> If your image is 240px * 240px at 240dpi, and you change the resolution in 
> photoshop to 72dpi, the image is still 240px * 240px and the quality isn't 
> changed. It is just a reference for if the image is to be printed.

Assuming you are talking about the menu option [Image > Image Size…], that 
sounds like pretty dangerous advice, it really depends on the settings used in 
Photoshop, if you have the 'Resample' option checked it will reduce your image 
to 72px x 72px and lose a great deal of data.

> If you're using an image onscreen you can ignore the dpi, it will only ever 
> display the number of pixels.

DPI (dots per inch) is the output device resolution, PPI (pixels per inch) is 
the image resolution, they will have an optimum relationship depending on the 
output device and the final quality required. Unfortunately DPI is often used 
in the wrong context and it can cause confusion.

DPI vs PPI terminology is explained here…

https://forums.adobe.com/thread/370714

> which begs the question: when saving for retina 2048 X1536: what PPI should 
> we choose on save?

PPI is not really relevant to LC, but the pixel dimensions of your image are. 
If you have an image that is 240px X 240px at 72ppi and place it on a card in 
LC, assuming you don't resize it, it will be 240 pixels by 240 pixels, the same 
image at 240ppi will still be 240 pixels by 240 pixels on the card.

Try it and you may understand better, make 2 images in Photoshop, both 240px 
square and set one to 72ppi and the other one to 240ppi then place them side by 
side in LiveCode, they will both import at the same size when placed on the 
card.

So if your image was to take up 70% of the screen area then an image of 1434px 
x 1075px would be correct. LC places images at 1:1 pixel size so just choose a 
resolution that you are comfortable with and stick to it, it won't make any 
difference to LC and is not important unless you intend scaling images or 
producing stacks that print high resolution images, then you just need to do 
the maths.

If you are making re-sizeable stacks for different devices, then you may need 
to either work to the highest resolution device and allow the images to scale 
down, or use the system described here…

http://lessons.runrev.com/m/15262/l/156530-how-do-i-support-different-device-screen-densities
(Especially note the paragraph titled Example towards the end of the article).

The benefit of this approach is that you can ignore "Retina" and design for 
standard screen sizes and the high-res stuff takes care of itself.

HTH

Paul
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