How to use the Reference Flip & Grayscale Tool
- Open a reference. Click Open a reference, drop an image on it or paste one. The still life loads by default so you can try things first. Everything happens in your browser; the image is never uploaded.
- Transform it. Use the toolbar or the keys: Flip H, Flip V, Rotate, Grayscale, Invert, Contrast and the Squint slider. They stack, so you can view a mirrored, grayscale, blurred version at once. The badge in the corner lists what is active; Reset clears it.
- Zoom without cropping. +, − and Fit zoom around the center. Ctrl + scroll, a trackpad pinch or a two-finger pinch on a tablet zooms around the cursor, double-click zooms in, and dragging pans. The whole image is always one Fit away.
- Compare with your drawing. Under Compare with my drawing, load a photo of your work. Choose Overlay and set the opacity, Difference to light up mismatches, or Side by side. Tick Drag moves the drawing and use the Scale and Rotate sliders to line it up. Try a sample drawing shows how it works with a deliberately flawed sketch.
- Download. Download PNG saves exactly what you see at full resolution, minus the zoom: flips, filters and the overlay included.
Why a flipped reference shows you new mistakes
After twenty minutes of drawing, you stop seeing your drawing. Your brain fills in what you meant to put down, and a nose that drifts left or a shoulder that sits too high starts to look normal. A mirror breaks that familiarity instantly; Leonardo da Vinci’s notes on painting already advised checking your work in one. Every lopsided ellipse and leaning vertical jumps out because the picture suddenly looks like someone else’s.
Each transform here does a different job:
| View | Key | What it exposes |
|---|---|---|
| Flip horizontal | H | Lopsided faces, leaning verticals, tilted horizons, drifting eye lines |
| Flip vertical | V | Shapes and negative spaces, because you can no longer read the subject |
| Rotate 90° | R (Shift+R back) | Angles and proportions of long forms such as limbs, trees and buildings |
| Grayscale | G | Value structure, separated from hue and saturation |
| Squint | B | Big value masses; details vanish and only the main shapes remain |
| Contrast | C | Faint edges and subtle value shifts in flat or hazy photos |
| Invert | I | Negative shapes and edge placement, in an unfamiliar form |
The grayscale view uses a perceptual formula, keeping each pixel’s luminance, so a saturated red reads as the middle-dark value it really is rather than the pale gray a channel average would give. The squint blur scales with the image size, so level 5 looks about the same on a phone snapshot as on a 40-megapixel file.
Getting a useful overlay of your drawing
The comparison is only as good as the photo of your drawing, so a few minutes of setup pay off.
- Shoot straight on. Hold the phone parallel to the paper and centered on it. Tilting it even a little turns a rectangle into a trapezoid, which you would then misread as a drawing error.
- Step back and zoom in. Phone wide-angle lenses bend edges at close range. Shooting from farther away with the 2× lens, if you have one, keeps lines straighter.
- Light it evenly. One window to the side leaves the far edge gray and hides light pencil marks.
Then align on the biggest shape, not on a detail. Set Overlay at about 50%, tick Drag moves the drawing, and match one landmark, for example the top of a head or a table edge. Use Scale until a second landmark at the other end of the picture also lines up, then Rotate if the photo was tilted. Whatever still disagrees is a real error in the drawing. Arrow keys nudge the drawing one pixel at a time, or ten with Shift.
Difference subtracts one image from the other: where both match, the result goes dark; where they disagree, edges glow. It works best when the two images are similar in value, such as a painting against its photo. For a pencil line drawing on white paper the whole sheet differs from the photo, so stay with Overlay and hold the Space bar to blink the drawing off and on.
Tips from practice
Flip early, not at the end. Check the flipped view once the big shapes are blocked in. Moving an eye is a two-minute fix in the lay-in and a two-hour fix after rendering.
Squint before you add detail. With Squint at 5 and Grayscale on, the reference shows the three or four value masses that matter. If your drawing doesn’t read the same way at that blur, adding detail won’t fix it. The Notan & Value Study Tool turns those masses into a printable plan.
Rotate when you draw long forms. Angles near vertical are hard to judge. Turning both the reference and the overlay 90° makes a nearly upright arm or tree trunk into a nearly horizontal line, and small tilts become obvious.
Keep a flipped copy in your workspace. Download the mirrored reference and work from it now and then. Drawing from the reversed image for a while keeps the habit-blind spot from settling in.