VGA Overscan/border

I've noticed something interesting about Doom this week. On my DOS machine (using a CRT of course) I've noticed that when Doom performs any palette change, there is significant overscan on the sides (and maybe some on the top as well).

Is there any particular reason this happens? I know in Wolfenstein 3D it's possible to change this overscan border color with a cheat code. In doom's case, is this just an oversight? Most source ports probably wouldn't bother with this, but it is an interesting effect, to say the least.

It surely grabbed my attention when the screen seemed to "grow" as I was being damaged.

On some CRT monitors it seems the screen contents can affect the position of the image. My parents had a computer with a monitor where this was particularly noticeable- to the extent that if you opened a new browser window, the white background would make the picture visibly bulge out like you describe. A palette change in Doom might have a similar effect.

In general, hardware of the time put palette color 0 in the dead areas outside the active display but also outside hsync/vsync.


As far as being intentional, it probably was. Even if not, they certainly saw it on their hardware of the time, and if they had wanted to remove it, it would have been a simple matter to simply fix color 0 in playpal.


Edit: This is distinct from the effect that fraggle is describing.

natt said:

In general, hardware of the time put palette color 0 in the dead areas outside the active display but also outside hsync/vsync.


As far as being intentional, it probably was. Even if not, they certainly saw it on their hardware of the time, and if they had wanted to remove it, it would have been a simple matter to simply fix color 0 in playpal.


Edit: This is distinct from the effect that fraggle is describing.

Ah, thank you. I've always liked the effect, but in LCD monitors, it's completely invisible.

Csonicgo said:

Ah, thank you. I've always liked the effect, but in LCD monitors, it's completely invisible.

Correct. There is no underscan of the active image. Also, it's possible that modern video card hardware output forces black on the back porch no matter what. Even if it didn't, you most certainly would only get the correct effect in a vga compatibility mode, so only running the original game in DOS...

Low quality CRT's exhibit a distortion effect because of the voltage difference due to different screen content(bright/dark).

A new idea for Chocolate Doom, perhaps? :)

I honestly don't know what you people are talking about. Could you post a photo of what's happening (if the CRT flashing doesn't ruin it)?

printz said:

I honestly don't know what you people are talking about. Could you post a photo of what's happening (if the CRT flashing doesn't ruin it)?

There is a term for the sides of the screen on a CRT Television signal, called "porches". There's a front and back. most monitors now (LCD, TFT, LED, whatever) do not show these porches. They are part of the video signal itself. they are, in doom's case, index 0 in the pallete (black)

When Doom undergoes a pallete change, whatever's in 0 (black) changes as well. when this happens, the front and back porch also change color, and the sides of the screen "glow" (that is, they're not black anymore).

And now for the correct explanation.

CRT computer monitors are designed to show the full image sent to them (underscan), unlike CRT televisions, which cut off the sides to fill the entire picture tube (overscan).

In VGA, the color of the border around the screen (called the overscan, but there is no displayed image there) is actually changeable, not forced to be black.

Michael Abrash, popularizer of Mode X, gives an explanation of how it can be changed on a page mostly about screen blanking.

You cannot see the front or back porch, as they are not part of the active video signal. Analog television#Structure_of_a_video_signal has a short explanation of why they were used.

LCD monitors with analog inputs have circuitry to convert the analog VGA signal to a digital one so that the LCD panel can display it, which removes the overscan and cuts off the border.

Surely you've used a CRT computer monitor before, printz? Never remember adjusting the geometry settings so that you actually got a square picture with the minimum amount of border on the sides?

Bloodshedder said:

You cannot see the front or back porch, as they are not part of the active video signal. Analog television#Structure_of_a_video_signal has a short explanation of why they were used.

I was trying to simplify it for printz, but you're right. it's the parts next to them.


I remember when I got flamed on an enthusiast's forum for asking for "NTSC's resolution". Hoo, boy, don't do that.

Changed title.

Edit: nope, title was right. that WAS overscan I was seeing.

Bloodshedder said:

You cannot see the front or back porch, as they are not part of the active video signal. Analog television#Structure_of_a_video_signal has a short explanation of why they were used.

In the context of analog and digital signals over VGA and DVI-D, what would you call those areas outside the active resolution but also outside the sync pulses? Wikipedia XFree86_Modeline calls them porches, and you can definitely see them on a modern CRT unless you overscan the active area.

I agree with you that in the context of NTSC itself, porch may have a slightly different meaning, but the term was re-appropriated.

Csonicgo said:

I remember when I got flamed on an enthusiast's forum for asking for "NTSC's resolution". Hoo, boy, don't do that.

Funny thing, I actually wrote a reply to that sort of question a few days ago. I can reproduce it for you here if you like =p

Bloodshedder said:

Surely you've used a CRT computer monitor before, printz? Never remember adjusting the geometry settings so that you actually got a square picture with the minimum amount of border on the sides?

I admit I never paid attention to those details, and dismissed the glitchy effects as artifacts.