Rendering is the final step in creating a 3D image from computer graphics, transforming raw models, textures, and lights into a finished image. For both traditional offline rendering and real-time rendering in games and interactive media, the rendering pipeline plays a central role. While offline rendering prioritizes high fidelity and is less concerned with speed, real-time rendering pipelines aim to strike a balance between visual quality and performance. Understanding how both pipelines operate will help you as an artist, technical director, or developer harness the full potential of 3D rendering.
A rendering pipeline is the process that converts 3D data—comprising models, textures, lights, and camera parameters—into 2D images. This workflow is essential in both offline and real-time rendering, but their goals differ. Offline rendering, used in films, animations, and pre-rendered cutscenes, seeks the highest possible quality, with little concern for time. Real-time rendering, on the other hand, aims to maintain a balance between quality and speed, especially in applications like video games and VR.
In both cases, the pipeline breaks down complex tasks into manageable stages, allowing GPUs and CPUs to process large amounts of data efficiently. This sequence includes transforming 3D models, applying lighting, handling textures, and eventually displaying the final image.
For the more technically inclined, Clickety Clack gives a very comprehensive explanation here:
The purpose of the rendering pipeline is to systematize the rendering process, ensuring that all aspects of a scene, from geometry to lighting, are processed in a logical order. This modular approach allows for optimization at each stage, ensuring that even complex scenes with millions of polygons and textures can be rendered.
In traditional offline rendering, where quality matters more than speed, the pipeline enables effects like ray tracing for realistic shadows and reflections. Real-time rendering pipelines are optimized for speed, using techniques like z-buffering, back-face culling, and level of detail (LOD) to reduce the computational load without sacrificing too much visual quality.
Though the specific steps may vary depending on the renderer or game engine being used, the core stages remain similar:
Rendering, whether for real-time applications like video games or offline scenarios like cinematic productions, follows a series of stages that process raw 3D data into the final visual output. Each step in this rendering pipeline contributes to the overall quality and efficiency of the render. Let’s take a deep dive into these stages, while highlighting the key distinctions between real-time and offline pipelines, especially focusing on the contrast between rasterization and other techniques used in offline rendering.
At the heart of geometry processing lies the transformation of 3D objects from model space to camera space. This process is achieved through several key matrix transformations, such as the Model-View and Projection matrices, which convert the 3D scene into a form that can be projected onto a 2D screen.
In both real-time and offline pipelines, z-buffering plays a critical role. This depth-buffering technique helps to determine which objects should be visible in the final render based on their distance from the camera.
Rasterization is the process where geometric data, primarily triangles, is converted into fragments (potential pixels) on a 2D screen. However, the implementation and reliance on rasterization differ significantly between real-time and offline rendering.
Shading is the calculation of the final color of each fragment or pixel based on the surface properties of objects and the lighting in the scene. This is where shaders come into play—small programs that determine how light interacts with surfaces.
Once the scene’s geometry has been processed, and each fragment has been shaded and lit, the final image is composed. This stage involves assembling all the data into the final 2D image that will be displayed.
While both real-time and offline rendering pipelines follow the same basic stages, the approach and focus differ dramatically. Real-time rendering emphasizes speed and efficiency, using techniques like rasterization and light approximations to achieve visually pleasing results at high frame rates. Offline rendering, on the other hand, prioritizes accuracy and detail, often forgoing speed in favor of techniques like ray tracing that deliver high levels of realism.
As technology advances, the gap between the two pipelines is slowly closing. Real-time ray tracing is becoming more prevalent, allowing for more accurate lighting and reflections in real-time applications, though it is still a far cry from the fidelity achieved in offline rendering. Both approaches continue to evolve, each pushing the boundaries of what’s possible in their respective fields.
OpenGL provides an excellent example of a real timel rendering pipeline:
In offline rendering, this pipeline can be expanded with ray tracing, whereas OpenGL in real-time contexts focuses more on speed.
Shaders are small programs that define how vertices, fragments, and pixels are processed. In both offline and real-time pipelines, shaders allow artists and developers to create highly customized effects.
In offline rendering, shaders can be used for complex materials, subsurface scattering, or advanced lighting models. In real-time applications, the fragment shader plays a pivotal role in rendering textures, colors, and lighting effects at high speed, often using simplified approximations to achieve acceptable quality without sacrificing performance.
Older rendering systems used a fixed-function pipeline, where the sequence of operations (vertex transformations, lighting, etc.) was hardcoded. Modern APIs like Vulkan and Direct3D offer a programmable pipeline, where developers can write custom shaders to control every stage of the rendering process, leading to more flexibility and visual quality.
In traditional offline rendering, the programmable pipeline allows for intricate custom shaders that can simulate everything from skin to glass. In real-time pipelines, it allows more control over the performance-to-quality tradeoff.
The rendering pipeline is the series of steps that transform a 3D scene into the 2D image you see on the screen. Whether it's used for offline rendering in high-end visual effects and animation, or for real-time rendering in video games, the fundamental process remains similar, though the approaches differ depending on whether the focus is on achieving the highest possible realism or delivering frames quickly enough for smooth interaction.
