# Subtitle Preview Scale Design

**Goal:** Make the subtitle sample inside the small preview screen visually proportional by scaling the rendered preview text down, while keeping the real subtitle size values unchanged for export and editing.

## Context

The current subtitle defaults card renders the preview text using the raw subtitle font size value. In a compact sample screen, that makes the text appear nearly as tall as the preview display itself, which breaks visual proportion and makes the mock preview feel unrealistic.

The user approved the "preview scale mapping" direction instead of enlarging the screen or rebuilding the preview as a strict video-canvas simulator.

## Approved Direction

- keep the stored subtitle size value unchanged
- keep the slider values unchanged
- keep exported subtitle defaults unchanged
- only scale the preview rendering inside the small sample screen
- make the preview text visually smaller relative to the sample screen

## Recommended Behavior

Use a lightweight preview-only mapping:

- compute a preview font size from the real subtitle size
- apply a clamp so small values do not become unreadable
- keep the label beside the slider showing the real value such as `24px`
- keep upload/export using the real value such as `24`

This preserves the product meaning of the controls while making the small preview believable.

## Rendering Rules

- real subtitle size remains the source of truth in state
- preview font size is a derived value for UI only
- preview outline strength should scale down with the preview text so the stroke does not overpower the sample
- subtitle bottom offset behavior remains unchanged

## Testing Strategy

Update upload-screen tests to verify:

- the preview text no longer renders at the raw `24px` size
- the slider can still change the real font size value
- the preview updates to the mapped display size
- upload confirmation still sends the unmodified real subtitle size

## Out of Scope

- changing the export subtitle engine
- changing slider ranges
- redesigning the subtitle defaults card
- introducing full video-canvas simulation

## Success Criteria

Success means the sample subtitle looks appropriately smaller than the preview screen, while the UI still communicates and submits the real subtitle size values unchanged.