"This Steel Sheet Has a Defect. Can You See It?" FORESIGHT STEREO Reveals a Dent Invisible in Color as "Shape"

The surface of this steel sheet actually has a defect. Can you tell where it is? Your eye may go to the arrow plate on the right, but the defect itself should be hard to find. That is because the defect is a dent (an indentation flaw), a "shape" defect where the surface is very slightly depressed, with almost no color contrast. We captured this sample just once with FORESIGHT STEREO, TOMOMI RESEARCH's stereo 3D imaging technology, and took four types of output from that single shot at the same time. See the moment a dent that cannot be seen in color appears as "shape."

LIVE image: the dent to the left of the arrow plate looks like nothing more than a faint white pattern

Why this dent sample?

The subject is a limit sample of a dent (indentation flaw) on steel sheet (a reference sample that marks the boundary between pass and fail). It carries a label reading "Manufacturer roll indentation limit / NG depending on intended use" and an arrow plate pointing to the location.

The actual limit sample, with its label and the arrow plate pointing to the dent

An arrow is needed because without it, you could not even point to where the defect is. A dent is not a change in color; it is an almost purely "shape" defect formed where the material has been pressed in. With no color contrast, it is easily missed in 2D images or by visual inspection, and pass/fail judgment tends to become a person-dependent sensory inspection that varies with the inspector's skill.

FORESIGHT STEREO: four outputs from one capture

FORESIGHT STEREO generates four outputs simultaneously from a single capture. The first two lean toward "color," and the last two toward "shape."

1. LIVE image: as it appears
2. 2D Texture: color and pattern only (albedo, the material's own color)
3. 3D Surface: reconstructs only the shape (relief)
4. Surface Normal: encodes the orientation of the surface (normals) as color

Invisible in color: LIVE image and 2D Texture

2D Texture: even with only color kept, the dent remains very faint

In both the LIVE image (at the top) and the 2D Texture, which keeps only color and pattern, the dent remains very faint. A "color" defect should stand out precisely in this color layer. That it cannot be seen is the flip side of the fact that a dent carries almost no color information and exists only as shape.

Appearing as shape: 3D Surface and Surface Normal

3D Surface: flat areas become noise-like, and only the dent emerges in black as a sharp shape

With the 3D Surface, the view changes completely. The flat steel sheet becomes a uniform, noise-like background, and within it only the dent emerges clearly in black as a sharp shape (sensitive enough to pick up scuffs on the arrow plate and a small separate mark at the upper left). Color that merely sits on a flat surface disappears, and only the physically deformed areas remain. This subtraction is what makes the dent visible.

Surface Normal: within a blue-violet gradation, only the dent glows in rainbow colors

The source data for that reconstruction is the Surface Normal. Flat areas appear as a blue-violet gradation, and only the dent glows in vivid rainbow colors. This shows that the slope of the surface changes abruptly, which is evidence that the surface is depressed. It is the most visually striking of the four images, and a fitting finale.

What this means on the inspection floor

• Capture depression defects that cannot be seen in color as "shape" (dents and depressions become clear features in the 3D outputs)
• Support person-dependent judgment against limit samples (adding a shared yardstick, shape, to sensory inspection that relies on eyes and touch)
• Four viewpoints from one shot: color and shape acquired at the same time, without recapturing

This is the exact reverse of the semiconductor demo we introduced earlier (where discoloration disappears in 3D, revealing that it is color). This time, the dent is invisible in 2D but appears in 3D, revealing that it is shape. It is another way of showing the same idea: "look at color and shape separately." Note that this is an observation demo on a single sample; it is not a performance guarantee for all materials and defects, nor a measurement of depth or dimensions.

Would you like to try FORESIGHT STEREO on your own samples with "hard-to-see relief defects" (dents and depressions, or workpieces that cause trouble in sensory inspection against limit samples)? We welcome inquiries about demos and sample evaluations. [https://www.tomomi-research.com/en/contact]