How to sharpen an image in gimp

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How to Change the Resolution of an Image with GIMP

Image editing programs such as GIMP (GNU Image Manipulation Program) can provide you with resolution information and allow you to change the resolution of an image. One would change the resolution of an image especially for printing purposes, because the quality of the print depends on the resolution of the image.


1. With GIMP open, go to File > Open and select an image

Tip: You can right-click and save the tiger image below to use as a practice image.




2. Go to Image > Print Size



3. A Set Print Image Resolution dialog box will pop up as shown below.

If the print size width and height are not displayed in inches, select the drop-down menu next to height and select “inches”.



4. In the X and Y Resolution fields, enter the desired resolution.

You will notice that when you enter a value in the Resolution field, the width and height values ​​of the document also change. This is because GIMP only changes the resolution of the image and does not add extra pixels (which happens when you resize an image). To change the resolution, we do NOT change the number of pixels in the photo, only the number of those pixels that will be displayed per inch.



5. Click OK to accept the changes.

Congratulations! You have successfully changed the resolution of an image! 

In this example, we had an image with a resolution of 300ppi. I wanted to print this image in a professional publication and the image had to be at least 600 dpi. 

Remember that the number of pixels in the image remained the same because we didn’t add or subtract pixels from the image, we just determined how many of those pixels to display per inch.

Note however that the width and height were halved when the resolutions were doubled. This means that to print at 600 dpi and maintain the best quality, I can only print this image at a size of 5″ x 3.33″.


What happens if we lower our resolve?


As you might have guessed, our width and height doubled when we halved our resolutions. Now my image will print larger, but the quality will be much lower.


What does all this mean?

It’s a game of give and take!

On remarque que les dimensions des pixels ne changent jamais. Nous commençons avec une image de 3000 x 2000 pixels (px) et terminons avec des dimensions de 3000 x 2000 px. Il est important de s’en souvenir, car lorsque nous modifions la résolution, nous ne modifions que le nombre de pixels qui seront affichés par pouce de l’image, et non le nombre de pixels qui composent l’image. Étant donné que nous n’ajoutons ni ne soustrayons aucune information (pixels) de notre image, notre image doit toujours être équilibrée avec ses 3000 x 2000 pixels d’origine. Si nous augmentons la résolution, alors nous devons diminuer de quelque part ! Étant donné que les dimensions en pixels ne peuvent pas changer, le seul autre endroit où diminuer est la taille de notre document (largeur et hauteur de l’image).

Here’s the math to make it clearer:  sample image is 3000 x 2000 px

600ppi : 3000 pixels / 600 pixels per inch = 5 inches

                  2000 pixels / 600 pixels per inch = 3.33 inches  

300ppi:  3000 pixels / 300 pixels per inch = 10 inches

                 2000 pixels/300 pixels per inch = 6.667 inches 150ppi:  3000 pixels/150 pixels per inch = 20 inches

                 2000 pixels / 150 pixels per inch = 13.33 inches


72ppi:   3000 pixels / 72 pixels per inch = 41.67 inches

                2000 pixels / 72 pixels per inch = 27.78 inches


How does resolution affect printing?

In this example, our image was sent to print from a laser printer on standard 8.5 x 11 sheets of paper.


72 dpi:  The document size is too large to fit on an 8.5 x 11 sheet and is cropped. Print quality is extremely poor, leaving the image very fuzzy or “soft”. 

150 dpi:  The document size is still too large for the 8.5 x 11 sheet and the quality is poor, making the image acceptable, but not very desirable.

300 dpi:  The image fills almost the entire sheet of paper and the print quality is very good, with crisp, sharp detail.

600ppi –  The image is significantly smaller than other files, but the quality is extremely high.

Which one to choose ? 72ppi and 150ppi images are too low quality to produce a high quality print, so they are rejected. The 300ppi and 600ppi images looked very sharp, but the 600ppi image was too small. For this example, the 300 dpi image would work best due to its larger print size and high quality.

The key to printing images is finding the best resolution that will produce both the size and quality you need. 


Video How to sharpen an image in gimp

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