Tag Archive for 'image'

A JPEG picture doesn’t care about no DPI

Published by Peter on April 21, 2008 in photography. 0 Comments

Every now and then I get a request to use pictures of mine for a poster, a flyer, a book. People usually find the pictures they want in my Flickr sets, where they are available in a max resolution of 1200 pixels for the longest side (so e.g. 1200 x 800 for photos in 3:2 aspect ratio). When this is not enough, people ask me for higher resolution versions. And that question comes in two versions:

  • the logical i’m-used-to-this-digital-stuff version: “could I get those pictures in 1800×1200” / “Can I have at least 2 megapixels“?
  • the weird I-used-to-work-in print version: “can you send them in 300dpi“?

DPI (dots-per-inch) only make sense for me if I would know on what size you want to print them. If you’re making an A4 flyer, that’s 8½ × 11 inch, and you need 300 dpi, then that means you need 3300×2550 pixels. If you want to print only an A6 size, that’s 1650×1275 pixels. So don’t tell me what DPI you need, tell me what pixel dimensions you need. Yes, you can save the DPI parameter in a JPEG file, but it changes nothing to the data. My 1200×800 picture with a DPI value of 72 or 300dpi is still, pixel by pixel, the same picture. Your image viewer might decide to show it as a bigger picture on the screen, but the image data is identical.
jpeg_dpi

Beginning to see the pattern? No matter what DPI you set (or, as it happens, what size in inches) so long as you don’t let Photoshop resample the image up or down, it’s STILL 504 by 144 pixels.

Let’s save this one as 12dpi.TIF before we forget, then have a look at the file sizes. You’d think that a 300dpi file would be higher resolution than a 12 dpi file, and because of that a lot bigger, right?

Sorry. All three files are exactly the same size.

(from pptfaq.com)

So, if you’re asking someone for a high-resolution version of a digital picture, ask for minimum pixel dimensions, not for DPI! Otherwise you just show that, while you might have experience in managing print, you have no clue how digital imagery works.

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What is HD-JPEG?

Published by Peter on October 8, 2007 in audio-video and photography. 3 Comments

While reading the specs of the PackardBell Store & Play, I fall upon “Foto formaten: JPEG, HD JPEG” As I am working with several aspects of HD (High Definition) video all the time, I am of course intrigued. There is a standard for HD JPEG? Wow, tell me more!

HD JPEG = HD Photo = JPEG XR (my guess)

Well, it’s not that easy, actually. When you do a search for “HD JPEG” on Google, all you see is promo talk. Vendors like Philips and Panasonic use it as a feature on their devices, but there is no official definition of what it stands for. What I think happened is:

  • Microsoft developed a standard “HD Photo” (formerly Windows Media Photo) that corrects some of the disadvantages of JPEG:
    • It can store color information at 32 bit per color instead of 8 or 12. (In current JPEG, “24-bit colour” means: 3 x 8 bits for R, G and B.)
    • All encoding steps of the algorithm are lossless (except for quantisation). So it has a true lossless mode (all coefficients = 1). JPEG does not have this (at least, not implemented).
    • All kinds of tech mumbo-jumbo reasons (read the Wikipedia article)
    • “HD Photo offers image quality comparable to JPEG-2000 with computational and memory performance more closely comparable to JPEG”
  • The JPEG committee announced that they would (consider to) adopt HD-Photo as a standard, with as a working name JPEG XR. Provided that, of course, …

    One important aspect regarding the standardization of HD Photo is Microsoft’s commitment to make its patents that are required to implement the specification available without charge. Microsoft’s royalty free commitment will help the JPEG committee foster widespread adoption of the specification and help ensure that it can be implemented by the widest possible audience. The JPEG committee hopes and encourages all participants in its meetings to consider this royalty free approach when offering patented technology as a candidate for standardization.

  • The hardware vendors didn’t like the “JPEG XR” name (it doesn’t really show clearly that it’s an upgrade from JPEG), so they combined it with an acronym that consumers know and associate with “better” and “more expensive”, so was born “HD JPEG”.
  • in short: Technically, it does not exist. Practically, it sells flat screens.

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