Skia Rasterization and Paint Pipeline

1. Skia 2D Engine and Memory Footprint

Chromium delegates rasterization to the Skia 2D Graphics Library (//third_party/skia).

• Color Allocation: Textures are allocated in 32-bit depth using kRGBA_8888_SkColorType.
• Byte Depth: Each pixel represents 4 channels: $\text{Red (8-bit)} + \text{Green (8-bit)} + \text{Blue (8-bit)} + \text{Alpha (8-bit)} = 32\text{ bits} = 4\text{ bytes}$
• Every rasterized layer uploaded to the GPU as an OpenGL/Vulkan texture consumes: $\text{Bytes} = \text{PixelWidth} \times \text{PixelHeight} \times 4$

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2. The Paint Invalidation Boundary

According to Paul Lewis and Paul Irish's CSS Triggers:

• Properties that affect visual styling without changing geometric boundaries bypass the Layout stage, but trigger immediate Paint (Rasterization) and Compositing:
  • Surface styling: background-color, background-image, box-shadow, border-color, border-radius, outline.
  • Text decorations: color, text-decoration, text-shadow.
  • Dynamic variables: CSS Custom Properties (--*), because their mutation necessitates computed style recalculation and broad subtree repaint.

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3. Fillrate and Surface Area Scaling Law

In computer graphics and GPU architecture, Fillrate is the rate at which a GPU can render pixels to the frame buffer:

$\text{Raster Cost} \propto \text{Area}_{\text{paint}} = W_{\text{element}} \times H_{\text{element}} \times \text{DPR}^2$

• Small surface area: A $32 \times 32\text{px}$ toggle switch animating background-color invalidates roughly $1\text{KB}$ of pixel data. The GPU handles this in sub-millisecond raster passes.
• Large surface area: A full-screen dialog ($1920 \times 1080\text{px}$, DPR 2) animating box-shadow invalidates $\approx 8.3\text{ million physical pixels}$ per frame ($\approx 33\text{MB}$ of raw raster throughput per tick).
• Conclusion for Scoring: A linear or clamped scaling multiplier based on $\text{PaintArea} / \text{ViewportArea}$ is mathematically essential to prevent disproportionate penalties on small UI widgets while aggressively flagging large viewport repaints.