Image Resize

Resizing an image changes its pixel dimensions by resampling. Each new pixel is computed from nearby source pixels with an interpolation method such as nearest neighbor, bilinear, bicubic or Lanczos.

Downscaling combines several source pixels into one, so fine detail is lost and files get smaller. Upscaling invents in-between pixels by interpolation. It makes the image larger but cannot add detail that was not captured.

To keep the aspect ratio, both dimensions must be scaled by the same factor. Resizing is different from changing the DPI value, which only changes print size, and from changing the canvas size, which adds or removes space around the image.

Resize vs crop

Resizing keeps all of the content and changes its scale. Cropping keeps the scale and removes content. For a docs image of one dialog, crop to the dialog first and then resize if needed, so the text stays as large as possible.

Where resizing is used

• Fitting screenshots to a web page or document column.
• Producing 1x and 2x versions for different screens.
• Meeting upload limits on pixel size.
• Creating thumbnails for galleries.

How resizing affects quality

Retina screenshots have twice the pixels of the point size in each direction. Scaling them to exactly 50 percent gives a clean 1x image, because every 2×2 block becomes one pixel. Scaling by an odd factor, such as 63 percent, spreads thin lines across pixel boundaries and makes text look soft. Upscaling a small capture for print gives blurry edges.

Nearest neighbor keeps hard pixel edges, which suits pixel art or exact UI zoom-ins. Bicubic and Lanczos give smoother results for general images.

Common mistakes

• Changing only the width or the height and distorting the image.
• Resizing several times in a row instead of once from the original.
• Upscaling to hide a low-resolution capture.
• Resizing a JPG and re-saving it at a low quality.

Have an image to fix right now?

Check the current pixel size with Screenshot Resolution Checker, and work out a new size that keeps the ratio with Aspect Ratio Calculator.

Common questions

Why are my Mac screenshots twice the size I expected?

On a Retina display each point is 2×2 pixels, so a 1000 point wide capture is 2000 pixels wide.

Does resizing reduce file size?

Yes. Fewer pixels need fewer bytes, often far more than a change in compression setting.

Sources

• Image scaling (Wikipedia)

Related resources

• Documents export: image sizing in PDF export.
• Pixel density: why Retina captures have more pixels.