How can I compress in terms of pixels not file size?

Started 2 months ago | Questions thread
NAwlins Contrarian Veteran Member • Posts: 5,938
Very specific suggestion
4

Is there a (free and hopefully not too complex) way to compress an image from 'a' x 'b' pixels to 'c' x 'd' pixels?

Context:

I'm trying to take a high-res satellite picture of Earth at 2500x1187px, and lower its resolution by a factor of 1.6 in each dimension.

Satelite images have what is called spatial resolution meaning that every pixel in the image represents some distance on the ground, so a really good camera might have spatial resolution of 1m, and a bad camera might have 3000m. The image I have has spatial resolution of 10m, but the camera that my project's satellite is using will have a spatial resolution of 16m. Hence the need to adapt the original image to a lower resolution so it can better represent what our satellite's images might look like.

So I need a way to change the image dimensions to:

2500/1.6 = 1563px width

1187/1.6 = 742px height

As others have said, there are a bunch of pieces of free software that can do this, and a bunch of different ways to do it. Given that maybe you're a newbie and need specific directions instead of general suggestions, I'll be specific with directions for one of the IMO better ones, at least without getting too complicated:

1. Go to the official GIMP downloads page (https://www.gimp.org/downloads/) and download it.

2. Install GIMP.

3. Run GIMP.

4. Do a File -> Open... and find the file you want to work on. Here's one at 2500x1187 pixels:

5. Do an Image -> Scale Image ..., then in the dialog box that appears, next to Width, change the 2500 to 1563, from the Interpolation drop-down choose NoHalo, then click the Scale button.

6. Do a File -> Export As..., by Name type in whatever file name you want (maybe just keep the original and add -rev. or sometime like that), click Export, and then for some file types you may want to adjust Quality (larger number means less risk of artifacts but bigger files sizes--I would not go below 85% unless you really need small files), then click the second (i.e., newer dialog box) Export button.

Your as-reduced file should be in the directory by the original file, under the new name you chose. If you download the files above, you'll see the proper sizes.

By the way, this process is not (here at least) typically called "compression", with "resampling" being maybe the most usual term.

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