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ADR-0011: ADR-0011: Faithful representation of diagram source

Status​

Accepted

Context​

When generating natural language descriptions for diagrams, we need to decide how literally we should represent the source code versus adding computed or inferred information.

The key principle: "Faithful" means matching what sighted users see, not just the raw source code.

Example 1: Pie Chart Percentages​

Mermaid pie charts with showData display both raw values AND percentages visually:

Since Mermaid displays "JavaScript: 40 (57%)" in the rendered diagram, our description should match: "JavaScript: 40 (57%)". This is faithful to what sighted users see.

Example 2: Activity Diagram Step Numbers​

PlantUML activity diagrams do NOT automatically number steps in the visual:

:Read input;
:Process data;
:Write output;

Our description should NOT add "Step 1: Read input, Step 2: Process data..." because sighted users don't see those numbers.

Example 3: Sequence Diagram Auto-numbering​

PlantUML only shows message numbers when autonumber is specified:

autonumber
Alice -> Bob: Request
Bob -> Alice: Response

Only with autonumber should our description include "1: Request, 2: Response".

Similarly, for class diagrams, should we infer relationships that aren't explicitly stated, or add information about design patterns we detect?

Options​

Option A: Faithful representation (chosen)​

Generate descriptions that closely mirror what is written in the diagram source code. Do not compute, infer, or add information beyond what the author explicitly specified.

Pros:

  • Predictable output - users know what to expect
  • No risk of incorrect calculations or inferences
  • Respects the author's intent
  • Simpler implementation
  • Easier to test and verify

Cons:

  • May miss opportunities to provide "helpful" additional context
  • Raw values may be less immediately meaningful than percentages

Option B: Enhanced/computed descriptions​

Compute additional information (percentages, totals, inferred relationships) to provide richer descriptions.

Pros:

  • Potentially more informative for end users
  • Could provide insights the visual representation shows implicitly

Cons:

  • Risk of incorrect calculations
  • May not match author's intent
  • More complex implementation
  • Harder to test edge cases
  • May conflict with what the visual actually shows

Decision​

We choose Option A: Faithful representation - faithful to what is visually rendered, not just the source code.

The a11y description should accurately reflect what sighted users see in the rendered diagram. We do not:

  1. Add information not in the visual - Don't add step numbers to activity diagrams that don't show them
  2. Infer relationships - We only describe explicitly stated relationships
  3. Add design pattern names - We describe the structure, not our interpretation of it
  4. Correct or normalize - If the source has inconsistencies, we reflect them

When to compute values​

We DO compute values when the diagram tool itself computes and displays them:

  • ✅ Pie chart percentages: Mermaid pie showData displays percentages, so we include them
  • ✅ Sequence message numbers: Only when autonumber directive is present
  • ❌ Activity step numbers: PlantUML doesn't show these, so we don't add them

Exceptions​

The following transformations are acceptable as they improve readability without changing meaning:

  • Converting visibility symbols (+, -, #) to words (public, private, protected)
  • Translating diagram keywords to natural language
  • Structuring output with proper grammar and punctuation

Consequences​

Positive​

  • Descriptions are predictable and verifiable
  • No risk of computational errors
  • Implementation remains simple
  • Testing is straightforward
  • Users can trust the description matches the source

Negative​

  • Requires understanding what each diagram tool renders visually
  • Descriptions may be less "polished" than hand-written alternatives
  • Edge cases like pie without showData require careful consideration (see issue #5632)

Mitigation​

For cases where computed values would be helpful, users can use the a11yDescriptionOverride attribute to provide their own enhanced description.

References​

  • ADR-0009: No automatic step numbering in activity diagrams (related principle)
  • W3C WCAG: Alternative text should convey the same information as the visual
Single Source of Truth

This ADR is automatically copied from docs/adr/0011-faithful-source-representation.md in the repository. Edit the source file to update this page.