High-DPI displays and Linux

Posted Nov 12, 2014 16:23 UTC (Wed) by proski (subscriber, #104)
Parent article: High-DPI displays and Linux

Projectors were mentioned once in the story, which brings an interesting question. What is the DPI of a projector? Technically, it very high on the lens and very low on the screen. In practice, it should be something in between and it should vary dependent on the audience and the quality of the projector, the screen and the lighting.

Also, people with vision problems would probably want to see bigger fonts and pictures.

DPI is great for applications like on-screen rulers, but in most cases, scaling should be variable based on the user needs. DPI could provide a good default setting for newly connected hardware.

High-DPI displays and Linux

Posted Nov 12, 2014 17:54 UTC (Wed) by raven667 (subscriber, #5198) [Link]

> What is the DPI of a projector?

You can't get an exact answer on current equipment AFAIK, it is entirely dependent on how far you are projecting which determines the size of the resulting image, you should be able to guess an approximate range through based on how you expect a particular model to be used. To get a more specific answer you'd need a laser rangefinder or autofocus in the projector and send the calculated display geometry back to the computer, maybe you could even get away with instrumenting a manual focus.

High-DPI displays and Linux

Posted Nov 13, 2014 3:55 UTC (Thu) by flussence (guest, #85566) [Link] (3 responses)

Very good point, and a good reason why we shouldn't call it "DPI" at all, IMO. What we ought to be describing to the software is some number for perceived image clarity - the size of an individual pixel matters to that just as much as the user's eyesight or viewing conditions.

FWIW, I like E17's approach to this: it just presents a grid of scaling levels on first run, normalised around 1.0, and lets the user pick whatever's most comfortable. You have to go out of your way to ask for the DPI-based mode.

High-DPI displays and Linux

Posted Nov 13, 2014 13:19 UTC (Thu) by SLi (subscriber, #53131) [Link] (2 responses)

I think it should be called DPI, but it should be recognized that DPI is not the quantity we're ultimately interested in. Instead what is more interesting is the portion of the human field of view occupied.

For example, a comfortable human field of view is maybe around 155° horizontally and 135° vertically. If you fill this view with a line of 155 characters, each character has horizontal size of about 1°, and this is independent of whether you are talking about a monitor, a phone or a projector.

High-DPI displays and Linux

Posted Nov 13, 2014 18:13 UTC (Thu) by zev (subscriber, #88455) [Link] (1 responses)

Yes, dots-per-degree does seem like the more relevant unit -- though this also has the complication that most of the display devices we deal with are flat, not spheres centered on the viewer, so the number of field-of-view degrees per display-surface inch is different at the center of the display than at the edges. (Though I guess in hypothetical practice it probably wouldn't be enough difference to matter much, at least in "normal" cases.)

High-DPI displays and Linux

Posted Nov 13, 2014 19:28 UTC (Thu) by alexl (subscriber, #19068) [Link]

Nobody wants to specify actual measurements in their apps in units like "dots per degree", so instead what you use is something of the same kind of measure, but with a different scale. Its called a "reference pixel":

http://www.w3.org/TR/css3-values/#reference-pixel

The reference pixel is the visual angle of one pixel on a device with a
pixel density of 96dpi and a distance from the reader of an arm's length.
For a nominal arm's length of 28 inches, the visual angle is therefore
about 0.0213 degrees. For reading at arm's length, 1px thus corresponds to
about 0.26 mm (1/96 inch).

This is essentially what gnome uses. Following the recommendation above that link:

For lower-resolution devices, and devices with unusual viewing distances,
it is recommended instead that the anchor unit be the pixel unit. For
such devices it is recommended that the pixel unit refer to the whole
number of device pixels that best approximates the reference pixel.