How Libvpx Separates VP8 and VP9 Codebase

This article provides an overview of how the Google-maintained libvpx repository structures its codebase to cleanly separate the VP8 and VP9 video codec implementations. We will examine the top-level directory layout, the internal folder structures for each codec, the shared utility libraries, and how the public API routes calls to the appropriate codec version.

The Top-Level Directory Layout

The primary method of separation in the libvpx codebase is directory-based segregation at the root of the repository. Rather than mixing the algorithms for both standards, the developers isolated the source code for each codec into its own dedicated directory:

• vp8/: Houses all source code, algorithms, and logic specific to the VP8 video compression standard.
• vp9/: Houses all source code, algorithms, and logic specific to the VP9 video compression standard.

Internal Codec Structures

Within both the vp8/ and vp9/ root folders, the code is further subdivided using a consistent architecture. Each codec directory contains three core subfolders:

1. common/: Contains code that is utilized by both the encoder and the decoder. This includes color space conversion, block-size partitioning logic, quantization parameters, loop filtering algorithms, and basic entropy coding structures.
2. encoder/: Contains the code exclusive to video compression. This includes motion estimation, rate control, mode decision algorithms, and bitstream packaging.
3. decoder/: Contains the code exclusive to video decompression. This includes bitstream parsing, inverse transform logic, and motion compensation.

Shared Infrastructure: vpx_dsp and vpx_scale

While VP8 and VP9 are distinct video standards, they share low-level math operations and hardware acceleration routines. To avoid code duplication, libvpx isolates these shared components outside of the vp8/ and vp9/ directories:

• vpx_dsp/: Contains digital signal processing (DSP) implementations, including loop filters, transforms (DCT/ADST), and quantization math. This folder is highly optimized for various hardware architectures (such as ARM NEON, Intel SSE/AVX, and MIPS).
• vpx_scale/: Handles frame scaling and multi-resolution-loop filtering, which are utilized by both codecs during encoding and decoding.
• vpx_ports/: Handles CPU detection and platform-specific wrappers, ensuring the correct SIMD assembly code is executed on target hardware regardless of whether VP8 or VP9 is running.

API Layer Routing

The public interface for libvpx is located in the vpx/ directory. It exposes unified headers like vpx_codec.h, vpx_encoder.h, and vpx_decoder.h.

The codebase separates the implementation at the binary level using function pointers. When a developer initializes an encoder, they pass a specific algorithm interface structure, such as vpx_codec_vp8_cx() or vpx_codec_vp9_cx(). These interfaces map the general API functions (like vpx_codec_encode) to the corresponding internal functions inside either the vp8/encoder/ or vp9/encoder/ directory, keeping the runtime execution paths of both codecs entirely independent.