How to Enable OpenCL Scaling in FFmpeg

Accelerating video scaling in FFmpeg using OpenCL offloads compute-heavy resizing operations from your CPU to an OpenCL-capable graphics card or integrated processor. This guide covers how to verify OpenCL support in your FFmpeg build, configure the hardware device, and use the scale_opencl filter within a transcoding pipeline to achieve faster rendering speeds.

1. Verify FFmpeg OpenCL Support

Before running OpenCL filters, verify that your FFmpeg binary was compiled with OpenCL support. Run the following command in your terminal:

ffmpeg -filters | grep opencl

If OpenCL is enabled, you will see a list of available OpenCL filters, including scale_opencl, overlay_opencl, and tonemap_opencl. If no results appear, you must install an FFmpeg build configured with the --enable-opencl flag and ensure proper OpenCL runtime drivers for your GPU (Intel compute runtime, AMD ROCm/AMDGPU-PRO, or NVIDIA drivers) are installed.

2. Identify Your OpenCL Device

FFmpeg needs to know which OpenCL platform and device to initialize. You can list available platforms and devices by running:

ffmpeg -v verbose -init_hw_device opencl

Look at the console output to find the device index. The notation is typically platform_index.device_index (for example, 0.0 represents the first device on the first platform).

3. Basic Command Using scale_opencl

Using an OpenCL filter requires initializing the device, transferring standard CPU pixel frames into GPU memory (hwupload), running the OpenCL filter, and transferring them back to system memory (hwdownload) unless you are encoding directly on the GPU.

Here is a standard command to scale a video to 1080p using OpenCL:

ffmpeg -init_hw_device opencl=gpu:0.0 -filter_hw_device gpu -i input.mp4 \
-vf "format=yuv420p,hwupload,scale_opencl=1920:1080,hwdownload,format=yuv420p" \
-c:v libx264 -c:a copy output.mp4

Breakdown of the Filter Chain:

  • -init_hw_device opencl=gpu:0.0: Initializes the OpenCL context using the specified platform and device index (0.0) and labels it as gpu.
  • -filter_hw_device gpu: Instructs the filtering pipeline to run on the initialized gpu device context.
  • format=yuv420p: Ensures the input pixels are in a format compatible with the OpenCL buffer transfer.
  • hwupload: Uploads host memory frames to OpenCL device memory.
  • scale_opencl=1920:1080: Performs the resize operation directly on the GPU using OpenCL kernels.
  • hwdownload: Retrieves the processed OpenCL frames from GPU memory back to host CPU memory.
  • format=yuv420p: Re-establishes the pixel format required for standard CPU-based software encoders like libx264.

4. Adjusting Scaling Algorithms

By default, scale_opencl uses bilinear interpolation. You can specify a different interpolation algorithm using the interp parameter.

Available options include:

  • bilinear (default, fastest)
  • bicubic (higher visual quality, slightly more compute-intensive)
  • nearest (nearest-neighbor, fastest, lower quality)
  • lanczos (sharpest, most compute-intensive)

To use bicubic scaling, update the filter definition:

scale_opencl=1920:1080:interp=bicubic