libplacebo

libplacebo is essentially the core rendering algorithms and ideas of mpv turned into a library. This grew out of an interest to accomplish the following goals:

• Clean up mpv's internal RA API and make it reusable for other projects.
• Provide a standard library of useful GPU-accelerated image processing primitives based on GLSL, so projects like VLC or Firefox can use them without incurring a heavy dependency on libmpv.
• Rewrite core parts of mpv's GPU-accelerated video renderer on top of redesigned abstractions. (Basically, I wanted to eliminate stateful APIs like shader_cache.c and totally redesign gpu/video.c)

API Overview

The public API of libplacebo is currently split up into the following components, the header files (and documentation) for which are available inside the src/include/libplacebo directory. The API is available in different "tiers", representing levels of abstraction inside libplacebo. The APIs in higher tiers depend on those in lower tiers. Which tier is used by a user depends on how much power/control they want over the actual rendering. The low-level tiers are more suitable for big projects that need strong control over the entire rendering pipeline; whereas the high-level tiers are more suitable for smaller or simpler projects that want libplacebo to take care of everything.

Tier 0 (context, raw math primitives)

• colorspace.h: A collection of enums and structs for describing color spaces, as well as a collection of helper functions for computing various color space transformation matrices.
• common.h: A collection of miscellaneous utility types and macros that are shared among multiple subsystems. Usually does not need to be included directly.
• context.h: The main entry-point into the library. Controls memory allocation, logging. and guards ABI/thread safety.
• config.h: Macros defining information about the way libplacebo was built, including the version strings and compiled-in features/dependencies. Usually does not need to be included directly. May be useful for feature tests.
• dither.h: Some helper functions for generating various noise and dithering matrices. Might be useful for somebody else.
• filters.h: A collection of reusable reconstruction filter kernels, which can be used for scaling. The generated weights arrays are semi-tailored to the needs of libplacebo, but may be useful to somebody else regardless. Also contains the structs needed to define a filter kernel for the purposes of libplacebo's upscaling routines.

The API functions in this tier are either used throughout the program (context, common etc.) or are low-level implementations of filter kernels, color space conversion logic etc.; which are entirely independent of GLSL and even the GPU in general.

Tier 1 (rendering abstraction)

• gpu.h: Exports the GPU abstraction API used by libplacebo internally.
• swapchain.h: Exports an API for wrapping platform-specific swapchains and other display APIs. This is the API used to actually queue up rendered frames for presentation (e.g. to a window or display device).
• vulkan.h: GPU API implementation based on Vulkan.
• opengl.h: GPU API implementation based on OpenGL.

As part of the public API, libplacebo exports a middle-level abstraction for dealing with GPU objects and state. Basically, this is the API libplacebo uses internally to wrap OpenGL, Vulkan, Direct3D etc. into a single unifying API subset that abstracts away state, messy details, synchronization etc. into a fairly high-level API suitable for libplacebo's image processing tasks.

It's made public both because it constitutes part of the public API of various image processing functions, but also in the hopes that it will be useful for other developers of GPU-accelerated image processing software.

Tier 2 (GLSL generating primitives)

• shaders.h: The low-level interface to shader generation. This can be used to generate GLSL stubs suitable for inclusion in other programs, as part of larger shaders. For example, a program might use this interface to generate a specialized tone-mapping function for performing color space conversions, then call that from their own fragment shader code. This abstraction has an optional dependency on gpu.h, but can also be used independently from it.

In addition to this low-level interface, there are several available shader routines which libplacebo exports:

• shaders/av1.h: Helper shaders for AV1 decoding, currently only implements a film grain synthesis shader.
• shaders/colorspace.h: Shader routines for decoding and transforming colors, tone mapping, dithering, and so forth.
• shaders/sampling.h: Shader routines for various algorithms that sample from images, such as debanding and scaling.

Tier 3 (shader dispatch)

• dispatch.h: A higher-level interface to the pl_shader system, based on gpu.h. This dispatch mechanism generates+executes complete GLSL shaders, subject to the constraints and limitations of the underlying GPU.

This shader dispatch mechanism is designed to be combined with the shader processing routines exported by shaders/*.h, but takes care of the low-level translation of the resulting pl_shader_res objects into legal GLSL. It also takes care of resource binding, shader input placement, as well as shader caching and resource pooling; and makes sure all generated shaders have unique identifiers (so they can be freely merged together).

