How to resize an image without losing quality
Image Size in Paint by Zaweba offers Lanczos-3, Mitchell, Preserve details and Pixel art scaling, and Smart Scale resizes by seam carving.
Every resize changes pixels, so the real aim is to lose as little as possible. In Paint by Zaweba, Image > Image Size (Ctrl+Alt+I) lets you pick how the new pixels are worked out: Lanczos-3 and Mitchell for clean general resizing, Preserve details for enlarging, and Pixel art for sprites. Shapes and text are scaled as vectors, so they stay sharp at any size, and Smart Scale changes the proportions of a picture without squashing what matters.
Getting smaller is easy. Getting bigger is where quality suffers, because the software has to invent pixels that were never there. Choosing the right method makes a visible difference to both.
Making a picture smaller
Open the picture and choose Image Size. Type the new width or height and keep the proportions linked. For photos, Lanczos-3 gives crisp results with good fine detail. Mitchell is a little softer and less prone to bright halos along strong edges, which can suit pictures with lots of high contrast lines.
Shrinking can soften a picture slightly. A light pass of sharpening afterwards, such as Unsharp Mask at a low amount, brings back the snap. Do this as the last step, at the final size.
Making a picture bigger
Choose Preserve details when enlarging. It combines denoising, Lanczos resampling and an edge following sharpening step, so edges stay defined instead of turning soft and blocky. It can't add detail that wasn't captured, but it makes the most of what is there.
Enlarge in one go to the size you need rather than in several small steps, and start from the best original you have. A picture that has been saved as a JPEG many times will show its artefacts more when enlarged.
Which other methods are for
Smooth and bilinear are quick general methods. Hard edged keeps each pixel's colour without blending, which is right for screenshots of icons and for pixel art you are scaling by whole numbers. Pixel art uses Scale2x and Scale3x, which double or triple sprites while rounding off their staircase edges in the style of classic game art.
Changing the shape, not the size
Sometimes you need a different aspect ratio, a wide banner from a square photo say. Stretching distorts people and objects. Edit > Smart Scale instead removes or adds seams of least importance, such as sky or plain wall, and leaves the important parts alone. An amount slider mixes in normal scaling, you can protect a selection, and skin tones can be protected by colour range so faces don't distort.
Getting a good result
Keep your full size original and resize copies. Shrinking throws pixels away, and you can't get them back by enlarging again. If you need the same picture at several sizes, make each one from the original.
Mind the layers too. Pixel layers are resampled, but shape and text layers are scaled as vectors, so a logo built from shapes and type comes out sharp at any size. That's a good reason to keep logos and lettering as live layers rather than flattening them early.
Resizing a whole folder
For a batch of images, File > Image Processor converts a folder to JPEG, PNG, WebP or TIFF and fits each one within a width and height you set. For one picture, though, Image Size with the right method is the way to keep as much quality as Paint by Zaweba can.
