Visual quality of keynote images--what is minimum for large auditorium?

Most projectors don't have resolutions higher than 1024x768. If you switch your display to 1024x768, and the presentation still looks good, it will very likely look good in the auditorium as well.

Pixel count is no more important on a big screen than on a small one. What matters is the angle subtended by each pixel at the viewer's eye. If the audience right up the front has to swing their heads from side to side, they are going to notice 😉. If the screen is small, and everyone is up the back of the hall, then 640*480 will look as good as 1920*1080. You will want to consider the brightness of the projector and the ambient room lighting too. There is no substitute for actual testing. If you are going to waste 400 person hours, you owe 'em a rehearsal. If you can't get the hall for a rehearsal, you need some elementary maths on similar triangles. Sitting 12 inches from a 24" screen is the same as sitting 12 feet from a 24 foot screen. Horrible at 800*600. Barely acceptable at 1600*1200. Sit 'em up the back!

The same applies to your DVD. It is sorta 768*576 give or take a letterbox and the odd rectangular pixel, but you can get away with poorer quality in a moving image.

On your next post, you are right. Up-resing a pixel starved image ends in tears, whatever the screen resolution. If it looks good at home, it will look good in the hall. But it won't, in either place.

Keynote usually does a pretty good job of upscaling images, but you might be able to do better by upscaling it yourself. It depends on the size and type of the original image, how much you're scaling it up, and what kind of upscaling algorithms are available in your non-Keynote tool. In most cases, I doubt the improvement will be worth the extra effort.

The best thing you can do is get better quality source images. Try to use vector-based graphics wherever possible.

See Wikipedia for a definition of vector graphics:

http://en.wikipedia.org/wiki/Vector_graphics

As an example, take a look at the elephant on page 3 of this document:

http://santos.cis.ksu.edu/schmidt/Escuela03/WSSA/talk1p.pdf

See how the image is all blocky and fuzzy? That's a raster image that has been upscaled (made bigger).

Now look at the elephant in this document:

http://dk77.com/vecteezy/elephant.eps

See how much clearer and cleaner that looks? That's because it's a vector image. You can make it as big as you want without losing any quality.

As for "non-Keynote tool", I was being a little too vague. I just meant a graphics manipulation program that is able to upscale images. For example, GraphicConverter lets you choose from a variety of different algorithms for the upscaling: bicubic, b-spline, Lanczos 3, and others, some of which may produce slightly better results than Keynote's algorithm. You can also use the program to manually edit the image to clean up the artifacts that result from upscaling. But again, this is a time-consuming and tedious task that is probably not going to be worth it for your situation.

The critical issue for you to figure out is: what is the resolution of the projectors in the auditorium. That's what will limit what you can do.

The old standard was 800x600 and that's not good for an images where you want any real detail.

The new, relatively safe minimum standard size seems to be 1024x768. I'm currently doing all of my presentations at this size and have not run into any problems anywhere, yet.

As for DVD playback quality, that all depends on how big the screen is (and/or how many screens you have -- some places will run two or more screens).

As for the size of the images to use, I always make them at least as big as the resolution that I use (i.e., 1024x768). I do screenshots at the exact resolution. Upscaling images pretty much always leads to ugliness -- it's much better to have images that are too big that you crop and/or shrink to size.

I completely disagree with what Elliott said about "horrible" and "barely acceptable". If your images are already 800x600, they should look perfectly fine in a Keynote presentation. Remember that most projector resolutions don't go past 1024x768, so a 1600x1200 image will actually have to be downscaled, meaning all those extra pixels will just go to waste!

You should only be concerned if your images are small, say 320x200 or less, and you are making them fill the screen. Even then, a small image will still look fine if it stays small on your Keynote slide (in the corner instead of full screen).

DVD resolutions are usually 720x480.

As for books or other tutorials on this topic, there's not that much to say. "Higher resolution images look better than lower ones" is all the advice you really need. The minimum acceptable resolution for your situation depends on the resolution of the source, the resolution of the display, the size of the image on the slide, and your personal definition of "acceptable". It's not a cut-and-dry thing you can write down in a book.

Also, the real problem here is that you're taking images off the Internet. Use your own sources and the whole issue becomes moot, since you'll have the original high-res image.

Sorry about the delay. I've been away. Everyone seems to have missed the 'similar triangles' test I mentioned. If you sit really close to a large 800*600 screen it does look horrible. If you are miles away, so you can't see the pixels it looks as good as a higher resolution screen of the same size. You can test how your projection will look if you know the screen size and how far away the audience will be sitting. A useful first stab is to call feet in the auditorium inches in front of your computer. e.g. If there is an 8 ft screen and the audience is sitting 15 feet away, test with an 8 inch picture with the right number of pixels with your nose 15 inches from your computer screen. You probably can't easily adjust the pixels per inch, but you can get the same result by sitting closer or further away and doing the arithmetic.
I did a show last weekend with a 1920*1080 projector shining on a tiny 6 foot screen. You couldn't see the pixels from 3 feet away. It would have been just as good at 800*600 because the nearest viewer was about 15 ft from it. The venue would not let me use my own 10 ft screen. My high res projector was wasted.
I messed up with your dvd dimensions. 720*480 is all you get in California. Moving images need far fewer pixels per foot than detailed diagrams with lots of text. Go with your gut feel on it. If a DVD looks ok at home on a 30 inch screen when you are sitting 10 feet away, it will look as good on a 300 inch screen when you are sitting 100 feet away if the room is dark enough and the projector is bright enough.

You could do worse than Wikipedia for more references. Use "display resolution" or "home cinema" or "video projector" as a starting point.