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Sath
Member
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# Posted: 4 Feb 06 12:20
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Hi, I'm new to posting pictures on the internet and I was wondering if anyone could help me out with this technical problem I've got...
Whenever I upload a picture onto the site I can see a significant difference in image quality (colour changes, less detail in the shadows,...) between the posted and the original pic.
Is there a way of reducing this quality deterioration?
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frnz64
Member
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# Posted: 4 Feb 06 13:12
Reply
I'm not shure, but you should try to download one of your images and then reopen it with your favourite photo editor. Than look at the differences between the original and the copy. This because sometime is just a question of the software tool you're using.
let us know ;)
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Sath
Member
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# Posted: 4 Feb 06 14:13
Reply
I downloaded a picture and compared it with its original and it turns out that there isn't any difference in colour or contrast, but there is in the sharpness. I also saw that the 'site picture' is a lot smaller (kb-wise) and this is probably the result of a compression that happens when the pic is uploaded.
Less kb >>> less sharpness. So far everything makes sense...
But I've also got some pics with a colour-space different from sRGB. Adobe RGB for instance, and those pictures (when uploaded) DO display the symptoms that I discribed in my first post.
Does anyone know why this is the case?
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Chris Ayriss
Member
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# Posted: 4 Feb 06 16:09
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Sath
I noticed this, but it was after the server went offline a number of weeks ago. I think they recompressed the images, and is noticeable on photo's with large blocks of colour but I haven't noticed any change in the 8Bit colour space used for the web.
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joris
Member
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# Posted: 5 Feb 06 11:02
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when your pic is larger than 1280 x 1024 pixels we compress it....
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Sath
Member
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# Posted: 5 Feb 06 21:54
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Joris,
I don't want to come across as a wise-guy but I think you are confusing 'compressing' with 'resizing'
The first has to do exclusively with with the amount of bits - a compressed image has the same dimensions as the original - while the second affects both dimensions and amount of kb (the image looks smaller on your screen).
For example : I uploaded a pic of dimensions 830x580 and is 370 kb
The uploaded version of the original pic has the same dimensions, but is only 80kb.
So it seems that you do compress EVERY picture, even when it doesn't exceed the limitations.
Chris,
thanks for answering my question.
If I'm not mistaken you are saying that there is such a thing as a standard colour space for internet pictures (sRGB). And so I should therefore only post picture with that colour profile...
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Wilfred
Member
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# Posted: 6 Feb 06 09:31
Reply
But I've also got some pics with a colour-space different from sRGB. Adobe RGB for instance, and those pictures (when uploaded) DO display the symptoms that I discribed in my first post.
Does anyone know why this is the case?
Yes, you should convert any image which is not in sRGB to sRGB before you post it. Some photo-editing programs include a 'save for web' option which does this automatically. Yet, even when converting the images or saving for web I do always notice a slight colour shift when it is displayed on Woophy. This may have to do with the (absence of) colour management in your browser.
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joris
Member
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# Posted: 6 Feb 06 09:56
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Sath I send your question to our technical man and he will tell you what exactly happens with your picture....
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Sath
Member
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# Posted: 6 Feb 06 21:54
Reply
Thanks Joris.
Thanks Wilfred
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Marcel
Admin
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# Posted: 9 Feb 06 18:47
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Hi Sath,
This is what happens to your pics:
The original is copied to 3 different sizes, each with a jpg compression of 10 (quality 90%). We had to draw the line, and 10 seemed acceptable, balancing the quality against the file size.
The compression has an effect on the quality:
-overall lightening of the image.
-less saturation (especially with images with a lot of points of different color within a few pixels of each other)
I hope this answers your questions.
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Wilfred
Member
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# Posted: 9 Feb 06 21:39
Reply
@Marcel,
That sounds strange, I mean, that the JPEG compression has influence on lightness and saturation. If I compress an image in Photoshop, the overall colours don't change - even at the strongest JPEG compression only the level of detail decreases, and artefacts may occur. It almost seems you are using some algorithm from a programming library, which assumes a particular colour space that is even narrower than sRGB.
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Marcel
Admin
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# Posted: 10 Feb 06 11:23
Reply
Yes, the lightness is caused by the image library we use (php gd library if you want to know), but the loss of saturation is a result of how jpg compression works. This can vary depending on the algorithms used, but it happens in photoshop too.
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