"Usable" screen resolution

MrSpeed wrote on 5/24/2004, 7:45 AM
I created some still photos for a slide show and made them 480 pixels high. I placed a white border around them to make them look like traditional photos kind of like a polaroid.

It seems like on my TV the top and bottom of the still image is getting clipped by about 20 pixels top and bottom. My white border is not visible on the photos on the TV

Is this normal ? Is is a screenblast thing or a DVD NTSC thing ?
Do the sides get clipped as well ?


-Rich

Comments

Chienworks wrote on 5/24/2004, 8:42 AM
This is a television thing and it is completely normal. The image on the television screen is slightly larger than the opening in front through which you can see the picture. This is done for a few reasons, one of which is that the image tends to shrink when the television gets older so this way the picture can still fill the screen. Another is that the edges of a broadcast image tend to be wavy and fuzzy so it's nice to have them hidden.

The term for this is overscan. It varies from TV to TV, and will even change depending on how old the TV is, how long it's been on today, how bright the image is, the temperature of the room, the voltage coming in from the power company, what other electronic devices you have on in the room, how loud you have the volume control on the TV set, what you had for breakfast, the alignment of the north star over the giant pyramid at Giza, and the color of your cat's retina. (OK, maybe not so much the last three, but i bet they do have some very very slight effect!) Some TVs cut of parts of the image on all four sides, some more on one side than the other, some not much at all, and some even show the complete edge in some places.

Since it varies so much you can't do anything with your video to always make your image look exactly the way you want it to. Don't even bother trying. The average amount is about 10% lost on all four sides. What most folks try to do is make sure that the picture is big enough to fill the whole frame but keep anything important away from the edges so it won't get cut off. If you have a 4x6" photograph fill the frame then chances are that about 1/4" to 1/2" all the way around may be hidden. Then again, it may not be. You just never know.

If you really want the white borders then make them wider. This means that on some TVs you may have a very wide border, but that's the price you pay to make sure it shows up on other TVs too.

The part of the picture in the middle that can be seen is called the "safe area". Usually this border is arbitrarily placed 10% of the way in from the edges. In addition there is also a "title safe area" which is about 20% of the way in. Anything super-important, such as titles, should be inside this inner area just to be safe. I've got a few graphics that may help you.

http://www.chienworks.com/media/safeareas-ntsc.gif
http://www.chienworks.com/media/safeareas-pal.gif

Download whichever of these is appropriate and place the .gif file on the top track. This will show a pair of dashed yellow rectangles in the preview window. The outer one is the safe area line and the inner one is the title safe area line. This may help you position your images and borders. Keep in mind that they are only estimates and your actual milage will vary from one TV to the next.

Remove the images from the timeline when you've got everything set up.
MrSpeed wrote on 5/24/2004, 1:48 PM
Cool. Is there a reason why your image is 655 x 480 pixels instead of 740 X480 ?
Chienworks wrote on 5/24/2004, 2:02 PM
Because 655x480 (or, actually 654.545454...x480) is the right size! ;) If you look carefully you'll see that it fills the frame just right.

If you were to measure the absolute dimensions of the frame you would find that the size is not quite 4:3 but is actually a tiny bit wider, 1.363636... times as wide as it is tall, not 1.333333... DV frames are 720x480 because the DV pixels are a little bit narrower than they are tall. If they were square then the picture would be 1.5 times as wide as it is tall instead of the more normal approximately 4:3 of a television. In fact, the pixels are 0.909090... times as wide as tall. If you multiply 720x0.909090... you get 654.545454. Since still image files do use square pixels, this is the size that they should be to fill the frame.
MrSpeed wrote on 5/24/2004, 4:51 PM
Chienworks-
Thanks for the explanation. BTW I just noticed on the placement tab for text that there is a dropdown to select the desired safe zone.

I don't see any options like that for video so your gif will come in handy.