Scanline Rendering: Exploring Visual Realism Through Scanline Rendering Techniques

By Fouad Sabry

What is Scanline Rendering

Scanline rendering is an algorithm for visible surface determination, in 3D computer graphics, that works on a row-by-row basis rather than a polygon-by-polygon or pixel-by-pixel basis. All of the polygons to be rendered are first sorted by the top y coordinate at which they first appear, then each row or scan line of the image is computed using the intersection of a scanline with the polygons on the front of the sorted list, while the sorted list is updated to discard no-longer-visible polygons as the active scan line is advanced down the picture.

How you will benefit

(I) Insights, and validations about the following topics:

Chapter 1: Scanline rendering

Chapter 2: Painter's algorithm

Chapter 3: Rasterisation

Chapter 4: Texture mapping

Chapter 5: Z-buffering

Chapter 6: Graphics pipeline

Chapter 7: Clipping (computer graphics)

Chapter 8: Hidden-surface determination

Chapter 9: Shader

Chapter 10: Shadow volume

(II) Answering the public top questions about scanline rendering.

(III) Real world examples for the usage of scanline rendering in many fields.

Who this book is for

Professionals, undergraduate and graduate students, enthusiasts, hobbyists, and those who want to go beyond basic knowledge or information for any kind of Scanline Rendering.

• Language: English
• Publisher: One Billion Knowledgeable
• Release date: May 14, 2024

Book preview

Scanline rendering

Scanline rendering (also scan line rendering and scan-line rendering) is an approach for determining visible surfaces in 3D computer graphics that operates on a row-by-row basis as opposed to a polygon-by-polygon or pixel-by-pixel basis. Each row or scan line of the image is computed using the intersection of a scanline with the polygons on the front of the sorted list, while the sorted list is updated to eliminate no longer visible polygons as the current scan line is progressed along the image.

The primary advantage of this method is that it decreases the amount of comparisons between edges by sorting vertices along the normal of the scanning plane. Additionally, it is not essential to translate the coordinates of all vertices from the main memory to the working memory; only vertices forming edges that cross the current scan line need to be in active memory, and each vertex is read in only once. Main memory is frequently much slower than the link between the central processing unit and cache memory; hence, avoiding re-accessing vertices in main memory might result in a significant speedup.

This type of algorithm can be easily merged with a variety of other graphics techniques, including the Phong reflection model and the Z-buffer algorithm.

Edges of projected polygons are typically placed into buckets, one per scanline; the rasterizer maintains an active edge table (AET). Entries preserve sort links, X coordinates, gradients, and references to the bound polygons. To rasterize the next scanline, irrelevant edges are eliminated and fresh edges from the current scanline's Y-bucket are placed in order of X coordinate. In the entries of the active edge table, X and other parameters are increased. Entries for active edge tables are maintained in an X-sorted list, causing a change when two edges intersect. After updating edges, the active edge table is traversed in X order to emit only visible spans, while retaining a Z-ordered active Span table and inserting and deleting surfaces when edges are crossed.

A hybrid of this and Z-buffering eliminates active edge table sorting and rasterizes one scanline at a time into a Z-buffer while keeping active polygon spans from one scanline to the next.

In a second form, an ID buffer is rasterized in an intermediary stage, enabling the resultant visible pixels to have their shading delayed.

In 1967, Wylie, Romney, Evans, and Erdahl likely published the scanline rendering technique for the first time. Ivan Sutherland's graphics group at the University of Utah and the Evans & Sutherland company in Salt Lake City did much of the early work on these technologies.

The early Evans & Sutherland ESIG line of image-generators (IGs) utilized the approach in hardware on the fly to generate images one raster-line at a time without a framebuffer, hence eliminating the requirement for expensive memory at the time. Later variants utilized a mixed strategy.

The Nintendo DS is the most recent piece of hardware capable of rendering 3D scenes in this manner, with the ability to cache rasterized graphics in VRAM.

The sprite hardware ubiquitous in 1980s video game consoles is a rudimentary kind of scanline rendering.

The approach was utilized for software rendering of surroundings in the original Quake engine (but moving objects were Z-buffered over the top). Prioritization of static scenery was based on BSP-derived sorting. It showed superior than Z-buffer/type painter's methods in handling scenes with costly pixel operations and high depth complexity (i.e. perspective-correct texture mapping without hardware assist). This practice predated the broad adoption of Z-buffer-based GPUs, which are currently prevalent on PCs.

During the development of the PlayStation 3, Sony experimented with software scanline renderers on a second Cell processor before settling on a standard CPU/GPU configuration.

Similarly, in tiled rendering (most notably the PowerVR 3D processor), primitives are sorted into screen space and rendered one tile at a time in rapid on-chip memory. In the spirit of hardware scanline rendering, the Dreamcast had a mode for rasterizing one row of tiles at a time for direct raster scanout, eliminating the requirement for a complete framebuffer.

Some software rasterizers employ span buffering (or coverage buffering), wherein a list of sorted, clipped spans is stored in scanline buckets. A series of primitives would be applied to this datastructure before rasterizing only the visible pixels.

The primary advantage of scanline rendering over Z-buffering is that the number of times visible pixels are processed is kept to an absolute minimum, which is always one if no transparency effects are applied, which is advantageous in cases of high resolution or costly shading computations.

Similar improvements can be obtained in current Z-buffer systems using crude front-to-back sorting (approaching the'reverse painters method'), early Z-reject (in conjunction with hierarchical Z), and less frequent deferred rendering techniques available on programmable GPUs.

Scanline approaches that operate on the raster have the disadvantage of not handling overload gracefully.

It is not believed that the technique scales effectively as the number of primitives rises. This is due to the size of the intermediate datastructures necessary during rendering, which, for complicated scenes, might exceed the capacity of a Z-buffer.

Consequently, the Z-buffer has become ubiquitous in interactive graphics applications of the present