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vox
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A recent mention of image quality on the Calendar topic prompted this.

I've thought before about posting it somewhere.

Correct me if I'm wrong but:

It seems to me that the meaning of High Res. is not fully understood by some.

I say this purely in the interest of preserving our images in the best possible way.

This is how I understand it to work.

(Hopefully, someone who knows more about it will either agree, correct a few details, add more information or tell me to go back to school.) :)

Many images posted on here seem to be scanned at 75 ppi or there abouts.

If you scan at 75 pixels the image will appear ok on screen at it's original size.

(75 pix is about what the human eye can discern I believe.)

However if you need to magnify the image for closer inspection, it will pixelate and become fuzzy. This becomes evident when trying to make out a road sign or similar from an old postcard. It's also evident when scanning newsprint or handwriting.

Similarly, increasing the print size of an image can result in a similar situation if it was scanned at a resolution as low as 75 ppi.

It's better, I think, to scan at 200 or 300 pixels. If this results in an image which is too large in file size to post, it can be converted to a smaller file size with jpeg compression without loosing pixel detail.

Jpeg compression works partly by reducing from the 16million original colour shades, in barely discernible steps, untill the best compromise is reached between size and truenes of colour .

In short, in almost all instances, high resolution (high number of pixels to preserve detail) and high compression (to reduce file size) is better than Low resolution and low compression.

I'll do a few examples to illustrate what I mean at some point soon.

As I said before I'm open to being corrected, and welcome anyone's more informed opinion.

.

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I always reduce the dpi of my photographs that I post (scanned or from the camera)

a 256 MB sized image will load onto screen a lot faster than say a 1GB image.

And I will only post up large high resolution images if there is need for it.

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I always reduce the dpi of my photographs that I post (scanned or from the camera)

a 256 MB sized image will load onto screen a lot faster than say a 1GB image.

And I will only post up large high resolution images if there is need for it.

How do you do that Steve?

The site has a maximum image size of 2MB anyway.

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I always reduce the dpi of my photographs that I post (scanned or from the camera)

a 256 MB sized image will load onto screen a lot faster than say a 1GB image.

And I will only post up large high resolution images if there is need for it.

But that's better achieved by compression, not by resolution.

A 200 dpi image at say 10 meg will (Jpeg) compress to 1 or 2 meg or less without any major effect at all.

The pixel detail is retained.

A 6" x 4" postcard for instance, at 75 ppi will only display clearly at 1 to 1. Ie. full size.

If you want to increase to Full Screen or more, in order to see more detail, it will pixelate.

That is to say, if you zoom to 200% you are only viewing at 37.5 pixels per inch.

I think this should illustrate what I mean.

A 100, 200, and 300 ppi version of the same picture.

All the same dimensions and the 200 and 300 versions are compressed to approximately the same file size as the 100.

Now try saving them to your computer and zooming to 2, or 3 times.

The low res one pixelates and becomes fuzzy much earlier than the others.

100

200

300

.

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There seems to be a little confusion between ppi and dpi. I think that vox is talking about the ppi that the scanner is set to--this affects how much of the fine detail the scanner will pick up. On the other hand SteveHB is talking about dpi, which is an instruction to the device displaying the image telling it how to display the image. The dpi of your computer monitor is fixed (usually 72 or 96 dpi), so for on-screen viewing dpi is irrelevant. What really matters for on-screen viewing is the absolute number of pixels--more being better, though I would avoid compressing the jpgs too much (say more than 50%) as it can make the images fuzzy.

Jeremy

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There seems to be a little confusion between ppi and dpi. I think that vox is talking about the ppi that the scanner is set to--this affects how much of the fine detail the scanner will pick up. On the other hand SteveHB is talking about dpi, which is an instruction to the device displaying the image telling it how to display the image. The dpi of your computer monitor is fixed (usually 72 or 96 dpi), so for on-screen viewing dpi is irrelevant. What really matters for on-screen viewing is the absolute number of pixels--more being better, though I would avoid compressing the jpgs too much (say more than 50%) as it can make the images fuzzy.

Jeremy

Thanks Jeremy that's a better description,

I often find that thechnical stuff is difficult for me to explain.

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There seems to be a little confusion between ppi and dpi. I think that vox is talking about the ppi that the scanner is set to--this affects how much of the fine detail the scanner will pick up.

Jeremy

Yes Jeremy - pixels. (I've never quite got to the bottom of the difference between dots and pixels.) lol

A lot of the scans that appear on here are only 72 pixels per inch. Like this for instance.

Fine at 1 to 1 viewing, but very blurred at anything larger.

I agree that too much compression is not advisable but, (depending on the complexity of the image) quite modest compression will usually give a very usable result which covers all bases, so to speak. I think my three "quick-pix" of the knife blade go someway to illustrating this.

As I understand it jpeg works this way. (Very simply put)

An uncompressed image with say, a block of 10 red pixels and then 10 blue pixels in a particular position, will need to store 20 bits of information.

Jpeg Compression would read the first red pixel, and when it sees that they're all the same untill it comes to the blue section. Just records 1 red pixel and some information to say "copy this 9 times. And so on across.

