Photo Compression and re-sizing

In reply to

www.irishfishkeepers.com/cms/component/o...limitstart,50/#88888

(as I don't want to interrupt voting).

I have the same problems, John, when I have images sent to me for publishing. Some have been re-sized from a ‘photobucket’ or similar storage system. In our photo contest of last year, the web-master receiving the images re-sized them before sending to myself and other judges….. all 16-bit TIFF images from the Canon and Nikon pro-cameras where ringing with artefacts because of the re-sizing.

I noticed a Gibbs phenomena in some of the pictures here that were probably originally from higher resolution cameras; or cases of maze moiré in cameras of lower resolution.
They can make the images look a bit ‘not good’.
Without going into the technical side of this, a high resolution image would need heavy compression to make it smaller. If that compression is, say, JPEG then it would have had specific subsampling applied (4:2:0 and its particular problems) and would maybe lack high-frequency chromanance.
If that compressed image is re-sized and pixel-ratio are not maintained or if there is a loss of 8x8 pixel units anywhere within the entire image then the ‘discrete cosine transform’ will cause a ‘ringing’ (Gibbs Phenomena) within the area of lost high-frequencies. Eg a chrominance may be lost and the algorithm to show the image will simply fill it in with something else (=could be square blocks of a general colour of white minus one or 2 CYMK colours within the region of edges).
Another problem is when the spatial frequency of the object or image if greater than the resolution frequency of the image or imaging capabilities, but that is more likely to cause maze moiré effects.

And...although a tad boring (unless one is into fourier transform mathematics I'm nerdy, so I am), digital images can be a major source of disappointment and there are more reasons to mosquito noise than ringing.
Ever tried watching an old non-HD source football game on a 1080p plasma screen? A bit duff.

The corrected images now look good, John.

ian

When we're all using 40" monitors we can start thinking about non compressed digital images in raw format.

One limitation is the 499 size limit of images on the forum whiich is the main drawback to the photo competition, thankfully postimage will let you upload a 4000x3600 (example res) image but the thumbnail representation of 499 is not great but at least you can then click it and open it at full resolution though you will have to scroll to see it all unless you have the above mentioned screen size that can match the resolution of the photo.

If the 499 size limit were increased on the forum for posting images will indeed look better, they will be larger so it is a balance. The resize by the host like photobucket is a problem but postimage will let you upload huge shots without resizing, if you want to see it all at one on your 22" screen in a program like MS image viewer, that program will resize it to fit your screen.

Also, resizing to different aspect ratios than that of which the photo was taken in without cropping will distort the picture as well.


Being nerdy about it, more bits per pixel are needed.


Mark

John, the maths behind photography in general is pretty complex to say the least.
I did one of my degrees in Optical and Imaging Physics (I'm not just a pretty face and a toxicologist ya know), and the principles of all them years ago still hold true today with modern digital cameras and fourier transform maths.

There is nothing wrong with JPEG.... it's horses for courses, and its just a case of knowing the limitations.

@mark.... if I'm doing any serious work on photos, I use a 52 inch monitor so that I can see what is on screen. My laptop is rubbish, but it does a convenient job. I also tend to use the image graphs for doing work rather rely on my poor eye-sight to spot things in the image.

Pixel ratio is important.... and loss of a pixel is a problem as the discrete cosine transform of jpeg codec requires every original 8x8 pixel area to be in-place in order to deconvolute the data ie the full image must be made up of a integral number of 8x8 pixels. (a bit like the data requirements in the convolution and deconvolution of ultrasound scans and laser holograms etc)

I wouldn't use the photobucket system for anything other than storing images, and even re-sizing in photoshop can introduce problems if not done properly.

I suppose the biggest problem of RAW is the shear size of the images, and the fact that camera manufacturers have made the RAW formats non-standard. (I believe Nikon still hold it as illegal to view a Nikon RAW format on non-nikon software).
Some of the images that I deal with for publishing are from 50megabytes to a gigabyte in size.... I couldn't send them via e-mail.
As a photographer, I need something that will print like a film camera and not just look good on-screen.

