Scanline

In computer graphics, scanline refers to a technique used in the rendering process to determine which pixels on the screen need to be colored or filled. It involves scanning each horizontal line of the image or viewport and determining the intersections of polygons or objects with that line.

The scanline algorithm is typically used in conjunction with other rendering techniques, such as rasterization, to render 3D scenes or 2D images. Here’s an overview of how the scanline algorithm works:

1. Polygon Setup: Before the scanline algorithm begins, polygons or objects in the scene are typically processed and prepared for rendering. This includes tasks like transforming 3D vertices into screen space, applying perspective projection, and determining other necessary calculations.
2. Edge Table and Active Edge List: The scanline algorithm maintains two data structures: the edge table and the active edge list. The edge table stores information about the edges of the polygons, including the minimum and maximum y-coordinates and the x-coordinate of the intersection point with the current scanline. The active edge list keeps track of the edges intersecting the current scanline.
3. Scanline Processing: The scanline algorithm proceeds by scanning each horizontal line or scanline of the image or viewport. For each scanline, the algorithm performs the following steps:a. Initialization: The edge table and active edge list are initialized for the current scanline. The edges that intersect the scanline are added to the active edge list from the edge table.b. Sorting: The edges in the active edge list are sorted by their x-coordinate at the intersection point with the current scanline. This determines the order in which the pixels on the scanline will be filled.c. Pixel Filling: The pixels between pairs of edges in the active edge list are filled or colored. This is typically done using interpolation techniques, such as linear interpolation, to calculate the colors or attributes of the pixels between the edges.d. Edge Updates: The active edge list is updated to account for changes in the edges as the scanline progresses. The x-coordinates of the edges are updated based on their slopes, and edges that are no longer intersecting the scanline are removed from the active edge list.
4. Scanline Completion: Once all scanlines have been processed, the rendering process is complete, and the final image is displayed on the screen.

The scanline algorithm is efficient and widely used for rendering polygonal objects, as it eliminates the need to process every pixel in the scene. It focuses on the intersections of polygons with the horizontal scanlines, enabling efficient and fast rendering of complex scenes.

It’s worth noting that modern graphics pipelines and rendering techniques have evolved beyond basic scanline algorithms, incorporating additional optimizations and algorithms to handle various rendering challenges and achieve more advanced effects and realism.