How to increase image resolution?

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I need to obtain a print quality image from an existing image. The image dimensions are 1024 x 768 pixels, and the print size is 12 inches x 8 inches.

The current resolution is 64 x 64 pixels per inch (ppi).

The menu options Image | Print Size | X resolution and Image | Print Size | Y resolution allow me to change the ppi setting.

When I change the ppi to 300 x 300, the print size of the image shrinks to 3.4 x 2.5 inches. I want to keep this at 12 x 8 inches.

How can I keep the same print size (inches) but a higher ppi?

abhi

Posted 2014-08-20T12:55:18.837

Reputation: 277

You can't get reasonable quality, much less print quality, taking a 64x64 image and increasing the size to 1024x768. Even an increase to 300x300 will have distortion. – Tyson – 2014-08-20T13:03:39.637

At this point, I am ok with the distortion. The image is a charcoal sketch. I think I can probably fill in the gaps by applying black color. – abhi – 2014-08-20T13:05:37.207

464x64 = 4096 pixels. 300x300 = 90,000 pixels. 1024x768 = 786,432 pixels. Stretching a 64x64 image to 300x300 means there is enough original information for 4.5% of the pixels the other 96.5% of the picture has to be "guessed". Increase the size to 1024x768 and that means the original image makes up 0.5% of the new picture and 99.5% of image would be "guessed". It just doesn't work. – Tyson – 2014-08-20T13:18:37.860

1It's like creating energy out of nothing. If you'd succeed, you will be the hero – Lorenz Meyer – 2014-08-20T13:57:29.207

1Normal interpolation to increase image resolution basically just ensures that you don't get "blocky" results. However, it's a bit like looking at a photograph under magnification, everything is just spread out and softened. You might want to investigate fractal enlargement. It can sometimes do a better job at preserving edges and sharpness. – fixer1234 – 2016-01-26T17:39:17.170

Answers

11

I am assuming you mean the current DPI is 64ppi x 64 ppi. You want GIMP to add pixels to the 1024x768 image without changing the size in inches. The only way I know to do this is in two steps, both of which use Image | Scale Image.

  1. In Image | Scale Image, set Interpolation to None and change the X and Y resolution. You will see that the Image Size in pixels doesn't change. Hit Scale, and the image will have the new resolution.

  2. Go back into Image | Scale Image. Set Interpolation to Cubic. This time, change the Image Size to be what you want in inches or cm. GIMP will compute the resulting pixel resolution and resample the image.

This way you don't have to manually convert between inches, dpi, and pixels. Hope this helps!

cxw

Posted 2014-08-20T12:55:18.837

Reputation: 1 389

Thanks for replying. I will try that soon. In the meanwhile I used a tool called SmillaEnlarger to do this. This is available on sourceforge.net – abhi – 2014-08-20T20:01:27.637

Here for those interested. – cxw – 2014-08-20T20:05:42.660

Asked: 2014-08-20T12:55:18.837

Viewed: 56 455 times

Active: 2019-10-16T20:00:59.943