I'm experimenting with image resizing techniques and algorithms. Specifically, I'm significantly downsizing images, e. g. from 2048x1536 to 64x48 - 32 times.
Now, say I'm using a 4x4 kernel. Right now I'm upscaling the image to be by a factor of power of 4 larger than the target (2048x1536 becomes 4096x3072), and then process the image 3 times, each time obtaining a new image 4 times smaller than the source. Seems ineffecient. Is there a trick to it?
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It is arguably inefficient. You could achieve the same results with a single processing step using a single kernel. You can do this by convolving all of the kernels together, then applying your resize using this larger kernel.
A similar process to this is used in most digital painting packages. The resizing usually offers some fixed kernel sizes (e.g. bilinear / bicubic - these may be referred to as 'smoother' / 'sharper') as well as some form of intelligent resize (sometimes called 'smart' or 'best quality') where the resampling kernel size is a factor of the image size.
If you need more information you should add some details about what precisely you are trying to achieve.
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$\begingroup$ So basically I need to construct a kernel that has just the right size for the target image? I simply need to downsize images, the twist being that the target size is basically an icon. $\endgroup$ Jun 3, 2014 at 19:05
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$\begingroup$ By the way, can you, by any chance, advise the most suitable algorithm / kernel for retaining a recognizable (not too soft) image after extreme downscale? $\endgroup$ Jun 3, 2014 at 19:07
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1$\begingroup$ I would investigate Lanczos resampling - en.wikipedia.org/wiki/Lanczos_resampling. For background you may want to read up on sinc filters. $\endgroup$– PAK-9Jun 4, 2014 at 10:33