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Scanline Rendering

Renders polygons row by row across the image.

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Quick Facts

Compact metadata for implementation and discovery.

Category: Rendering
Difficulty: Intermediate
Family: Raster Rendering
Tier: Core
Status: Verified
Review: Source Checked
Sources: 1
Formulas: 1
Relationships: 3
Code example: Not ready
Visualization: Ready

Overview

Scanline Rendering processes horizontal lines of pixels, finding polygon spans that cover each row.

Core Concepts

rendered imagevisibility orderingraster renderingscene representationvisibility testpixel output

Quick facts

Complexity

Runtime and memory behavior at a glance.

Best

O(p + e)

Average

O(p + e)

Worst

O(p + e)

Space

O(e)

When to Use

Use Scanline Rendering when you need to walks across image rows.
Use it for software rasterizers with inputs like polygons.
Use it when the scene representation and quality target match the rendering tradeoff.
Use it when the Raster Rendering tradeoff is acceptable for the problem size.

When Not to Use

Avoid it when the cost per pixel or sample is too high for the desired frame rate.
Avoid it when the required output is not rendered pixel buffer.

What It Does

Walks across image rows
Fills visible spans between polygon edges

Use Cases

Software rasterizers
2D polygon filling
Classic rendering pipelines

Inputs

Polygons
Image dimensions

Outputs

Rendered pixel buffer

Key Formulas (1)

Edge intersection

x = x₁ + (y − y₁) · (x₂ − x₁) / (y₂ − y₁)

x = x1 + (y - y1) * (x2 - x1) / (y2 - y1)

Finds where a polygon edge crosses the current scanline.

Pseudocode

1.Build an edge table
2.Move scanline from top to bottom
3.Track active polygon edges
4.Fill spans between edge pairs
5.Update active edges for the next row

Canonical Example

Input: scene = one triangle, one camera, one light
Output: image pixels showing the visible triangle
Explanation: Scanline Rendering: The algorithm converts scene data into visible pixel results.

Implementation Notes

Keep the input contract explicit: Polygons; Image dimensions.
Use this step as the implementation spine: Build an edge table.
Then preserve the next state change: Move scanline from top to bottom.
Build tiny scenes with known geometry before testing complex materials or lighting.

Common Pitfalls

Scanline methods are efficient for polygon filling and classic software renderers.
Ignoring coordinate spaces, depth precision, or sampling noise while debugging image artifacts.
Returning rendered pixel buffer without handling empty, invalid, or boundary inputs.

Interactive Mechanism Walkthrough

Spatial Primitive & Sweep-Line Canvas

Show horizontal rows filling polygon spans.

Step 1 of 5: Build an edge table

scanline-spans

Notes

Scanline Rendering operates in O(p + e) time and O(e) space within the Raster Rendering family. Key governing relation: x = x1 + (y - y1) * (x2 - x1) / (y2 - y1). Often compared with Rasterization and Z-Buffering.