OpenGL vs. Vulkan: What are the Key Differences?

The first issue every newbie in the Graphics API world faces is the choice of the right API. When it comes to open-source multi-platform APIs, OpenGL and Vulkan are the main considerations.

How do they differ, and which one is worth going for? This article answers these questions with an in-depth analysis of the differences between the two APIs.

OpenGL vs. Vulkan: A Side-by-Side Comparison

OpenGL vs. Vulkan: What’s the Difference?

Definition

OpenGL

Silicon Graphics released the Open Graphics Library, OpenGL, in June 1992 as an open-source API to provide an interface for graphics programming. Over the years, improvements and the introduction of other extensions saw it evolve into a cross-language API.

The turning point was in the introduction of the version 4.2 extension that allowed the use of compute shaders, which made thread synchronization and memory sharing possible. It transformed OpenGL into what it is today, an API for the hardware-accelerated rendering of 2D and 3D vector graphics.

Vulkan

The Khronos Group, AMD, and DICE released Vulkan in 2016 as a cross-platform open-source API that provides an interface for programming and rendering video games and other real-time 3D graphics applications. It was to be a modern version of OpenGL, one that would meet the fast-changing needs of the gaming industry while also exploiting the full capability of modern GPUs. It was initially known as Next Generation OpenGL before being renamed Vulkan.

Design

The design of OpenGL made it suitable for graphics workstations that have direct renders. As a result, it could not be suitable for desktop platforms, irrespective of the OS. The threading model inhibits parallel command generation and execution. Consequently, it could not run on certain platforms such as tablets, smartphones, and consoles.

Unlike OpenGL and other APIs before it, Vulkan is primarily designed to run on diverse platforms, especially mobile and consoles. Its layered design allows code validation, profiling, and debugging without affecting the performance; hence it can sustain innovative use of cross-vendor GPU tools.

Due to its layered design, Vulkan offers direct control of the GPU, a capability that most sophisticated game engines demand. It also relies on command buffers, which are pre-generated commands, thereby eliminating the need to compile or validate during each render loop. As a result, it significantly reduces CPU overhead.

CPU Utilization

In OpenGL, executed calls are not cached; so, many calls to the kernel mode are required to change and validate the driver’s state. On the other hand, Vulkan uses command buffers, thus improving CPU resource utilization.

The second difference is Vulkan’s parallel buffer generation feature. With it, Vulkan harnesses the power of all the CPU cores, something that OpenGL struggled with, given that it was designed at a time when multiple-core processors were unheard of. With modern processors having at least eight cores, Vulkan is the only API that can suitably make use of the possibilities they offer.

Shader Compilers

Compute shaders help make the most of the GPU’s (Graphic Processing Unit) power instead of depending only on the power of the system’s CPU. However, these APIs manage these shaders differently.

OpenGL uses C++, a high-level language, with a GLSL compiler. The compiler runs at the driver level and writes the shaders, which are then translated by the application runtime into machine code for the GPU.

Vulkan, however, shortens the process using a standard portable intermediate representation, SPIR-V. It has a compiler-source translator for GLSL, HLSL, and LLVM that converts the codes from all the other high-level APIs into SPIR-V. Vulkan then takes the ready-to-execute binary-intermediate input and utilizes it in the shader stage. In addition to GLSL, Vulkan supports various domain-specific languages, tools, frameworks, and OpenGL C kernels.

Predictability

The level of abstraction is very high in OpenGL, as most of the operations occur behind the scenes. They include resource management, error checking and validation, pre-compiling shaders, and some synchronizations.

This implicit design ensures that the developer does not need to write codes for every action, but also makes it less predictable; you never know if your code will do what it is supposed to do until it does. Also, the driver schedules the submissions of jobs for processing, which leads to rendering interruptions.

Vulkan is different. It is an explicit API, which means the driver does not track resources or define their relationships. This is done at the application level and requires the developer to write code for every process.

The advantage of this is that jobs are submitted upfront for execution. As a result, job processing in Vulkan is predictable, streamlined, and uninterrupted. Thus, rendering in Vulkan does not suffer lags and hitches as is common in OpenGL.

Driver and Application Layer

OpenGL, as an implicit API, leaves the responsibility of resource management and state tracking to the driver layer. As a result, the application layer is very thin in OpenGL. Rendering takes longer than it would if most of the resource allocation tasks were done at the application layer.

On the other hand, Vulkan is designed to be as close to the graphics hardware as possible. To achieve this, it leaves the management of resources, logic, and states to the applications.

By letting the application layer do the resource allocation, Vulkan gives the application developer full control of the computer resources of the host system. Hence, allowing the software to access the graphics processor directly makes the GPU render as fast as possible and improves the processor’s performance.

Applications Developer Control

Another key difference between the two APIs is the level of control that the developer has over the system’s memory. In OpenGL, such control is nonexistent as the drivers use internal heuristics to allocate the system’s resources, including memory. The main challenge with this memory control mode is that it varies with the vendor and can result in suboptimal allocations or hitches when the resource is moved.

