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Thread: Need Alpha Channel Support in PNG images

  1. #11
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    This transparency issue has always been bothering me, but this is the first
    time i see it stated that conclusive.
    .png is my main storage format and i was encouraged to use irview for the
    extra pngout-plugin.

    I think most users aren't even aware of the loss that's happening to them.
    An example: if you use the 'select color' option to define transparency
    you might even get a result being only marginally different from the original,
    on a closer look however you can make out jagged borders and the lost
    smoothness.

    I consider this flaw in transparency handling quite severe, rather a bug than
    a feature request.

    irview is great, i'd rather not have to give up on it.

  2. #12
    IV Beginner TheOS's Avatar
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    I concur, but hopefully Irfan will implement support in a general way. Last time I ran into this limitation I tried to convert a png image into a Windows icon.

    Being a programmer myself I have to remark that how a user sees a program and how something is actually implemented are two (sometimes very) different things. In this case "preserving" the alpha can create complications, but not in the way a user might expect. As Midora remarked there are different ways to store the alpha information. pre-multiplied alpha is a great way to quickly show images, but are not so great for editing, especially when combining multiple images (or layers). Some editing commands are easy to extend with alpha support (e.g. cropping) others commands become complex (e.g. color reduction). This means that IrfanView has to work internally with a "neutral" alpha channel format suitable for editing. Then when reading a file convert to this internal format and on writing convert to the alpha format (if any) supported by the output format. All the plug-ins have to be examined by either Irfan and/or the original authors to make this work. Definitely not something I would do on a single Saturday afternoon.

  3. #13
    Moderator Sam_Zen's Avatar
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    Quote Originally Posted by TheOS
    how a user sees a program and how something is actually implemented
    Right on. A user states 'this must be easy-does-it' while the programmer knows better, even with apparent simple things.
    Your explanation makes this quite clear.
    Quote Originally Posted by Flit
    on a closer look however you can make out jagged borders and the lost smoothness
    Why 'rather a bug'? It just does what the command says : select pixels with exactly the same color.
    So, with e.g. a photo, this will not work correctly because of slightly different colors at edges of transition.
    It could be possible with a more 'graphical' image, but even then. Color reduction if acceptable should be first.
    0.6180339887
    Rest In Peace, Sam!

  4. #14
    Moderator Enterprise User Bhikkhu Pesala's Avatar
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  5. #15
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    great...thank you

  6. #16
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    Default full alpha channel transparency. (+mp3 mpg pdf)

    full alpha channel transparency
    i'm surprised that i couldn't found this with search expelicitly asked in a purpose made thread (was mentioned a very few times), i was expecting tons of such ones, and that it isent even in the site faq (!), i thougt of making one...

    somewhere i read recently that its because iv uses bmp (dib) as an internal format. why dont u change it into tga? ppl says its simplier anyway than bmp, 1ot simplest bitmap formats internally; while bmp has its quirks.

    would this be a much horrid rework job? if not too much... we'd love it... its the ONLY missing from iv lol. imo. well, ok, apng would be nice natively. but who cares absolutely not that important. and mng-crap is (maybe more than) enough via a plugin. hm... maybe improved svg support. also not imptt @all, just if we are at missing stuff.

    ok, and maybe the option to disable certain formats opened when browsing through directories with next and prev buttons... 'd be much useful to dont have mp3s and mpegs pdfs et c. got opened, only when we weant them.
    Last edited by Bhikkhu Pesala; 07.03.2012 at 04:43 PM.

  7. #17
    Moderator Enterprise User Bhikkhu Pesala's Avatar
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    Quote Originally Posted by aaa3 View Post
    ok, and maybe the option to disable certain formats opened when browsing through directories with next and prev buttons... 'd be much useful to dont have mp3s and mpegs pdfs et c. got opened, only when we weant them.
    Already possible. Check out the options available in Properties, Extensions.

  8. #18
    Plugin Author MItaly's Avatar
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    Quote Originally Posted by aaa3 View Post
    somewhere i read recently that its because iv uses bmp (dib) as an internal format.
    DIBs theorically do not support alpha transparency, however they do support 32bpp bit depth, even if in the BITMAPINFOHEADER documentation the high byte of each DWORD is specified to be "not used". Still, it can be used internally to store the alpha value; moreover, the use of that byte as alpha value is well accepted, also because it's used by the AlphaBlend and TransparentBlt API functions. So, there's no reason to switch to TGA, which by the way would require the rewrite of a vast part of IV and of the plugins. Still, I don't know if IV and the various plugins are prepared to work a 32 bit DIB; moreover, the AlphaBlend and TransparentBlt functions are present from Windows 2000 onward, and Irfan still wants to mantain compatibility with the Windows 9x family.

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  9. #19
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    thank you all for your replies.
    i see now...

    (but... maybe... maintaining a separate 9x version which would only be different in this? ok it starts to feel too much i know, was just an idea and nvm

    or... in a single version, having by default the old behaviour, and an option to enable rgba with a notifying text that dont do this if ure on 9x... again, i dont feel like pushing this too hard just another thought)

  10. #20

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    Have been trying to resize some PNGs with an alpha channel, only to find out it loses the alpha channel.

    A quick look in google and I found this thread from 2 years ago raising this issue. Is this something that is going to be resolved in IrfanView?

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