This is obviously more complicated when it comes to matching "similar" but not identical colours is concerned. That's where the compression ratio comes into play.

As I said. I'm willing "and anxious" to learn more from anyone who can enlighten me further.

.

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dpi vs. ppi

A digital image is made up of pixels. At the time of digitization the scanner (or camera, but I'll just talk about scanners) converts an analogue image into an array of pixels. At this point ppi (pixels per inch) refers to the density of those pixels. So if I have a 6 X 4 inch postcard and I scan it at 100 ppi I will end up with an image that is 600 X 400 pixels. If I scan the same postcard at 200 ppi I will end up with an image that is 1200 X 800 pixels. Obviously getting more pixels from the same postcard means that each pixel represents a smaller portion of the postcard, so more fine details are captured in the resulting image.

My monitor displays pixels at 96 ppi, but for the sake of ease lets pretend that it displays at 100 ppi. So, if I take the image scanned at 100 ppi and display it at 100% on my monitor I will see a 6 X 4 inch image (the same size as the original because the input and output resolution are matched). On the other hand, if I take the image scanned at 200 ppi, that is 1200 X 800 pixels, my monitor will display this at 100 ppi so at 100% I will see an image that is 12 X 8 inches.

The scanner, when it scans the image embeds within the file various pieces of information about the image (called metadata). One of these is the ppi that the scanner was set to. Looking at the image information in most image editing programs will show you this information some call it ppi some call it dpi, photoshop just calls it resolution. This means that when I print the image, the printer can read this information to help it make a print that is about the same size as the original postacard. Printers don't make pixels, they make dots, and unlike a monitor the output resolution of a printer is not fixed. So when I print, the resolution is referred to as dpi (dots per inch). If I take the image that I scanned at 100 ppi and print it at 100% size, the printer will print at 100 dpi and the resulting print should be 6 X 4 inches. Likewise, if I take the image that I scanned at 200 ppi and print it at 100% size, the printer will print at 200 dpi, the resulting print should be 6 X 4 inches.

In photoshop I can change the ppi/dpi. If I take the 200 ppi image and change it to 100 ppi I have two options. 1) I could maintain output size at 6 X 4 inches, in which case photoshop will reduce the number of pixels from 1200 X 800 to 600 X 400 (downsampling). 2) I could maintain the number of pixels, in which case all photoshop does is change the image metadata. Now, the original 200 ppi file will still have 1200 X 800 pixels and the new 100 ppi file will still have 1200 X 800 pixels, so when displayed on my computer monitor at 100% they will be exactly the same size because my monitor is fixed at 100 ppi. However, if I print them the original 200 ppi image will still print at 200 dpi and so be the same size, but the one that I changed to 100 ppi will now print at 12 X 8 inches.

I think that what SteveHB is doing when he says that he reduced the dpi of his images is option 1, which results in downsampling of the image. The reason the resulting image has a smaller file size is not because the output resolution was changed, but that in doing so he reduced the number of pixels in the image. This is why I say that the most important thing to look at once the file has been created is the pixel dimensions and not the ppi. Vox is right though that when scanning ppi is important because the input resolution affects how much detail is recorded in the file.

The manufacturers of some scanners confuse things a little, because they allow you to set a zoom level. My scanner does this. If I scan the 6 X 4 postcard at 200 ppi I get an image that is 1200 X 800 pixels and the metadata tells the printer to output at 200 dpi to get the original size. However, if I scan at 200 ppi, but tell the scanner to do a zoom of 2X, the resulting image is 2400 X 1600 pixels, but the stored resolution is still 200 ppi, so the printer printing at 100% will give me an image that is 12 X 8 inches. I think that this is confusing, because when I set the 2X zoom, my scanner really scanned the image at 400 ppi and then changed the stored resolution to 200 ppi in the same way that I described in option 2 above.

This explanation that turned out longer than I initially intended, so I hope that it simplifies rather than confuses more. In short, resolution (ppi or dpi) is important when taking something from the real world and scanning it onto your computer, and when talking something from your computer and recreating it in the real world as a print, but whilst the file is actually on your computer (including being displayed on your monitor) the important thing is the actual total number of pixels that you have.

Jeremy

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jpg compression:

The way that Vox described jpg compression is essentially correct. However, you are describing what is known as lossless compression--in the case you describe, no information is lost by the compression, so it would be easy when decompressing the file to exactly reproduce the number and position of the pixels. The jpg compression algorithm is actually a lossy compression--information is lost during compression and cannot be recovered on decompression. I won't pretend to even slightly understand the exact details of the jpg compression algorithm, but basically it looks for information in the image that can be thrown away without affecting image quality. The more you compress, the more the algorithm throws away. This works fine for pictures with very few areas that are blocks of colours, for example most photographs, but it is particularly bad for line art with text such as maps. Although I generally upload maps to this site as jpgs, it would actually be better to use a different format such as gif or png.

Because jpg compression is lossy, every time you open, change, and then resave the file more information is lost and the file can degrade. For this reason, it is best to keep the image in a lossless format like tiff until you have finished editing it and only then save it as a jpg for web uploading.

Jeremy

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