How to resize photo and images

Resize photo or resample photo?
What is the difference?

In short we could say that resampling does use an algorithm, while resizing does not.

Resampling produces smoother photos, while simple resizing may produces some steps in the images.

The advantages and limits of resampling versus resizing images and photos:

Resizing

When you enlarge an image using the resize photo or resize image function, some pixels will be doubled or more if necessary.
When you reduce an image using the resize image function, some pixels will just be thrown away. Hence thin lines may completely disappear.

Resampling

When you enlarge an image using the resample image function, the color of the added pixels will be the result of an interpolation. Good photo editors offer a choice of many different interpolation algorithms.
When you reduce an image using the resample image function, all pixels will be re-calculated. A kind of mean value of the neighbouring pixels. Here too some photo editors offer a choice of many different algorithms.

Not every photo editor call it resample, some call it resize photo, but if there is a method or an algorithm to select, then it IS resampling.
A current algorithm used to reduce photos is bilinear.
A current algorithm used to enlarge photos is bicubic.

Photos should always be resized through a resampling algorithm. With other images results may be blured when resampling and sharper when resizing. But again, thin lines may completely disappear when you reduce the size without going through a resampling algorithm.

Where is the resize image or resample image function?
Most photo editors either have a resize button in the tool bar or you will find the function under the menu image.

In most cases you can check a keep proportions check box. If this box is checked you only need to give the new width in pixels and the height that keeps the proportions of the image will be calculated or vice-versa.

Often you will also be able to choose a percentage for the size reduction instead of a width/height in pixels.

The resample function works only on images with 24 bit color definition (16 millions of colors). Images with 2, 16 or 256 colors can only be resized, not resampled.

color depth	# of colors	jpg	png	gif	bmp	can be resized	can be resampled
1	2	N	Y	Y	Y	Y	N
4	16	N	Y	Y	Y	Y	N
8	256	N	Y	Y	Y	Y	N
8	grey tones 256	Y	Y	Y	Y	Y	Y
24	16 777 216	Y	Y	N	Y	Y	Y
32	4 294 967 296	still not spread out on a large scale

The table above shows which color depth can be saved in jpg, png, gif and bmp image formats. It shows also which color depth can be resampled versus resized only.

Examples:

resized photo

resampled photo
resampled photo, bilinear algorithm

Read also the page resize photo in jpg format and resize photo in gif format, where I will tell you how you can resample gifs and what are the advantages and the disadvantages of doing it.