Seeing Your Own Error

"The number tells you that you missed. Only your own eyes, shown the miss, can fix it."

The Score Is Not the Point

After all the machinery — Procrustes, sub-scores, estimated exaggeration — the grade is still just a summary. A summary doesn't retrain your eye. What retrains your eye is seeing the specific drift: the jaw you set five degrees too steep, shown right beside where it should have gone. The score is the doorway; the visual comparison is the room where learning actually happens. An engine that stopped at the number would have done the hard geometry and skipped the part that makes you better.

Onion-Skin, Diff, Flash

So the grader offers three ways to see the difference, each surfacing it to perception differently. Onion-skin lays your construction and the reference over each other, semi-transparent, so misalignments shimmer. Diff highlights exactly where the two disagree, so the error has a color and a place. Flash toggles rapidly between yours and the answer, exploiting the fact that your visual system detects change far better than it compares two still images — the jaw that looked fine at rest suddenly jumps. Three tools for one job: move the error out of the score and into your eyes.

Reference-Space Arrows

The sharpest tool is the arrow field. After the Procrustes alignment, the grader draws a vector from where you placed each part to where it belongs, in the reference's frame — little V-arrowheads pointing your construction toward the truth. And because the errors that matter most are often too small to notice at actual size, there's a deviation-gain slider that amplifies the arrows: turn it up and a subtle, consistent drift you'd never catch at 1x becomes an unmistakable pattern of arrows all leaning the same way. Amplifying the error is what makes an invisible habit visible enough to break.

Feedback Has to Be Perceptual

Underneath all three modes is one principle: to change a perceptual skill, the feedback has to reach perception, not just cognition. 'You scored 72' is a cognitive fact — you can know it and still not see what to do. A flash-compare is a perceptual event — your eye catches the jump and your hand starts correcting before you've even reasoned about it. Drawing is a seeing skill, so its feedback loop has to close at the level of seeing. That's why the grader's last and most important act isn't printing a number — it's putting your error in front of your eyes until you can't unsee it.

Code

Move the error out of the score and into the eye·python

# The grade is a summary. The LEARNING is in SEEING the specific drift.
compare_modes = {
    "onion": overlay(yours, reference, alpha=0.5),   # both at once, semi-transparent
    "diff":  highlight_difference(yours, reference), # color where they disagree
    "flash": toggle(yours, reference, hz=2),         # the eye catches CHANGE, not stills
}

# Reference-space arrows: after Procrustes, draw a vector from where you placed
# each part to where it belongs, in the REFERENCE frame (little V arrowheads).
arrows = [Arrow(src=your_pt, dst=ref_pt) for your_pt, ref_pt in aligned_pairs]

# Errors that matter are often invisible at 1x, so AMPLIFY them:
arrows = scale_from_anchor(arrows, gain=deviation_gain)  # make the drift unmissable
# You learn to SEE by seeing -- the loop closes at perception, not at the number.

Exercise

Take two nearly-identical images — a spot-the-difference pair, or your own drawing next to its reference. First, try to find every difference by staring back and forth. Then rapidly toggle between them (flip in an image viewer, or blink one eye then the other with them aligned). Count how many more differences the flashing surfaces that staring missed, and explain why that's the mechanism the grader's flash mode exploits.

Hint

Staring forces your slow, comparative cognition to hold two images in memory and diff them — error-prone and effortful. Flashing hands the job to your fast, involuntary motion-detection, which fires on any change. The differences 'pop' because change is exactly what that system evolved to catch. Loomis borrows that reflex to make your construction errors impossible to ignore.