"In this session I'm going to describe some efforts to bring OpenCL acceleration to the General Graphics Library (GEGL) and the GNU Image Manipulation Program (GIMP). I intend to show the current state of the project, some implementations techniques used and performance comparisons among common GPUs."

1. OpenCL in GEGL and GIMP
Victor Oliveira
Libre Graphics Meeting 2012

2. The goats are finally running!

3. GIMP was designed to process images with 8-bits per channel.
• Artifacts
• Noise
GEGL is optimized to do it in high-depth formats, like floating-point.

4. Processing images in floating-point is intensive.
How can we improve GEGL performance?

5. The solution: making floating-poing processing
faster using GPUs, multi-threading and vectorization!
(All to do with OpenCL)

6. But what is OpenCL?

7. ✔ A language for writing functions that
execute in OpenCL devices:
• GPUs
• Multi-core CPUs

8. ✔ An API to control these OpenCL devices

9. ✔ A specification

10. ✔ A shared library (you can use it in your C/C++ code)

11. for (i=0; i<n_pixels; i++)
{
out_pixel[0] = (in_pixel[0] - 0.5f) * contrast + brightness + 0.5;
out_pixel[1] = (in_pixel[1] - 0.5f) * contrast + brightness + 0.5;
out_pixel[2] = (in_pixel[2] - 0.5f) * contrast + brightness + 0.5;
out_pixel[3] = in_pixel[3]; /* copy the alpha */
in_pixel += 4;
out_pixel += 4;
}
__kernel void gegl_brightness_contrast(__global const float4 *in,
__global float4 *out,
float contrast,
float brightness)
{
int gid = get_global_id(0);
float4 in_v = in[gid];
float4 out_v;
out_v.xyz = (in_v.xyz - 0.5f) * contrast + brightness + 0.5f;
out_v.w = in_v.w;
out[gid] = out_v;
}

12. Common flow in a OpenCL program:
➡ Write code in the OpenCL language
➡ Detect the device
➡ Build your code
➡ Transfer data to the device
➡ Run your OpenCL program
➡ Transfer data back from device

13. AMD, Intel and Nvidia proprietary implementations.
Mesa open-source implementation is progressing fast.

14. What I have done:

15. ✔ Automatic data transference to the
OpenCL Device when necessary

16. ✔ Conversion between color spaces

17. ✔ A simple API for writing OpenCL
filters in GEGL

18. ✔ Benchmarking

19. ✔ Integrating this work in the next
release of GIMP (2.8)

20. Sharing data in the OpenCL Device among many operations

25. Color conversion in the OpenCL Device

27. Point Operations API

29. Benchmarks
(GEGL single-threaded x GEGL w/ OpenCL)

31. Profiling is hard.

35. ✔ OpenCL is a portable and efficient
way to write filters in GEGL/GIMP.

36. ✔ There is a huge performance boost
using GPUs.

37. ✔ On CPUs, we gained multi-core support
for free.

38. ✔ 91 commits - 14,161 lines changed,
code in GEGL 0.20, which is required
by GIMP 2.8.

39. What is left to be done:

40. ✔ Testing by users on different
cards/drivers/cpus and feedback

41. ✔ Porting more ops to OpenCL

42. ✔ Multiple GPUs support

43. ✔ OpenGL output node for GEGL

44. ✔ The more GIMP uses GEGL, more
performance gains

45. Thanks!
Victor Oliveira (victorm)
victormatheus@gmail.com