Perscanline

Perscanline is a term used to describe processing or rendering that is applied on a per-scanline basis, meaning for each horizontal line of a video frame or raster image. The expression is not standard in formal graphics literature; more common is the broader idea of per-scanline or line-based effects, which are particularly discussed in the context of raster displays and retro hardware.

In CRT and hardware video contexts, a frame is drawn line by line. Perscanline processing allows rendering parameters to change during the display of a single line or between lines. This enables dynamic effects such as altering color palettes, adjusting tile or sprite attributes, or shifting scrolling offsets on specific lines. Such line-based changes are useful for creating visual effects that would be costly or impractical to achieve with uniform frame-wide settings.

Common applications include line-based parallax scrolling, mid-frame palette swaps, sprite multiplexing, and other line-specific adjustments that produce parallax or depth cues within a single frame. In retro game development, per-scanline techniques were often implemented via interrupts or careful timing to modify rendering state as the beam scanned the screen. Modern emulation and some shaders replicate similar behavior by using the current scanline or vertical position as an input to select color lookups or offsets.

Implementation considerations involve precise timing and synchronization with the display, memory layout for per-line state, and the risk of artifacts if line changes are not aligned with the scan. Perscanline concepts remain relevant for historical discussion of raster graphics and for understanding line-based effects in contemporary graphics pipelines.