernel are coalesced.
Consider two alternative ways to linearize the global id:
auto rowMajorLinearId = (idx[1] * width) + idx[0]; // row-major
auto columnMajorLinearId = (idx[0] * height) + idx[1]; // column-major
Try using both of these and compare the execution time of each. Though note that the benchmark facility provided measures who application time which is less accurate than measuring the actual kernel times.
3.) Use vectorization
Now that global memory access is coalesced another optimization you could do here would be to use the SYCL vec class to present the pixels in the image.
You can reinterpret a buffer to be represented as a different type using the reinterpret member function template of the buffer class. When calling this function you must specify the new type as a template parameter and a new range that will represent elements of the new type within the same space in memory as a function parameter.
Try reinterpreting your buffer to use cl::sycl::float4 instead of float.