Cropping/Resizing/Scaling in photoshop without losing resolution

Started Aug 26, 2011 | Discussions
BlueTrain
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Cropping/Resizing/Scaling in photoshop without losing resolution
Aug 26, 2011

Hello,

I have a panoramic image dimensions are roughly 9500x6000 pixels.

I want to upload a version of this image to Flickr but not the original size (partly due to file size restrictions and partly because I dont like to have max resolution versions of my images on Flickr).

Is there a way to scale the image without losing to much of the quality?

When I use the Image > Resize > Image Size option and, say, resize to about 1500 pixels width, the image becomes blocky and horrible.

I presume this is because I am trying to cram too much information into a small space and so the resulting image can't cope. But I would've thought there is a way to do this without losing image quality - surely its best to have as much detail as possible in the original digital image so that when you come to edit/resize/crop/etc you dont lose too much detail. I seem to be losing more detail than when I resize a standard 4000x2500 image to 1500 pixel width!

WineO
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to BlueTrain, Aug 26, 2011

You do not say what software you are using to resize with and the aspect ratio of the image does not really seem to be a panorama but the size you are trying to downsize to should not be a problem if you are starting with a good quality image and using decent software to downsize. Are you viewing at 100% when looking at the downsized version?

If you don't have Photoshop try Faststone Photo Resizer a free download - just Google it.
Claude

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JJMacks
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to BlueTrain, Aug 26, 2011

Whenever you resize an image you loose some image quality in one way or an other.

If you look at the Image size dialog you will see there are two ways to resize. Resample an not resample.

When Resample is NOT checked not a single pixel changes. You will see the upper file size section gray out for nothing will change or can be chaned there. The only thing that changes is image resolution which determons print size as you increase resolution the image prints smaller and sharper and you decrease resolution the image prints larger and softer. If you have a lot of pixels and keep the resolution in a range above 200dpi the human eye is not going to notice a change for our eyese cam only resolve to around 300dpi up close.

When an image is resample the resulting image is a completely new image. Not a single original pixel remains. You either have more pixels or less pixels. If you have more pixels to hold detail this added detail has to be manufactured through interpolation. It not done with a bigger sensor with a better lens looking at the original seine. Its done by a computer the image quality will not be as good as if it would be if done with good optics and camera. If you reduce the number of pixels you have fewer to hold detail you will loose some. And again its done through interpolation. If your large image has been sharpened do not reduce its size using bicubic sharper use bicubic. How much detail you loose depends on how much your reducing the size. You wrote you loose more then you loose when you reduce other images to 1500 width. Yes going from 4000 to 1500 your loosing 62% of the pixels for details going from 9500 to 1500 your loosing 84% of the pixels for details. It like you 100 dollar bank account when you reduce it by 62% your left with 38 Dollars by 84% your left with 16
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joey_B
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to BlueTrain, Aug 26, 2011

When you resize you will not be able to view the picture as big as you would do the original. Nothing to be done about that, besides looking out for a host without a pixel limit...

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Allen Gerdes
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to BlueTrain, Aug 26, 2011

I have not tried it myself and there may be size limitations, but you might give the jpegmini tool discussed in this post a try:

http://forums.dpreview.com/forums/read.asp?forum=1006&message=39200001

Regards...Allen

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joey_B
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to Allen Gerdes, Aug 26, 2011

wouldn't make a difference... this is not about image quality, this is about trying to view a picture at a bigger scale than your monitors dot-pitch.

It's really quite simple. Your picture consists of a number of pixels. when you view the picuter 1:1, each pixel will be shown on one dot on your monitor. If you now zoom in, the PC can only show the pixels bigger, because there is no more information to show. The pixel will be interpolated, and will show up as a square 2x2, 3x3, 4x4 dots big and so on.

Bigger pictures can be viewed on a bigger part of a screen, and can be zoomed in more because they hold more data. If you scale down those pictures they will have less pixels, regardless if it is a TIFF, PSD, JPG, compressed or not...

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apaflo
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Re: Cropping/Resizing/Scaling in photoshop without losing resolution
In reply to JJMacks, Aug 26, 2011

JJMacks wrote:

Whenever you resize an image you loose some image quality in one way or an other.

If you look at the Image size dialog you will see there are two ways to resize. Resample an not resample.

The only way to resize an image is by resampling it. The question is just a matter of where the resampling takes place, not if it happens.

When Resample is NOT checked not a single pixel changes.

That is true of the image that is currently being worked on, and the one that would be save to file at that point. It is the image that will be sent to a printer or viewed on a computer monitor... where it will be resampled.

But both computer monitors and printers have fixed pixel per inch display rates, and if your 6000x4000 image is displayed in an 800x600 window on a monitor that displays at 96 PPI, the image is first resampled to be 800x600 pixels and only then is it displayed. Printers are not different! Most HP and Canon printers print at 300 PPI, and Epson printers at 360 PPI. If the print dimention requested for a 6000x4000 pixel image are 8x12 (which means the resolution has been set to 500 PPI) the print driver will resample the image data to 300 PPI (on a Canon printer for example), so that what is actually printed is a 3600x2400 pixel file, not the original 6000x4000 image.

Hence, if you change the resolution (the PPI value) the monitor will totally ignore it, and resample the image to fit the size of the display window. But print drivers use that value to determine the physical dimensions... and then resample to image to the correct size at the native resolution of the printer.

You can of course check the "Resample" box and have PhotoShop resample the image to start with... but be aware that unless the resolution is set to the native pixel per inch rate of the output device the image will still get resampled again. The significance is that if you want to preview what the resampled image actually looks like, for example in order to apply Sharpen or UnSharp Mask with critical precision, it is absolutely necessary to check "Resample" and set the PPI resolution to the appropriate value (the native PPI rate for the intended output device) in order to cause the image to be resampled before the edits are done.

[...] If you reduce the number of pixels you have fewer to hold detail you will loose some. And again its done through interpolation. If your large image has been sharpened do not reduce its size using bicubic sharper use bicubic. How much detail you loose depends on how much your reducing the size. You wrote you loose more then you loose when you reduce other images to 1500 width. Yes going from 4000 to 1500 your loosing 62% of the pixels for details going from 9500 to 1500 your loosing 84% of the pixels for details.

Keep in mind that if you view the image on a computer monitor at 1500 across, it necessarily has been resampled to that size . If you look at a 9000x6000 image or at a 1500x1000 image both of them are reduced to 1500x1000 for viewing. Hence if the second one was generated by resampling the first one, it should look exactly the same! If it doesn't, the resampling method is faulty. As noted, some algorithms are not as good as others. Photoshop uses various forms of BiCubic, which are not very good. (ImageMagick, as an example, defaults to Mitchell for upsizing and Lanczos for downsizing.)

A perhaps even more important lesson from the above is that unless viewed at 100% zoom it is impossible to judge the sharpness as it will be printed by displaying it on a computer monitor. The image is necessarily resampled for the computer display, and loses significant sharpness in the process.

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