SWFSize Explained: What It Measures and Why It Matters

SWFSize Best Practices: Compression, Assets, and Code TricksAdobe Flash (SWF) files are less common than they once were, but many legacy projects and interactive pieces still rely on SWF output. SWFSize is a tool and a set of practices aimed at understanding and minimizing SWF file size so deliverables load faster, use less bandwidth, and remain maintainable. This article walks through proven best practices—compression settings, asset management, and code-level tricks—that help you reduce SWF sizes without sacrificing user experience.

Why SWF size matters

Faster load times: Smaller SWF files download quicker, improving perceived performance.
Lower bandwidth costs: Reduced data transfer benefits both hosts and users.
Better compatibility: Some environments have size limits or performance constraints where lean SWFs perform more reliably.
Easier maintenance: Fewer and better-optimized assets make iterative development faster.

Measuring and analyzing SWF size

Before optimizing, measure. SWFSize tools (or SWF analyzers) break down file size by section—shapes, images, embedded fonts, bytecode (ABC), and metadata. Typical important metrics:

Image/bitmap bytes
Vector shape data
Embedded font bytes (especially for many glyphs)
ActionScript bytecode (ABC) size
Library/asset duplication

Use a profiler to identify the biggest contributors and focus optimization where it yields the largest gains.

Compression: settings and approaches

Use proper SWF compression

SWF supports built-in compression (zlib) for all tags after the header when the file is saved compressed. Make sure your build pipeline outputs compressed SWFs (commonly .swf with compressed header). For larger projects, compressing via a post-build zlib step can help.

Optimize JPEG/PNG assets

For photographic content, use JPEG with tuned quality (60–80%) to balance size and visual quality.
For interface graphics and flat-color art, use PNG-8 or indexed PNG to preserve sharp edges while reducing bytes.
Strip metadata (EXIF) from images before embedding.

Use image atlases / sprite sheets

Combining many small bitmaps into a single atlas reduces overhead (fewer bitmap tags and less per-image metadata) and can improve rendering performance. When using atlases:

Pack tightly to minimize empty space.
Group assets that are used together to avoid loading unused atlas regions.
Consider multiple atlases by level/section to avoid loading the entire app at once.

Vector vs bitmap tradeoffs

Vectors scale without loss and often have smaller sizes for simple shapes; complex vector artwork with many nodes can bloat SWF. When vectors become heavy, rasterize at appropriate resolution and use bitmaps instead. Hybrid approaches work well: keep UI as vectors, heavy artwork as optimized bitmaps.

Asset management: fonts, audio, and symbols

Embedded fonts: subset aggressively

Embedded fonts can become the largest part of an SWF. Always subset fonts to only include glyphs used in the project. Further reduce size by:

Using system fonts where acceptable (no embed).
Using font formats optimized for SWF (and convert only needed glyph ranges).
Replacing multiple weights with a single weight where design allows.

Audio optimization

Choose compressed formats suitable for SWF (e.g., MP3 for voice/music).
Use lower bitrates for background music (96–128 kbps) and even lower for voice (32–64 kbps) depending on acceptable quality.
Trim silence and remove unused channels/metadata.

Remove unused symbols and assets

Dead assets—unused movie clips, images, or code—often linger. Use your build tools to perform tree shaking and eliminate unused library items. Review the library and remove legacy or experimental assets that aren’t referenced.

Code-level tricks: ActionScript and bytecode size

Minimize ActionScript bytecode (ABC)

ActionScript bytecode contributes directly to SWF size. Reduce ABC size by:

Avoiding heavy use of reflection, dynamic features, or large metadata blocks.
Flattening deep inheritance hierarchies where methods are duplicated.
Reusing code and functions rather than duplicating inline logic.
Using smaller identifiers (minification) where your toolchain permits—shorter class/function names reduce bytecode size.

Compiler flags and optimization levels

Use your compiler’s optimization flags to strip debugging information and to optimize bytecode generation. For example:

Compile without debug info in release builds.
Enable bytecode optimization options if available.
Use conditional compilation to exclude debug or development-only code.

Lazy-loading modules

Break large applications into smaller SWFs (modules) loaded on demand. This reduces initial download size and speeds up startup. Best practices for modular loading:

Keep a very small bootstrap SWF with only UI shell and a loader.
Load modules when the user navigates to features that need them.
Cache loaded modules in memory if the platform and memory budget allows.

Build pipeline and tooling

Automate size checks

Integrate SWF size checks into CI so regressions are caught immediately. Fail builds when size thresholds are exceeded or when large new assets are added without justification.

Use post-build analyzers

Run analyzers that can show tag-by-tag breakdowns and identify redundant resources. Combine with visual diff tools to see what changed between builds.

Minify and obfuscate carefully

Minification shortens names and removes unreachable code, reducing size. Obfuscation can also reduce size but may interfere with reflection-based code—test thoroughly.

Practical checklist

Enable SWF compression (zlib) for releases.
Subset fonts and prefer system fonts when possible.
Optimize images: JPEG for photos, PNG-8 for flat artwork.
Use atlases to reduce per-image overhead.
Rasterize overly complex vectors.
Compress and trim audio; use appropriate bitrates.
Remove unused assets and dead code (tree shaking).
Compile without debug info; enable bytecode optimizations.
Break app into lazy-loaded modules.
Automate size checks in CI and run post-build analyzers.

Example workflow (concise)

Audit current SWF with an analyzer.
Subset fonts and convert large vectors to bitmaps where needed.
Re-encode images and audio with tuned settings; pack images into atlases.
Enable release compiler optimizations and strip debug info.
Split into modules and lazy-load large features.
Re-run analyzer; iterate until size targets are met.

When to accept trade-offs

Not all size reductions are worth the hit to visual quality or development speed. Prioritize based on:

User connectivity expectations (mobile vs desktop).
Frequency of asset reuse across the app.
Time and maintenance cost of advanced optimizations.

Compression, asset discipline, and intentional code design together produce the largest wins when optimizing SWF files. Focus on the biggest contributors first—fonts, bitmaps, and bytecode—and use build automation so your team keeps SWFs lean over time.