Bart_van_der_Wolf
pro member
Hi folks,
In another thread I was asked a question, and I think the answer deserves a thread of its own. Understanding the issues explained in this thread also helps with finding solutions for other recurring downsampling procedures.
The question was: "A question to Bart and offcourse anybody else: Do you mind having a look at that "downrez from PhotoAcute-action? It's here
and used to automatate best the downrez of the 380 MB tiffs, being generated out of PhotoAcute's big DNGs (with 2 x size, aka superresolution)".
PhotoAcute allows to increase the single image resolution, by combining multiple under-sampled images. In that 'SuperResolution' procedure the resulting image is enlarged 2x (to ensure under-sampled images), with a higher resolution than would be possible from enlarging a single image. While that is useful for large format output, it also poses a burden for other uses of such images such as (panoramic) stitching. Hence the need for down-sampling to 50% of the ' SuperResolution' size.
As I've explained in one of my webpages, proper downsampling requires precautions to do it without introducing disturbing artifacts. In short, one needs to apply a 'low-pass filter' prior to actual downsampling, to avoid problems.
Low-pass filters come in many different varieties, and they are all trade-offs between avoiding artifacts, and retaining resolution. The original question above, can be answered by applying the Photoshop downsampling action to my test image as introduced on my webpage, mentioned above. I have to repeat a warning from my webpage, the downloaded test target (1000x1000 pixel size (1294 KB)) needs to be converted to RGB mode in Photoshop, before using it!
Applying Michael Fontana's action (the downsampling part) to the test image produces the following result:
.
To rephrase the actual question at hand; Is this result good (enough) or not?
The answer; Well, it could be better... As an example of what could be considered to be an optimal compromise between resolution and artifacts, here is the IMHO better result from the ImageMagick application (as mentioned on my webpage) with 'Sinc' Low-pass pre-filtering:
One may ask; In what sense it that a better result?
Okay, what we need to look at, is the central square area of 50% size (250x250 pixels). In that central 50% square we will find spatial frequencies that can be reliably reproduced, and anything outside that square area should ideally be featureless gray because it cannot be reliably reproduced (it will show various low spatial frequency aliases of the original frequencies). The first image shows several aliasing artifacts inside the area that should ideally be flawless. However, the artifacts are of relatively low modulation, so could be acceptable due to the simpler workflow which can be kept 100% inside Photoshop.
Given the fact that the task at hand, resampling to 50% size while avoiding aliasing artifacts, is pretty well defined, it may be possible to create 'a better mousetrap' while offering the workflow benefits of staying in Photoshop.
I'll give it a try myself, but I hereby also challenge other OPF members to improve on the action provided by Michael and provide a better 50% downsampling action.
Remember, the goal is to produce an optimal 50% downsampling action which retains spatial frequency integrity inside the central 250x250 pixel area, and as low as possible modulation of aliasing outside that area. Don't forget to convert the GIF image target to RGB mode before you let Photoshop do its/your thing!
A side effect of such a Photoshop action can be that it is universally applicable to downsampling, e.g. from original size down to Web publishing size.
Good luck,
Bart
In another thread I was asked a question, and I think the answer deserves a thread of its own. Understanding the issues explained in this thread also helps with finding solutions for other recurring downsampling procedures.
The question was: "A question to Bart and offcourse anybody else: Do you mind having a look at that "downrez from PhotoAcute-action? It's here
and used to automatate best the downrez of the 380 MB tiffs, being generated out of PhotoAcute's big DNGs (with 2 x size, aka superresolution)".
PhotoAcute allows to increase the single image resolution, by combining multiple under-sampled images. In that 'SuperResolution' procedure the resulting image is enlarged 2x (to ensure under-sampled images), with a higher resolution than would be possible from enlarging a single image. While that is useful for large format output, it also poses a burden for other uses of such images such as (panoramic) stitching. Hence the need for down-sampling to 50% of the ' SuperResolution' size.
As I've explained in one of my webpages, proper downsampling requires precautions to do it without introducing disturbing artifacts. In short, one needs to apply a 'low-pass filter' prior to actual downsampling, to avoid problems.
Low-pass filters come in many different varieties, and they are all trade-offs between avoiding artifacts, and retaining resolution. The original question above, can be answered by applying the Photoshop downsampling action to my test image as introduced on my webpage, mentioned above. I have to repeat a warning from my webpage, the downloaded test target (1000x1000 pixel size (1294 KB)) needs to be converted to RGB mode in Photoshop, before using it!
Applying Michael Fontana's action (the downsampling part) to the test image produces the following result:
To rephrase the actual question at hand; Is this result good (enough) or not?
The answer; Well, it could be better... As an example of what could be considered to be an optimal compromise between resolution and artifacts, here is the IMHO better result from the ImageMagick application (as mentioned on my webpage) with 'Sinc' Low-pass pre-filtering:
One may ask; In what sense it that a better result?
Okay, what we need to look at, is the central square area of 50% size (250x250 pixels). In that central 50% square we will find spatial frequencies that can be reliably reproduced, and anything outside that square area should ideally be featureless gray because it cannot be reliably reproduced (it will show various low spatial frequency aliases of the original frequencies). The first image shows several aliasing artifacts inside the area that should ideally be flawless. However, the artifacts are of relatively low modulation, so could be acceptable due to the simpler workflow which can be kept 100% inside Photoshop.
Given the fact that the task at hand, resampling to 50% size while avoiding aliasing artifacts, is pretty well defined, it may be possible to create 'a better mousetrap' while offering the workflow benefits of staying in Photoshop.
I'll give it a try myself, but I hereby also challenge other OPF members to improve on the action provided by Michael and provide a better 50% downsampling action.
Remember, the goal is to produce an optimal 50% downsampling action which retains spatial frequency integrity inside the central 250x250 pixel area, and as low as possible modulation of aliasing outside that area. Don't forget to convert the GIF image target to RGB mode before you let Photoshop do its/your thing!
A side effect of such a Photoshop action can be that it is universally applicable to downsampling, e.g. from original size down to Web publishing size.
Good luck,
Bart
