Downsampling and stair interpolation

These are pixel-greedy days – the more pixels your DSLR has the better. Hardly anyone disputes that. But when it comes to showcasing ones work on the Internet the number of pixels has to be reduced, either to protect copyrighted work, for the sake of reducing file size, or just to be able to display the whole image on screen. Actually thinking about it I display my pictures on screen much more than on paper.

Image resampling is focused primarily on upscaling, e.g. increasing the number of pixels in an image to prepare it for printing, especially when making larger prints. Various resampling algorithms have been devised for blowing up images. Here I make an attempt to compare some of these methods, when reducing the size of the image.

Downsampling inevitably wipes out some of the detail – there are simply less pixels to store the data. Also due to the way interpolation works some degree of softness is introduced to the image when downsampling. Sharpness can be brought back by applying some amount of unsharp masking AFTER the image has been downsampled.

One of the things I’ve payed special attention to when looking at the results from down sampling is if the given method manages to preserve fine detail. For example the detail textures small patterns etc. The move to try different interpolation methods for down sampling was a method called stair interpolation. The basic idea behind this method is to perform the dawnsampling in incremental steps (hence stair interpolation), each step preceded by applying some USM. In theory applying USM before down sampling should fightback some of the softness introduced. Reading through some blogs there was a fair amount of discussion if this works, is useless or even degrades the quality of the resulting image.

Speaking of USM – sharpness is largely a thing of personal taste. Yet for pleasing results, edge halos and aliasing must be avoided.

I also found some references that many interpolation algorithms behave similarly at particular resampling ratios, like 50%, one third etc. This may be so but my tests did not reveal anything outrageous, so resample at whatever steps you want to, I guess.

In the test results I’ve also included crops from the originals so one would be able to judge what to expect from the crops. First example shot with Canon 50mm f 1.4 on tripod, second with Canon 17-40 f4 L on tripod, third and fourth Canon 100mm f2.8 macro hand held. All mounted on a 20D.

The last 3 step stair interpolation is intentionally quite over sharpened to show the perils of pushing USM way too far.

All in all I believe stair interpolation has potential to bring up really fine detail to downsampled images. It is interesting to note that the amount of sharpening needed varies hugely depending on the subject matter. For instance the second example shows some really ugly artifacts even at fair sharpening applied while the third example shows pleasing detail on the girl’s eyelashes at severe amounts of USM. If you are really mad about detail, you could do selective sharpening at each step and bring detail only where you need it. You might also need special medical help 🙂

Happy downsampling !

References

  • controlledvocabulary.com
  • americaswonderlands.com
  • contaxg.com
  • photo.net
  • prophotohome.com

    Landscape/architecture

  • method result
    original
    bell – irfan view 3.99
    BSpline – irfan view 3.99
    Hermite – irfan view 3.99
    Lancoz – irfan view 3.99
    Mitchell – irfan view 3.99
    Triangle – irfan view 3.99
    Bicubic – Photoshop CS2
    Bicubic Sharper – Photoshop CS2
    2 step stair bicubic (USM: 130/0.8/5, 140/0.7/5) – Photoshop CS2
    2 step stair bicubic (USM: 160/1/5, 140/0.7/5) – Photoshop CS2
    3 step stair bicubic (160/1/5, 150/0.8/4, 130/0,6/4) – Photoshop CS2

  • Landscape

  • method result
    original
    bell – irfan view 3.99
    BSpline – irfan view 3.99
    Hermite – irfan view 3.99
    Lancoz – irfan view 3.99
    Mitchell – irfan view 3.99
    Triangle – irfan view 3.99
    Bicubic – Photoshop CS2
    Bicubic Sharper – Photoshop CS2
    2 step stair bicubic (USM: 130/0.8/5, 140/0.7/5) – Photoshop CS2
    2 step stair bicubic (USM: 160/1/5, 140/0.7/5) – Photoshop CS2
    3 step stair bicubic (160/1/5, 150/0.8/4, 130/0,6/4) – Photoshop CS2

  • Portrait

  • method result
    original
    bell – irfan view 3.99
    BSpline – irfan view 3.99
    Hermite – irfan view 3.99
    Lancoz – irfan view 3.99
    Mitchell – irfan view 3.99
    Triangle – irfan view 3.99
    Bicubic – Photoshop CS2
    Bicubic Sharper – Photoshop CS2
    2 step stair bicubic (USM: 130/0.8/5, 140/0.7/5) – Photoshop CS2
    2 step stair bicubic (USM: 160/1/5, 140/0.7/5) – Photoshop CS2
    3 step stair bicubic (160/1/5, 150/0.8/4, 130/0,6/4) – Photoshop CS2

