Lecture 20 - LODs - Fall 2004

Oct 23, 2026

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Lecture 20 in the Fall 2004 Cornell CS course by Kavita Bala introduces rendering research on lighting with many lights and then transitions to Level of Detail (LOD). It motivates LOD by viewer distance, contrasting close-view detail with far-view approximations. The lecture explains standard game-style LOD models, typical issues (creation, selection, artifact-free switching), and two frame-based resolution selection strategies. It concludes with LOD switching artifacts like popping and flickering, and practical remedies such as more hierarchical resolutions and blending or morphing.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

Rendering with Environment Maps

Sampling Environment Maps

New Metric and Hierarchical Threshold

Summary: Many-Lights Rendering Complexity

Level of Detail (LOD)

Purpose of LOD

LOD Models

Standard LOD in Games

Issues with LODs

Selecting LOD Models

LOD Switching

Lecture 20: LODs Fall 2004 Kavita Bala Computer Science Cornell University Research on many lights • Ward ‘91 • Shirley, Wang, Zimmerman ‘94 • Fernandez, Bala, Greenberg ‘02 • Wald and Slusallek’03 • Environment Map Sampling © Kavita Bala, Computer Science, Cornell University Rendering w/ Environment Maps • High lighting complexity • Rich: captures real world © Kavita Bala, Computer Science, Cornell University High-Dynamic Range Imagery • Multiple Exposures © Kavita Bala, Computer Science, Cornell University | | | | --- | --- | | | | | © Kavita Bala, Computer Science, Cornell University | | Sampling Environment Maps • Say we have a fixed budget of samples – Say 300 or 3000 • Goal: sample environment maps La ∆ ω b • Basic idea: sample according to … – Illumination importance sampling: oversamples small bright lights – Area-based stratified sampling: distribute samples over area (under-samples bright lights) © Kavita Bala, Computer Science, Cornell University New Metric 1 • Instead L∆ ω 4 © Kavita Bala, Computer Science, Cornell University Hierarchical Threshold • Create regions from iσ to (i +1) σ • Distribute samples in region according to metric Metric N = N j j Metric map • Distribute samples in each region according to Hochbaum-Shmoys – Minimize maximum distance © Kavita Bala, Computer Science, Cornell University © Kavita Bala, Computer Science, Cornell University © Kavita Bala, Computer Science, Cornell University Summary • Rendering with many lights remains an open problem – Techniques are linear in number of lights – Hard to handle visibility • Some interesting new approaches needed for interactive rendering with many lights © Kavita Bala, Computer Science, Cornell University Complexity • Lighting: many lights, environment maps – Global illumination, shadows • Materials: BRDFs, textures • Geometry: Level-of-detail, point-based representations • All: impostors, image-based rendering © Kavita Bala, Computer Science, Cornell University LOD • What is Level of detail used for? • When close, want detail • When far, want approximation • Level of Detail (LOD) techniques draw models at different resolutions depending on their relevance to the viewer © Kavita Bala, Computer Science, Cornell University | LOD Models (I) | | | | | --- | --- | --- | --- | | | | | | | © Kavita Bala, Computer Science, Cornell University | | | | | LOD Models (II) | | | | --- | --- | --- | | | | | | © Kavita Bala, Computer Science, Cornell University | | | LOD Models (3) • Effect of LOD less obvious if smooth shaded • Note also that shading changes with LOD © Kavita Bala, Computer Science, Cornell University Standard LOD • Used in games – Finite set of models – Hand generated – Or using automatic decimation/simplification • Use of LODs: based on distance from eye or projected screen size © Kavita Bala, Computer Science, Cornell University Issues with LODs • LOD Creation • LOD Selection: Choosing which model to display • LOD Switching: Artifact-free switching between LODs © Kavita Bala, Computer Science, Cornell University Selecting LOD Models • For any given frame, decide what resolution to display • Option 1: Choose each object’s resolution independently – For example, use projected area or distance – The current standard practice: fast, simple – Potential problem: total \# polygons may be large • Option 2: Fix number of polygons and choose models to fit in budget – Ensures near constant frame rate, but harder to implement © Kavita Bala, Computer Science, Cornell University LOD Switching • Popping when models switched • Popping is visually disturbing – Why? – Flickering when at threshold • Solutions: – Have more resolutions in hierarchy – Blend two resolutions 􀂃 Image blend: draw both resolutions 􀂃 Geometry blend: morph between resolutions © Kavita

FAQs

Why is level of detail (LOD) used in rendering?

LOD techniques draw models at different resolutions depending on relevance to the viewer: more detail up close and approximation when far away.

What are the main issues when working with LODs?

Key issues are LOD creation, LOD selection (choosing which model to display), and LOD switching (avoiding artifacts during transitions).

How can LOD switching artifacts like popping and flickering be reduced?

Solutions include using more resolutions in the hierarchy and blending two resolutions, such as image blending (draw both) or geometry blending (morph between).

Oct 23, 2026

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