Why is the same image being scaled differently by photoshop and Gimp

Question

Heyo, I'm learning web optimization. I needed to scale a large image down from over a thousand pixels... ...to 51x72 pixels. I used GIMP, and simply changed the resolution to 72 ppi (naturally, no effect) and the size to height 72 (with the width being constrained to 51 as a result; height was the limiting factor) using the Image>Scale Image Tool: It looks excessively blurry. My friend did it in photoshop, saying he just did "Resize Image" or such: I'm not a graphics designer, and have only worked tangentially with such graphics concepts, however image optimization is important for web development, so it would help a lot if I had insight on why these two images are so radically different. I tried Cubic and Linear interpolation as well, to no effect (I'm guessing those are matrix functions that determine the processing of the image whilst scaling, I have no idea otherwise). Is it likely I'm using the wrong tool? Or is it simply that Photoshop costs $$ thus will produce better results? Thank you in advance for any guesses/answers. I understand if it's too vague to answer.

Answer 1

All image scaling introduces some blur since pixels are interpolated. You can mitigate this by sharpening the image after the scaling. For instance, just using Gimp Filters>Enhance>Sharpen (unsharp mask) with default values:

Answer 2

Ok so scaling is a signal processing thing. Conceptually it works as follows*

1. Discrete image is converted into a continuous image. This is called rebuilding it is done by convolving the discrete signal by a continuous function.

   Here you can see two different examples.

   

2. Then the signal is re sampled into a new image, Although the scaling up is shown you can sample any which direction you want.

   

Now the convolution shape affects how you image looks. And there are many many convolution shapes to choose from. The shape you choose is a trade-off between having a more blurry image and having a sharper image (also called ringing). Sometimes you want one over the other which is why you can choose differently in most editors.

* although there are methods that don't work this way, most processes can be reduced to the basic design of this form. Although steps 1 and two are usually baked into one to save time.