Tan Line Descriptor Test: Are tan lines and bikini tan lines Visible When Wearing a Bikini?

Key Takeaways

• Specifying tan lines while wearing a bikini produces no visible tan lines. Since the bikini covers the tan mark boundaries, the border between tanned and untanned skin is not visible.
• tanned skin slightly shifts skin tone toward a darker shade. However, no tan “lines” were observed.
• Showing tan lines requires a “mismatch between outfit and tan pattern.” For example, showing bikini tan lines while wearing a one-piece swimsuit or camisole — the tan marks and current outfit need to differ.
• This “no difference” result is valuable. Misusing tan line keywords wastes tokens without achieving the intended expression.

Experiment Design

Condition Design

Evaluation Criteria

• Tan line visibility: Whether a boundary between tanned and untanned skin is visible
• Skin tone: Whether tanned skin effectively makes the skin darker
• Difference between conditions: Whether the presence of tan lines keywords changes the appearance

Testing Each Condition

A00: Control (tanned skin only)

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Observations: White bikini with beach background. Skin is slightly tanned, though the effect of tanned skin is subtle. Rocky beach background compositions are consistently generated. No tan lines, as expected.

A01: tan lines

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Observations: No visual difference from control. 3/3 showed no visible tan line boundaries. Since the white bikini covers the same area as the tan marks, even if the model internally generated tan lines, they’d be hidden under the clothing.

A02: visible bikini tan lines (Detailed Specification)

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Observations: Despite specifying visible bikini tan lines on shoulders and chest with specific body parts, tan lines were still not visible. There should be tan boundaries on the shoulders and decolletage, but the bikini straps align with the untanned areas, making them impossible to see.

Note: Tan lines aren’t visible when wearing the same bikini that produced them. Changing into a different outfit, such as a camisole, is needed to make them visible.

Why Tan Lines Aren’t Visible

Tan lines are only visually meaningful when the current outfit differs from the tan pattern:

• Bikini tan marks → Viewed in a one-piece swimsuit or camisole → Strap marks become visible
• T-shirt tan marks → Viewed in a tank top or bikini → Sleeve marks become visible

In this experiment, the design was “viewing bikini tan marks while wearing a bikini,” so the tan boundaries were hidden under the clothing.

Future Testing Plans

To actually visualize tan lines, the following condition designs would likely be effective:

• bikini tan lines, wearing one-piece swimsuit — View bikini marks in a one-piece
• bikini tan lines, wearing camisole — View bikini marks in everyday clothes
• t-shirt tan lines, wearing bikini — View T-shirt marks in a bikini

These are to be confirmed in future tests.

Practical Tips

• Specifying tan lines while wearing a bikini wastes tokens. If the outfit and tan pattern match, the lines won’t be visible
• To show tan lines, design an “outfit mismatch” — The tan source outfit and current outfit should differ in the prompt
• “No difference” results are useful as tests — Helps avoid meaningless keywords and save tokens

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