  • Landscape/architecture

  • method result
    original
    bell – irfan view 3.99
    BSpline – irfan view 3.99
    Hermite – irfan view 3.99
    Lancoz – irfan view 3.99
    Mitchell – irfan view 3.99
    Triangle – irfan view 3.99
    Bicubic – Photoshop CS2
    Bicubic Sharper – Photoshop CS2
    2 step stair bicubic (USM: 130/0.8/5, 140/0.7/5) – Photoshop CS2
    2 step stair bicubic (USM: 160/1/5, 140/0.7/5) – Photoshop CS2
    3 step stair bicubic (160/1/5, 150/0.8/4, 130/0,6/4) – Photoshop CS2

    Camera profiles in Adobe Lightroom

    This simple test tries to illustrate the diffrences between the camera profiles avalable in Lightroom 2.2, in particular the profiles for Canon 20D. I was intereseted particulary in the differences that the profiles give to skin tones. To test a picked an image with pleasing skin tones (at least according to me). The image is taken with the magnificent EF 100mm Canon prime macro using available light. The images produced are out of the box – no tinkering done with any of the sliders within the develop module. I guess results can vary vastly from image to image, depending on factors like color balance, camera, etc. Anyhow here is what I got :

    adobe standard
    Adobe standard
    camera faithful
    Camera faithful
    camera landscape
    Camera landscape
    camera neutral
    Camera neutral
    camera portrait
    Camera portrait
    camera standard
    Camera standard


    P.S. This is really very unscientific. Not trying to prove anything here, test is done primarily for fun.

    Canon EF 100-300 f5.6 L

    First the facts from the Canon literature:

    Marketed June 1987
    Original Price 93,800 yen
    Lens Construction (group) 10
    Lens Construction (element) 15
    No. of Diaphragm Blades 8
    Minimum Aperture 32
    Closest Focusing Distance (m) 1.5
    Maximum Magnifcation (x) 0.26
    Filter Diameter (mm) 58
    Maximum Diameter x Length (mm) 75 x 166.6
    Weight (g) 695

    High-performance 3x telephoto zoom lens. Synthetic fluorite in group 1 and UD glass in group 2 achieve low refraction and low dispersion. They also effectively correct chromatic and other aberrations. Resolution is high throughout all focal lengths. Images are sharp and crisp. At the 300mm focal length, 0.25x magnification is possible.

    I did not know this lens existed when I got a chance to shoot a bit with it. Of course I was immediately attracted to the L designation. Zooming is done like the EF 100-400L monster, but here there are no lock rings. This means the lens creeps at the slightest tilting. This lens has a new definition of slow focusing for me… it takes ages to focus. The noise produced when focusing reminds me that of a plane retracting its landing gear.

    The test is made in a real world situation. The realworldness comes from the fact that the shots were taken raw and then processed to tiff with DPP 2.0.3.7 with its default params, meaning some sharpening has been applied, but anyway that’s most people’s workflow. All shots were taken with a 20D, mounted on a sturdy Manfrotto tripod using cable release and mirror lockup. Here is a quick set of tests and comparisons against the cheapo EF 100-300 f4.5-5.6.

    I won’t comment on the results presented here. It’s up to you to interpret the images.

    I apologize for my stupidity of not taking shots of the same part of the building with the two lenses.

    Center performance at 100mm

    F number 100-300 f5.6 L 100-300 f4.5-5.6
    f 5.6
    f 8
    f 16

    Corner performance at 100mm

    F number 100-300 f5.6 L 100-300 f4.5-5.6
    f 5.6
    f 8
    f 16

    Center performance at 300mm

    F number 100-300 f5.6 L 100-300 f4.5-5.6
    f 5.6
    f 8
    f 11
    f 16

    Corner performance at 300mm

    F number 100-300 f5.6 L 100-300 f4.5-5.6
    f 5.6
    f 8
    f 11
    f 16

    Изложба фотографии от Мароко и България

    Посолството на Кралство Мароко организира фотографска изложба с участието на български и марокански фотографи в рамките на честването на Националния празник на страната, от 25 до 29 юни 2008 г. Изложбата се помещава в залата на Съюза на българските архитекти, който по този начин сътрудничи на посолството. Повече на страницата на европа 2001

    Имаше десетина от моите снимки (още можете да видите в моята портфолио галерия от Мароко), и по още толкова снимки на Константин Буюклиев и Константин Иванов. Трябваше много набързо сами да си рамкираме снимките с каквото разполагаше ‘дом на архитекта’ – стари и мръсни, отвратително изглеждащи, криви рамки. Не че снимките бяха толкова свят, ама направо ни беше срам да си слагаме ‘творбите’ в тези рамки. Слава богу организаторите бяха достатъчно ‘мили’ да ни спестят неудобството, като дори да не ни споменаха имената на откриването. Хубава работа, ама Мароканска 🙂

    За сведение ‘Дом на Архитекта’ не става за фото изложби – стените са покрити с лакирани дървени плоскости и цветната температура вътре е сигурно 2000 келвина. Края на юни месец и температурата на въздуха вътре е горе-долу толкова. Истинска мароканска атмосфера!