Moiré disaster on the E-5

Started Jul 10, 2013 | Discussions thread
John Sheehy Forum Pro • Posts: 20,661
Re: That's just the downsampling algorithm of the program you are using...

Raist3d wrote:

Resize in a high quality program and hopefully this shouldn't happen that bad.

Another way around it is to blur the image before using the horrible downsizing method.  With just the right amount of blurring, the results are very good.  The problem with these horrible downsizing methods is that they work a lot like a grid of nails coming out of a rectangular board, and laid upon a mosaic.  Where the point of the nail falls on the mosaic, is what the resultant pixel is.  If there is one nail spacing for every 1.5 tiles (2 nails every three tiles), then 1/3 of the rows and columns of original tiles are ignored.  Those that are used, are shifted, since the nails are not falling in the center of the tiles; every other column or row of nails is shifted right/left or up/down, relative to the other rows and columns.

Now, do it where every nail spacing corresponds with a spacing of 3 tiles.  Now, 2/3 of the rows and columns are ignored; IOW, a total of 91% of the entire original image is ignored.  However, if you first blur the image sufficiently, the influence of the dropped pixels is present in the nail-selected ones.  Also, in the case where the nails are falling in different parts of the tiles (like our 2 nails for every three tiles above), the spatial error is less intense, but the results can be better simply by upsampling the image softly first.

Of course, it's easier to use a method that does it right in one step, but knowing that you can blur images and then point-sampling opens up possibilities beyond simple one-step resizing algorithms.  For example, if your original capture is very sharp except for a two-pixel vertical motion blur (vertical edges are very sharp, and horizontal edges are very soft), you can avoid over-softening the horizontal edges before downsizing by blurring the image only horizontally, before point sampling.

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