Perspective Correction Online

How this works

Perspective correction fixes the keystone distortion you get when a flat object is photographed at an angle, mapping the slanted, trapezoid-shaped page back to a flat, square-on rectangle.

It solves a 3×3 homography between the four corners you set and the corners of a true rectangle, then resamples every output pixel through it (bilinear sampling) so straight edges become straight again; the output rectangle is sized from the quad's own longest edges so the document keeps its real resolution.

What you get

Parallel edges that appeared to converge are made parallel again — the core trick behind every document scanner, and just as useful for whiteboards, posters, slides, paintings and signage.

This page opens with auto edge-detection already placing the four corners; export the corrected result as a PNG or PDF, with an optional enhance filter.

Tip

Accuracy depends entirely on the four corners — place each handle right on the object's corner tip, because even a few pixels off at one corner is the most common cause of a slightly skewed result.

Enhance filters explained

After the page is straightened, an enhance filter cleans it up. Each one is applied to the corrected scan in your browser; here is exactly what they do and when to use each.

Magic (colour)

Stretches contrast about 1.35× around mid-grey and adds a small brightness lift, so dull greys become white and text deepens — the classic colour-scan look. Colour is kept. Best for: Most documents, and anything with colour you want to keep (logos, highlights, coloured forms).

Black & White

Measures the page's average brightness, biases the cut-off toward white paper (≈0.82× the mean), then forces every pixel to pure black or pure white. The result is a clean, high-contrast bi-tonal page. Best for: Text-only pages, and the cleanest input for OCR; also gives the smallest PDF.

Grayscale

Converts to neutral grey using the standard Rec. 601 luma weights (0.299 red + 0.587 green + 0.114 blue), keeping soft tones without colour. Best for: Pages with photos or shading where pure black-and-white would lose detail, but colour isn't needed.

Original

Leaves the warped pixels exactly as photographed — perspective is still corrected, but no contrast, brightness or colour change is applied. Best for: When you only want the page straightened and cropped, or you'll do your own colour edits afterwards.

PDF page sizes

Save a PNG, or a single-page PDF in one of these sizes. Fit makes the page exactly the scan's pixels (no margins); A4 and Letter centre the scan on a standard page. Sizes are shown in millimetres, inches and PDF points (1 pt = 1/72 in).