The process starts with raw 3D data, including models made up of vertices, edges, and faces, as well as textures, lighting information, and the placement of the camera. These 3D models exist in model space, which is essentially a mathematical definition of where each object is located in relation to itself. To display this 3D data on a flat, 2D screen, a series of transformations occur. This stage involves converting the position of each vertex in the 3D world into screen space (2D coordinates) using matrix operations. These operations include viewing transformations, which orient the scene according to the camera’s position, and projection, which mimics perspective by making distant objects appear smaller.
Once the 3D models have been transformed into 2D coordinates, the next critical step is lighting and shading, where the interaction of light with surfaces is calculated. The results determine how surfaces reflect light, how shadows are cast, and how textures and materials should appear based on the lighting environment. In offline rendering, this process is handled with extreme precision. A technique like path tracing is commonly used to simulate realistic lighting by tracing rays of light as they bounce off objects in the scene, capturing subtle interactions like reflections, refractions, and diffuse lighting. This method is computationally expensive, often requiring hours or even days to render a single frame, but the result is photorealistic imagery. Real-time rendering, on the other hand, emphasizes speed. To meet the performance demands of interactive applications like video games, lighting calculations rely on a variety of approximations.
Techniques such as screen-space reflections, which simulate reflections based on visible data, and baked lighting, where lighting is pre-calculated and stored, allow for faster processing. While these methods sacrifice some of the realism achieved in offline rendering, they enable the system to maintain high frame rates. Recent advances in GPU technology, such as real-time ray tracing, are closing this gap, allowing for more realistic lighting effects while still keeping the frame rate manageable, though not to the same level as offline rendering.
The next stage of the pipeline involves turning the 2D screen coordinates into actual pixels through a process known as rasterization. Here, geometric shapes in the scene, primarily triangles, are converted into fragments or pixel data. Each pixel on the screen is then "colored" based on the 3D object’s material properties, such as textures, colors, and reflectivity, and how light interacts with it. In simpler terms, this stage determines how each pixel in the final image should look based on where it falls on the object’s surface and how the lighting interacts with that surface.
In offline rendering, once the scene has been rasterized, multiple render passes may be performed. These passes separate the image into distinct layers, each focusing on different elements like reflections, shadows, or lighting, which are later fine-tuned individually in post-production software. This layered approach allows for a high degree of control, enabling artists to achieve stunning levels of detail and realism. For instance, one pass might capture direct lighting, while another focuses on indirect light bounces, ensuring that even the softest shadows and the subtlest lighting effects are represented accurately.
Real-time rendering, however, must complete the entire pipeline in a fraction of a second—typically under 16 milliseconds to maintain a smooth frame rate of 60 frames per second. Due to this time constraint, real-time engines cannot afford the luxury of multiple passes. Instead, post-processing effects like motion blur or bloom are applied quickly in a single pass to enhance the visual quality while keeping performance in check.
Ultimately, the rendering pipeline transforms raw 3D data into the final image we see on screen. The key difference between offline and real-time rendering lies in the trade-off between accuracy and speed. Offline rendering, with its emphasis on techniques like ray tracing and path tracing, delivers exceptional realism, while real-time rendering focuses on optimizations and approximations to generate images fast enough for interactive use. As technology advances, particularly with innovations in GPU-based real-time ray tracing, the gap between these two approaches continues to narrow, bringing greater realism to real-time applications.
In Unity, for example, developers can choose between the Universal Render Pipeline (URP), optimized for performance, and the High Definition Render Pipeline (HDRP), which focuses on achieving the highest possible quality using advanced shaders and rendering techniques. HDRP offers features like volumetric lighting and ray tracing, making it closer in quality to offline renderers, though it still can't match the level of detail achieved in pre-rendered scenes like those in film or animation.
For Unity users or those curious, Brendan Dickinson explores the different render pipelines offered by the software in his video:
Advanced techniques such as ray tracing, global illumination, and physically-based rendering (PBR) dominate offline rendering, producing images indistinguishable from real life. Real-time rendering is catching up with techniques like deferred shading and real-time ray tracing, though these are still in their infancy compared to the fidelity achieved in offline pipelines.
In offline rendering, the pipeline prioritizes quality with little regard for time, enabling effects like global illumination, caustics, and complex shader calculations that require hours of computation. In real-time rendering, the pipeline is heavily optimized to prioritize performance, relying on simplified lighting models and approximations.
With the rise of machine learning and AI-enhanced rendering, both offline and real-time pipelines are benefiting from improvements in noise reduction and optimization. Additionally, real-time ray tracing continues to push the boundaries of real-time rendering, slowly closing the gap with offline methods.
In both offline and real-time rendering, optimization is key to managing complex scenes efficiently. Techniques like clipping, LOD, and shader level detail can help optimize real-time pipelines, while sampling and adaptive subdivision are crucial for offline renders.
Industries from film studios like Pixar to game developers like Epic Games use advanced rendering pipelines to create stunning visuals. While films rely heavily on offline rendering for detailed, high-fidelity images, game developers use real-time pipelines optimized for fast, responsive performance.
By understanding the intricacies of both traditional offline rendering and real-time pipelines, 3D artists and developers can choose the right tools and techniques to meet their project's visual and performance goals.
