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I have taken a picture whose resolution is less. I can resize the image easily (i.e I can scale to higher resolution) but the image quality goes for a toss, which is true because the image might not have enough pixels to scale properly.

  1. Is resizing a means of increasing the resolution of the image ?

  2. Is it possible to increase the pixel density of the image and then resize, so that
    quality of the image is not affected much?

I mainly use GIMP and Picasa.

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1  
The following may be helpful. I used photoshop in the examples, however GIMP should also work fine. photo.stackexchange.com/questions/1715/…. To summarize the link...yes, I think it is possible to increase the size of an image and minimize the loss in quality, however you can't maintain all original detail. –  jrista Jun 7 '11 at 17:27
    
so you want to magically create data out of thin air to show things that were there in reality but aren't in the picture because your resolution wasn't high enough. Isn't going to work. At most you can interpolate between existing pixels and add more that have an average value of those around them (possibly weighted in some way). That gives the illusion of increased resolution when in fact it's not, it's just more pixels showing the exact same thing. –  jwenting Jun 9 '11 at 7:50

6 Answers 6

up vote 9 down vote accepted

In general it is not possible to increase the size/resolution/pixel density of an image after it has been captured. If the detail was never present it can't be replaced.

There are ways to increase the number of pixels whilst minimizing artifacts (an example being fractal based image resizing). These methods are useful when you need to print large without seeing pixel artifacts. But the results are nothing like what you'd get with an image that was higher resolution to start with.

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"There are ways to increase the number of pixels whilst minimizing artifacts" to a degree of course :) –  jwenting Jun 8 '11 at 5:11

No, not really. If we could, camera would not be increasing in megapixels all the time.

You need more data captured to show or print something bigger.

Scaling algorithms have sophisticated ways of inventing pixels, called interpolation, but without more data, those added pixels cannot increase image details. Clever algorithms may look slightly better than dump ones by adding pixels with more local contrast but that is it.

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Is resizing a means to increasing the resolution of the image ?

Resizing can increase pixel count if you want, but in general it cannot increase the amount of detail. Though in some situations upscaling is necessary (for example, if you want to combine two images). Use Sinc (Lanczos3) interpolation when upscaling the image in Gimp.

Even more, resizing usually leads to loss of image sharpness even if you downsize the image. Some resizing algorithms are better at keeping the image sharp, but are more likely to produce resizing artifacts. Usually, you would need to sharpen (e.g. apply unsharp mask) the image after the final resize. Cubic interpolation is usually working well for downscaling.

Is it possible to increase the pixel density of the image and then resize, so that quality of the image is not affected much?

I don't understand what you mean here.

If you mean pixel density as points per inch, and resize as an operation to change the number of pixels in the image, then you can change the pixel density freely (it affects only the size and the quality of the print), but it doesn't affect the quality of the resized image (which depends mostly on what algorithm you use to resize, whether you upscale or downscale, and how much you resize).

However, it is possible to increase the amount of detail and resolution of the image, if you have a stack of almost identical images with small shifts with respect one another. There is a technique known as super resolution. It is pretty advanced, and, as far as I know, it is not yet available in popular photographic software. But there are quite a lot of papers about it and research-grade code which can do it.

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Yes, it's possible to upsize an image. The most common application used for such work is Genuine Fractals (which I see has now been renamed to Perfect Resize). It can be used as a standalone application or as a Photoshop plugin. The software will enlarge and then interpolate to fill in the "missing" pixels. The company has some complex fractal algorithms that work pretty well. They offer a free trial (Windows or OS X).

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Review/comparison by Ctein at The Online Photographer: Part 1, Part 2, Part 3. (Summary — it works for some images, with caveats.) –  mattdm Jun 7 '11 at 16:38

It's not quite clear from your question what you mean by "resolution", as the term is often used to describe two not quite related things.

  1. Image size in pixels - i.e. 3000x2000 pixels. This parameter describes how much information an image can contain - i.e. the above image has 6 million physical pixels and there is no way to recover any details which were smaller than a single pixel when the image was shot. There are algorithms (using fractals, wavelets etc.) which may trick the eye to make the image look like it has additional details but it's not possible to recover the actual information. Obviously, this trickery only works for a limited range of up-scaling. So, for example, if you shot an image at 640x480 physical pixels, there's absolutely no way to make it look as if it was shot at 5000x3000 pixels.

  2. Image resolution in pixels per inch (PPI) or dots per inch (DPI). This parameter is just a number recorded somewhere in image's metadata which controls how the image should be scaled when reproduced on a physical medium (paper or screen). As a result, it controls the size of an image when printed (or even, I'd say, the default size as you can easily change the image's dimensions in a publishing program, for example).

Examples: a 5000x3000 image printed at 300 DPI will be 17x10". If some other image's metadata says its resolution is 100 DPI, it means nothing as long as its size in pixels is the same - you still can print it at 17x10" with the same amount of details.

Similarly, a 640x480 pixels image can be printed at 6.4x4.8", which gives us effective resolution of 100dpi. The larger, 5000x3000 image at 100 DPI can be blown up to 50x30" while preserving a comparable amount of detail/sharpness.

My point is: if you're concerned about your image's resolution-in-dpi being low - don't be, it doesn't matter at all, only the image size in pixels matters. Any image resampling in this case will result in deterioration of quality.

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Try using photoshop you can increase resolution there...that may help

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Can you explain how on might do that, beyond the simple resize which @sat notes in the question doesn't give very good image quality? –  mattdm Jun 7 '11 at 17:28
3  
Hi @Caroline, and welcome to PhotoSE. A quick word of advice regarding votes here. This community highly values rich, informative, and factual answers. I think the reason for the down votes on your answer are due to the terse answer that lacks any details about how one might increase resolution without much loss in detail with photoshop. Can you expand on your answer and offer more details about how one might increase resolution while maintaining as much quality as possible? –  jrista Jun 9 '11 at 3:13

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