If people use Photoshop for generating jpeg images for the net, then I recommend NOT firstly trying to get the file-size low, but to reduce the image size using the image re-sizer. Then after that to go to 'Save for Web...' to get the right file size. It may take several attempts to tweek the photos on first try, but if a large RAW or TIFF image at 16 bits and at maximum resolution from the camera is simply shrunk to a minimal file size then the quality will suffer (and it may even turn out worse than a mobile phone picture of similar file size).
If the 'image size' is set too low at first then artifacts will soon show as well.

And different forums tend to have different settings on image-placers.
Here, for example, the engine tends to default to showing images much smaller than we see on some of the, say, SMF forum engines for the same image-bucket source.

I haven't tried the other image-bucket system, Mark. Maybe I'll give it a review when my head is not feeling all snotted up.

ian

igmillichip wrote:

John, the maths behind photography in general is pretty complex to say the least.
I did one of my degrees in Optical and Imaging Physics (I'm not just a pretty face and a toxicologist ya know), and the principles of all them years ago still hold true today with modern digital cameras and fourier transform maths.

There is nothing wrong with JPEG.... it's horses for courses, and its just a case of knowing the limitations.

@mark.... if I'm doing any serious work on photos, I use a 52 inch monitor so that I can see what is on screen. My laptop is rubbish, but it does a convenient job. I also tend to use the image graphs for doing work rather rely on my poor eye-sight to spot things in the image.

Pixel ratio is important.... and loss of a pixel is a problem as the discrete cosine transform of jpeg codec requires every original 8x8 pixel area to be in-place in order to deconvolute the data ie the full image must be made up of a integral number of 8x8 pixels. (a bit like the data requirements in the convolution and deconvolution of ultrasound scans and laser holograms etc)

I wouldn't use the photobucket system for anything other than storing images, and even re-sizing in photoshop can introduce problems if not done properly.

I suppose the biggest problem of RAW is the shear size of the images, and the fact that camera manufacturers have made the RAW formats non-standard. (I believe Nikon still hold it as illegal to view a Nikon RAW format on non-nikon software).
Some of the images that I deal with for publishing are from 50megabytes to a gigabyte in size.... I couldn't send them via e-mail.
As a photographer, I need something that will print like a film camera and not just look good on-screen.

If people use Photoshop for generating jpeg images for the net, then I recommend NOT firstly trying to get the file-size low, but to reduce the image size using the image re-sizer. Then after that to go to 'Save for Web...' to get the right file size. It may take several attempts to tweek the photos on first try, but if a large RAW or TIFF image at 16 bits and at maximum resolution from the camera is simply shrunk to a minimal file size then the quality will suffer (and it may even turn out worse than a mobile phone picture of similar file size).
If the 'image size' is set too low at first then artifacts will soon show as well.

And different forums tend to have different settings on image-placers.
Here, for example, the engine tends to default to showing images much smaller than we see on some of the, say, SMF forum engines for the same image-bucket source.

I haven't tried the other image-bucket system, Mark. Maybe I'll give it a review when my head is not feeling all snotted up.

ian

Certainly know yer stuff matey when it comes to the photography. I am familiar with imaging not photography, have no idea what most settings are on my camera, lumix fz45, my first non phone camera:)


WHen I develope a raw image without compression it is about 4368x3660, not exact guesstimate, and I can upload this without compressing, but like I said once you open it, you need a monster screen, 52" would be nice:) What's the max res on that 52" monster?


MArk

Lumix are good cameras and have a good movie function from what I see, I'm not too sure from first-hand experience of the image inside one of them and what type of files they produce.

What dpi if printing (ppi if viewing) are the files? 72? 150? 300?

My monitor is a HD TV that works well as a monitor for us old uns.... but I'm used to working with graphs and getting contrast info etc from a graph. As for what resolution it is? I haven't the faintest I don't read the specification pages too much.
NTL HD channels and Blu-Ray looks fantastic on it.