The original 20×20 image.
Nearest-neighbor resizing.
Pixel mixing assumes that the original image is perfect, as opposed to a low-resolution approximation. It’s not good for enlarging images, unless you want a pixellated effect.
Linear interpolation. Fast, easy to program, and better than nothing.
Hermite is fast, and usually gives a clearer image than linear interpolation. It has good worst-case behavior, but it’s very blocky.
B-spline. Very blurry, but it makes even very jagged edges smooth, and has no halo or ringing artifacts.
Gaussian. Blurry, but it’s about as good as a non-adaptive resampling filter can be and not have any halo artifacts.
Quadratic filter. Designed to be a fast approximation of a gaussian filter.
Mitchell is a balanced filter, with only slight blurring and haloing. A good general-purpose filter.
Catmull-Rom spline. Has no blurring, but does have a halo effect. IW’s default filter when enlarging.
A 3-lobed Lanczos filter. This is the most popular of the “windowed sinc” filters. As with most such filters, it has no blurring, and minimal aliasing, but can have significant halo and ringing artifacts.
A type of windowed sinc filter.
A type of windowed sinc filter.
Mitchell filter in the horizontal dimension, and pixel mixing in the vertical dimension.
Convert to grayscale+alpha.
Apply a solid background color.
Apply a checkerboard background.
A smaller checkerboard.
A larger checkerboard, shifted.
Applying a background and dithering. Note that the background gets dithered.
Applying a background and converting to grayscale. Note that the background gets converted to grayscale.
Blurred, mostly to make more partially-transparent pixels for the demos.
Restricted to 4 levels of transparency.
Binary transparency, with partial transparency simulated by dithering the alpha channel.
Dither to an 8-color color cube, and to binary transparency. Note that the transparency and color dithering can interact in strange ways.
Dither to an 8-color color cube, but with a full alpha channel. This is silly, but included for completeness.
Did you know that BMP (v3) images can have transparency? Well, sort of. When using RLE compression, pixels can be left undefined, and some web browsers will interpret undefined pixels as being transparent. IW has a -bmptrns option to take advantage of this. As of this writing, it works in Firefox and Chrome. Other browser will probably display the background of this image as either black or magenta.