Notes on bazelifying an opencv release

Opencv has a lot of configs and optimizations to enable. Generating a native bazel build is quite tedious, but here are the steps I've followed.

These are defined mainly in four files: cv_cpu_config.h, custom_hal.hpp, opencv2/opencv_modules.hpp, and cvconfig.h.

CPU Optimization Flags

CPU optimization flags (e.g., SIMD operations) are toggled on or off in cv_cpu_config.h. These flags are configured based on guidance from the CPU optimizations wiki. Ensure that the appropriate flags are enabled for your target architecture to maximize performance.

Custom Hardware Abstraction Layer (HAL)

The custom_hal.hpp file allows you to define custom hardware acceleration layers. This is useful if you are targeting specialized hardware or need to implement custom optimizations. In an attempt to generalize, I've left this blank

Module Configuration

The opencv2/opencv_modules.hpp file specifies which OpenCV modules are included in the build. You can modify this file to include or exclude specific modules based on your requirements. These are modules you want (i.e core, calib3d, features2d, etc)

Build Configuration

The cvconfig.h file contains general build configuration options. This includes settings for third-party dependencies, build flags, and other global configurations.

Adding New/Maintaining Modules

The opencv_module macro creates a cc_library for each module in OpenCV. Note that for each new dispatched file in a module, you'll need to define a mapping of the dispatched file to the actual operation. For example:

dispatched_files = {
    "stat": [
        "avx2",
        "sse4_2",
    ]
}

These mappings ensure that the correct SIMD or hardware-specific implementation is used during runtime, based on the capabilities of the target architecture. Be sure to review the dispatched files for each module and update the mappings as necessary.

When adding a new module, follow these steps:

1. Add the new module modules/opencv/4.11.0/overlay/opencv.bzl to CONFIG
2. Use the opencv_module macro to define the module's build rules.
3. Update the dispatched_files mapping if the module includes hardware-specific optimizations.

If you encounter build issues:

• Verify that all required dependencies are correctly specified in the BUILD files.
• Check the cvconfig.h file for any missing or misconfigured options.
• Ensure that the dispatched_files mappings are accurate and complete.