Programmatically change image resolution

Question

I have calculated that if I want my generated image to be A4 size @ 600dpi for print purpose, it needs to be 7016x4961px @ 72dpi. So, I generate it programmatically, then test it in Photoshop and it seems to be fine so if I resize it, it gets proper size and resolution

Image size dialog in Photoshop .

What I wonder about is if it's possible to make this resizing programmatically, preferably with PIL, but not necessarily with it. I need to make it higher DPI.

If you truly are generating it, for the love of flying spaghetti monster, generate it at the wanted resolution. If you truly need upscaling, http://stackoverflow.com/a/1750331/180174 :) — Kimvais, Feb 07 '12 at 10:15
Actually I am generating it with html5 canvas, then save with image2canvas from http://www.nihilogic.dk/labs/canvas2image/ so I can't control DPI there. Any ideas? — Sergei Basharov, Feb 07 '12 at 10:25
In your question, the "72 dpi" in confusing. If you mean (as I suppose) "How to set resolution of a 7016x4961 pixels image to 600dpi so it is printed on A4 ?" then you can do it with PIL as I explain in my answer. — MatthieuW, Feb 07 '12 at 12:37

score 37 · Accepted Answer · edited Oct 09 '17 at 20:43

37

If you have generated your image 7016 x 4961 px, it is already A4 at 600 dpi. So you don't need to resize it, you just have to set resolution information in file.

You can do it with PIL:

from PIL import Image

im = Image.open("test.png")
im.save("test-600.png", dpi=(600,600))

edited Oct 09 '17 at 20:43

Rishabh Agrahari

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answered Feb 07 '12 at 10:42

MatthieuW

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I don’t know why but it does not work with png images( – Tymophiyy Dec 23 '22 at 14:23

score 22 · Answer 2 · edited May 11 '12 at 02:25

22

This code will resize a PNG image into 7016x4961 with PIL:

size = 7016, 4961
im = Image.open("my_image.png")
im_resized = im.resize(size, Image.ANTIALIAS)
im_resized.save("my_image_resized.png", "PNG")

Perhaps a better approach would be to make your canvas x times bigger prior to printing, where x is a factor you have to figure out (7016x4961 in size for this particular image).

edited May 11 '12 at 02:25

Juuso Ohtonen

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answered Feb 07 '12 at 10:28

Chewie

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score 0 · Answer 3 · answered Feb 13 '23 at 14:41

Here's how you can resize by batch (per folder) and skip other file types and Mac system files like .DS_Store

from PIL import Image
import os

Image.MAX_IMAGE_PIXELS = None

path = "./*your-source-folder*"

resize_ratio = 2  # where 0.5 is half size, 2 is double size



def resize_aspect_fit():
    dirs = os.listdir(path)
    for item in dirs:
        print(item)
        if item == '.DS_Store':
            continue

        if item == 'Icon\r':
            continue
      
        if item.endswith(".mp4"):
            continue
        
        if item.endswith(".txt"):
            continue
        
        if item.endswith(".db"):
            continue

        if os.path.isfile(path+item):
            image = Image.open(path+item)
            file_path, extension = os.path.splitext(path+item)
            new_image_height = int(image.size[0] / (1/resize_ratio))
            new_image_length = int(image.size[1] / (1/resize_ratio))

            image = image.resize((new_image_height, new_image_length), Image.ANTIALIAS)
            
            image.save("./*your-output-folder*/" + item)


resize_aspect_fit()

Programmatically change image resolution

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