Tiled rendering

기본 개념

Creating a 3D image for display consists of a series of steps. First, the objects to be displayed are loaded into memory from individual models. The system then applies mathematical functions to transform the models into a common coordinate system, the world view. From this world view, a series of polygons (typically triangles) is created that approximates the original models as seen from a particular viewpoint, the camera. Next, a compositing system produces an image by rendering the triangles and applying textures to the outside. Textures are small images that are painted onto the triangles to produce realism. The resulting image is then combined with various special effects, and moved into a frame buffer, which video hardware then scans to produce the displayed image. This basic conceptual layout is known as the display pipeline.
Each of these steps increases the amount of memory needed to hold the resulting image. By the time it reaches the end of the pipeline the images are so large that typical graphics card designs often use specialized high-speed memory and a very fast computer bus to provide the required bandwidth to move the image in and out of the various sub-components of the pipeline. This sort of support is possible on dedicated graphics cards, but as power and size budgets become more limited, providing enough bandwidth becomes expensive in design terms.
Tiled renderers address this concern by breaking down the image into sections known as tiles, and rendering each one separately. This reduces the amount of memory needed during the intermediate steps, and the amount of data being moved about at any given time. To do this, the system sorts the triangles making up the geometry by location, allowing to quickly find which triangles overlap the tile boundaries. It then loads just those triangles into the rendering pipeline, performs the various rendering operations in the GPU, and sends the result to the frame buffer. Very small tiles can be used, 16×16 and 32×32 pixels are popular tile sizes, which makes the amount of memory and bandwidth required in the internal stages small as well. And because each tile is independent, it naturally lends itself to simple parallelization.
In a typical tiled renderer, geometry must first be transformed into screen space and assigned to screen-space tiles. This requires some storage for the lists of geometry for each tile. In early tiled systems, this was performed by the CPU, but all modern hardware contains hardware to accelerate this step. The list of geometry can also be sorted front to back, allowing the GPU to use hidden surface removal to avoid processing pixels that are hidden behind others, saving on memory bandwidth for unnecessary texture lookups.
There are two main disadvantages of the tiled approach. One is that some triangles may be drawn several times if they overlap several tiles. This means the total rendering time would be higher than an immediate-mode rendering system. There are also possible issues when the tiles have to be stitched together to make a complete image, but this problem was solved long ago. More difficult to solve is that some image techniques are applied to the frame as a whole, and these are difficult to implement in a tiled render where the idea is to not have to work with the entire frame. These tradeoffs are well known, and of minor consequence for systems where the advantages are useful; tiled rendering systems are widely found in handheld computing devices.
Tiled rendering should not be confused with tiled/nonlinear framebuffer addressing schemes, which make adjacent pixels also adjacent in memory. These addressing schemes are used by a wide variety of architectures, not just tiled renderers.

초기 작업

타일 렌더링에 대한 초기 작업의 대부분은 Pixel Planes 5 아키텍처의 일부분으로 수행되었다.
Pixel Planes 5 프로젝트는 타일 접근 방식을 검증하였고, 현재 타일 렌더링에 대한 표준으로 간주되는 대부분의 기술을 발명하였다. 현장에서 다른 논문에서 가장 널리 인용된 작업이다.
타일 접근은 소프트웨어 렌더링의 역사 초기에 알려졌다. Reyes rendering 의 구현은 종종 "타일 버킷(tile buckets)"으로 이미지를 나눈다.