Vulkan exposes the various available memory types on the system and lets the developer allocate and deallocate the memories at their discretion. The application programmer must select the memory type best for the intended purpose.

In addition to memory types, Vulkan also grants the developer access to available extensions, command buffer queues, and physical devices. Therefore, since there is no interference by the driver, the developer can modify the behavior of Vulkan to meet the game’s demands.

Multithread Scalability

OpenGL has very poor multithreading capacity making it difficult to take advantage of the processing power of modern CPUs. Due to this, it also has high CPU overhead.

Here again, Vulkan takes the opposite trajectory. It is specifically designed to exploit the full multithreading capacity of the CPU in an explicit manner. Through its intentional resource control, jobs are separated into different threads, from creation to execution.

Cross-Platform Game Porting

Game porting between desktop and mobile platforms is impossible in OpenGL. Instead, there are two separate versions of the API; the desktop-based API (OpenGL) and OpenGL ES, an embedded API for mobile platforms. Even then, priority is often given to desktop platforms. Also, the GL APIs have evolved considerably slower than GPU hardware and technology; hence even the latest implementations are often only suitable for the GPUs of the previous generation.

Vulkan has a unified API for all platforms as it integrates both desktop and mobile rasterizers. It also has better integration tools than OpenGL, as the developer can independently enable the diagnostics and validation layer. All these make the API’s desktop and mobile versions similar, making gaming porting across the platforms possible. Also, unlike OpenGL, Vulkan places priority on mobile platforms.

Error Checking

In OpenGL, error checking is an automatic function of the driver layer. As a result, applications that are running well might suffer when error-checking is not triggered during execution. Excessive and random error checks also overburden the PC.

Whereas, in Vulkan, error checking comes as an add-on that can be turned on and off when necessary by enabling or disabling error-checking and other validation layers. Ideally, for debugging purposes, these layers are enabled on Vulkan during the development phase and then disabled during the release phase.

API Size

Vulkan is generally a thinner API than OpenGL. The advantages of a thin API are low overhead and a high level of resource management control. As such, developers on Vulkan can add frameworks and helper libraries to the application layer. Also, due to its low overhead, products built on top of it lose very little performance and control of the API. In fact, it is possible to build OpenGL on top of Vulkan.

Developer’s Hours

The main advantage of OpenGL over Vulkan is that it saves the developer some hours as the driver layer does the resource allocation behind the scenes. On Vulkan, however, the developer must write code for every intended action, choosing the memory type, memory size allocation, thread, etc. As a result, Vulkan requires more lines of code for the same application than OpenGL.

Most people use this to justify why machine-level coding is not worth it. However, for those to whom high hardware efficiency is the holy grail, the extra hours are worth it. Besides, in the real sense, you need to do this only once and not for every application due to the creation of command buffers.

Also, Vulkan offers a layered ecosystem for developers to choose the level of programming they can handle. Vulkan experts and those who need maximum flexibility and control will settle for using Vulkan directly. Those who wish to speed up the development phase go for libraries and layers. The majority of Vulkan users fall in this category. Then, there is the final category of those who use Vulkan to optimize ready-made game engines.

OpenGL vs. Vulkan: 5 Must-Know Facts

• The initial version of OpenGL was released in 1992, whereas Vulkan’s first version was released in 2016.
• Vulkan’s biggest merit is that it supports a variety of platforms.
• Leading game engines such as Unreal Engine 4, Unity, and CryEngine have full Vulkan support.
• Vulkan features batching to prioritize on-chip memory tanks and reduce CPU load.
• Many essential operations are hidden on OpenGL while they are accessible in Vulkan.

OpenGL vs. Vulkan: Which One is Better? Which One Should You Use?

If you must choose between the two APIs, your decision will depend on the following factors:

• Your level of expertise: If you are new to the Graphics world, you will probably be developing simple and small applications. In that case, start with OpenGL. As your skills grow, consider learning Vulkan. It is the API for anyone who desires to build an application for modern GPUs.
• The target OS and hardware: OpenGL is suitable if you only care about Windows and Linux OS or intend to use the compute shaders on iOS. Also, the target hardware is a critical consideration when using some tools, such as AMD’s integrated GPU profiler, that work only with  DirectX and Vulkan.
• Required control levels: If you need to have control over resource management as a developer, then Vulkan is the API to use. It is also vital to consider if your application is CPU-bound and distributes its operations on different cores.
• New projects: Vulkan is the best API for those working on a new project, especially on mobile platforms.

OpenGL was released at a time when GPUs were just graphic devices. Time has changed; today, most GPUs focus on accelerating computations and reducing CPU loads. As such, future-facing APIs such as Vulkan are the way to go.

Nonetheless, it is worth keeping in mind that Vulkan is a more complex API than OpenGL, and due to its high verbosity, the learning curve is very steep. It also requires a high level of responsibility and more maintenance to run effectively. But then, high performance has always come at a premium price.