Tier 4 (high level renderer)

• renderer.h: A high-level renderer which combines the shader primitives and dispatch mechanism into a fully-fledged rendering pipeline that takes raw texture data and transforms it into the desired output image.
• utils/upload.h: A high-level helper for uploading generic data in some user-described format to a plane texture suitable for use with renderer.h. These helpers essentially take care of picking/mapping a good image format supported by the GPU. (Note: Eventually, this function will also support on-CPU conversions to a different format where necessary, but for now, it will just fail)

This is the "primary" interface to libplacebo, and the one most users will be interested in. It takes care of internal details such as degrading to simpler algorithms depending on the hardware's capabilities, combining the correct sequence of colorspace transformations and shader passes in order to get the best overall image quality, and so forth.

Authors

libplacebo was founded by Niklas Haas (@haasn), but it would not be possible without the contributions of others. Special note also goes out (@wm4), the developer of mpv, whose ideas helped shape the foundation of the shader dispatch system. This library also includes various excerpts from mpv. For a full list of past contributors to mpv, see the mpv authorship page

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License

Since the code derives from several LGPLv2.1+-licensed parts of mpv, there's little choice but to license libplacebo the same way. It's worth pointing out that, except for some minor exceptions (e.g. filters.c and colorspace.c), most of the code is either original work or can be attributed to only a small number of developers, so a relicensing to a more permissive license might be possible in principle.

Installing

Gentoo

An ebuild is available as media-libs/libplacebo in the gentoo repository.

Building from source

libplacebo is built using the meson build system. You can build the project using the following steps:

$ DIR=./build
$ meson $DIR
$ ninja -C$DIR

To rebuild the project on changes, re-run ninja -Cbuild. If you wish to install the build products to the configured prefix (typically /usr/local/), you can run ninja -Cbuild install. Note that this is normally ill-advised except for developers who know what they're doing. Regular users should rely on distro packages.

Dependencies

In principle, libplacebo has no mandatory dependencies - only optional ones. However, to get a useful version of libplacebo. you most likely want to build with support for either opengl or vulkan. libplacebo built without these can still be used (e.g. to generate GLSL shaders such as the ones used in VLC), but the usefulness is severely impacted since most components will be missing, impaired or otherwise not functional.

A full list of optional dependencies each feature requires:

• glslang: glslang + its related libraries (e.g. libSPIRV.so)
• lcms: liblcms2
• opengl: libepoxy
• shaderc: libshaderc
• vulkan: libvulkan, python3-mako

Vulkan support

Because the vulkan backend requires on code generation at compile time, python3-mako is a hard dependency of the build system. In addition to this, the path to the Vulkan registry (vk.xml) must be locatable, ideally by explicitly providing it via the -Dvulkan-registry=/path/to/vk.xml option, unless it can be found in one of the built-in hard-coded locations.

Configuring

To get a list of configuration options supported by libplacebo, after running meson $DIR you can run meson configure $DIR, e.g.:

$ meson $DIR
$ meson configure $DIR

If you want to disable a component, for example Vulkan support, you can explicitly set it to false, i.e.:

$ meson configure $DIR -Dvulkan=disabled -Dshaderc=disabled
$ ninja -C$DIR

Testing

To enable building and executing the tests, you need to build with tests enabled, i.e.:

$ meson configure $DIR -Dtests=true
$ ninja -C$DIR test

Benchmarking

A naive benchmark suite is provided as an extra test case, disabled by default (due to the high execution time required). To enable it, use the bench option:

$ meson configure $DIR -Dbench=true
$ meson test -C$DIR benchmark --verbose

Using

Building a trivial project using libplacebo is straightforward:

// build with -lplacebo

#include <libplacebo/context.h>

void main()
{
    struct pl_context *ctx;
    ctx = pl_context_create(PL_API_VER, &(struct pl_context_params) {
        .log_cb    = pl_log_color,
        .log_level = PL_LOG_INFO,
    });

    // do something..

    pl_context_destroy(&ctx);
}

For a full documentation of the API, refer to the above API Overview as well as the public header files. You can find additional examples of how to use the various components in the demo programs as well as in the unit tests.

I will create more and expanded tutorials/examples once libplacebo is a bit more feature-complete.