If you want to change the resolution of an image without changing
its physical size in the document, you need to “resample” it.
Reducing the resolution without resampling will increase the
physical size of the image, because the number of pixels per inch
becomes smaller, but the total number of pixels remains the same;
i.e. less pixels per inch with the same number of pixels results in
more inches.
Downsampling
reduces the resolution of the image without increasing the physical
size of the image, by “throwing away” pixels and determining the
color of the remaining pixels based on the combined color of the
pixels that were discarded.
The following downsampling methods are available:
- Average downsampling computes the average pixel color of the pixels
in the sample area, and replaces the entire area with this average.
- Resampling replaces the entire area with the pixel color of the
pixel at the center of the sample area.
- Bicubic downsampling the value of the new dots is calculated on the
basis of a weighted average of the pixels in the sample area. This method offers the best
results with smooth tonal gradations, but takes the longest to compute.
- Bilinear resampling uses weighted interpolation between 4 pixels
in the original image.
- Bicubic B-spline is a modified bicubic interpolation with more
sharpening effects and is recommended for upsampling. Bicubic downsampling usually yields
better results than the simple averaging method of downsampling.