The Web is never concerned with the (inches, centimeters, ppi) at all, only ever the pixel dimensions. An image with 1200 pixels in each axis will appear exactly the same (all other things being equal and not specified) if it is set to have a 144ppi header or a 36ppi header. Only Photoshop and various DTP applications care at all about the ppi header. Printers care about the ppi, but screens generally do not.
The “high dpi” trick with Web pages works like this: you set the width and height of the image in HTML or CSS code, disregarding the original pixel dimensions of the actual file referenced. The browser reads the width and height you have specified and sets the display box accordingly, and squeezes the image on screen to fit that box.
<img src="1200x1200.jpg" width="600" height="600" alt="Retina" />
A “retina” screen has twice the physical pixels in each dimension as a normal screen, and when viewing normal low-resolution Web images, just “up-rezzes” the original image to fit the new pixel grid, effectively making four (slightly mushy, fake) pixels out of one actual source pixel. On a 2x or higher-resolution source image, it doesn’t need to do this magic, so the final image is higher quality (since those aren’t four fake pixels, but rather four actual source pixels, which can be and often are different from one another, and therefore convey more image data).
On a low resolution screen, the browser does the same sort of thing, but actually averages those four source pixels to make one display pixel on screen. This sort of averaging is the same sort of thing that happens in Photoshop (or Freeway) when you convert a large original image into a Web-friendly low-res version.
Walter
On Aug 13, 2013, at 1:01 PM, Gordon Low wrote:
Thanks Walter, I hope you’ll forgive my slowness. Are double-size images at 72ppi and normal size images at 144ppi one and the same? The knowledge-base talks about using the former but your method seems better to me as I can keep the physical sizes the same.
All the best
Gordon
http://www.gordonlow.net/
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