ACDSee Pro 2 and 2.5

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JPEG Options

(8 posts)

  • RobotDude
    Member

    I have been looking at the JPEG Options and have come to the conclusion to use:

    Image quality: 85% - as there is no noticable loss in the image quality but the file size is noticably smaller

    Encoding - Progressive: Not ticked - as this refers to how the image is downloaded on web sites
                    - Optimize Huffman codes: Ticked - recommended by many sources on the web

    Color component sampling - 2:1 Horizontal - ADVICE NEEDED!
                                                - 2:1 Vertical - ADVICE NEEDED!

    Unfortunately I can find no advice on Color component sampling in the ACDSee help or on the web.  Does anyone know what this does and what effect they have on the image?  The ACDSee default has 2:1 Horizontal ticked but not 2:1 Vertical - why?  A smaller file size results when the color component sampling options are used.

    FYI: The ACDSee help states: "2:1 Horizontal: Select to subsample color channels on the horizontal axis by a factor of 2:1." and similar for 2:1 Vertical, which I don't find very helpful!

    Posted On December 16, 2008 - 08:19 AM (11 months ago) (Permalink to this post)

  • Sam Dring
    Moderator

    I think it is something to do with Chroma Subsampling.  Google has a number of references but the subject stretches my limitations!

    Posted On December 16, 2008 - 09:01 AM (11 months ago) (Permalink to this post)

  • ctmk
    Member

    Color component sampling - 2:1 Horizontal
                                                - 2:1 Vertical

    This 2 features above degenerate the color quality to save space. This is not recommended for high quality JPGs. And the the space saved is can be neglected.

    Color component sampling 2:1 Horizontal - interpolate 2 color pixel Horizontally.
    Color component sampling 2:1 Vertical - interpolate 2 color pixel Vertically.

    And i think this won't affect the luminance of the image.

    Note: Color(chroma) Black&White(luminance)

    in pratical use, i actually use it to make JPG that looks more blur in color. (For avatars)

     

    In final words, you tick them to reduce the image quality to save space.

    Posted On December 16, 2008 - 07:14 PM (11 months ago) (Permalink to this post)

  • ctmk
    Member

    more about color sub sampling

    http://en.wikipedia.org/wiki/Chroma_subsampling

    Posted On December 16, 2008 - 07:16 PM (11 months ago) (Permalink to this post)

  • John Radcliffe
    Member

    I must confess that I have always just gone with the ACDSee defaults for everything other than the quality % setting (85% seems good to me, and gives me an edited image file that is typically about the same size or slightly larger than my original Jpeg). I've simply assumed that the people who designed the program understand the issues involved better than I do, and so came up with sensible defaults.

    Having read a Wikipedia article, my opinion is unchanged.

    That article said that:

    (1) The human visual system is more sensitive to variations in brightness than to colour, which is the idea behind sampling colour at a lower resolution than luminance.

    That suggests to me that it might be better to use one or both of the "Colour component sampling" options rather than to decrease the quality setting (i.e. to compress more) in order to achieve a required or acceptable image file size.

    When I tested this on a randomly selected "typical" image, the image file for the version with "2:1 Horizontal" de-selected was about 15% larger. To get the same image size I needed to reduce the "quality" setting down from 85% to 79%. The big question then was how any improvement in colour reproduction compared to any notice increase in compression artefacts. Well I did notice that in some places the colour did appear smoother, and compression artefacts were indeed slightly worse, but this was only noticeable when viewing the image on-screen at 100% or more. I suspect that in ordinary usage (full image on screen or printed to A4) I'm not going to notice any worthwhile difference.

    (2) It says that most commonly 2:1 sampling is used both horizontally and vertically. It didn't mention using just 2:1 Vertical, so my guess is that the eye may notice colour differences more easily vertically than horizontally. (From this I would deduce, rightly or wrongly, that one should go for both, neither or horizontal only, but not vertical only.)

    So in conclusion I think I'll just continue to go with the defaults, as being a sensible compromise.

    (And I still continue to be amazed at just how good Jpeg compression is!)

    Posted On December 17, 2008 - 03:17 PM (11 months ago) (Permalink to this post)

  • ctmk
    Member

    yup, but you will see differences if you are working with sharper CGs or resterized vector images. you see almost no differences in live image from cameras.

    Posted On December 18, 2008 - 09:57 AM (11 months ago) (Permalink to this post)

  • John Radcliffe
    Member

    I wouldn't want anyone to misunderstand me. I'm not suggesting that my conclusion is the definitive answer for everyone. Different people's needs (or wants) will differ.

    My advice in such situations would always be: Try it for yourself, and see what works best for you. But I'd also suggest that people shouldn't automatically assume that they always "need" the highest quality settings. Often they won't. It is usually the case with photography that image quality has to be set against some cost, whether in practicality, time, money, or any combination of these. (And typically the "law of diminishing returns" will also apply.)

    Posted On December 18, 2008 - 02:13 PM (11 months ago) (Permalink to this post)

  • antoinebrisebard
    Member

    John Radcliffe said:I must confess that I have always just gone with the ACDSee defaults for everything other than the quality % setting (85% seems good to me, and gives me an edited image file that is typically about the same size or slightly larger than my original Jpeg). I've simply assumed that the people who designed the program understand the issues involved better than I do, and so came up with sensible defaults.

    +1. 85% jpeq-quality, w/o checking color resampling gives top of the notch results. changing to 90 or 95%, jpg-filesize increases heavily (!, non-linear) w/o giving any visual quality increase.

    Posted On December 18, 2008 - 07:10 PM (11 months ago) (Permalink to this post)

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