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# Getting single pixel lines with GL_LINE_STRIP

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For some reason, i can't convince openGL to give me single pixel wide lines, and insists on producing slightly dodgy and flickery lines
They look sort of like this:
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Also, even dead vertical/horizontal lines 'shimmer' when i scroll my camera, they seem to flick between a single and a double line of pixels, with the intensity changing as well I've disabled antialiasing with:
		gl.lineWidth(1);
gl.disable(GL.LINE_SMOOTH);
gl.hint(GL.LINE_SMOOTH, GL.FASTEST);

but i think its something to do with my perspective projection perhaps. Maybe my aspect ratio is causing problems?
		gl.matrixMode(GL.PROJECTION);

double aspect = Display.getWidth()/Display.getHeight();
//				fov	aspect	nearZ	farZ
glu.perspective(90,	aspect,		0,		100);

gl.matrixMode(GL.MODELVIEW);

gl.viewport(0, 0, Display.getWidth(), Display.getHeight());

Anyone any ideas for a n00b to try? _@/ [edited by - OrangyTang on September 8, 2002 2:19:18 PM]

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This is normal behaviour AFAIK. There''s nothing a scanline renderer can do about it.

-Lev

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It's called aliasing (i.e. you haven't "disabled" antialiasing with your code; you've tried to enable it). Look up how to draw "antialiased lines" with OpenGL. For starters:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_LINE_SMOOTH);
// draw lines.
glDisable(GL_BLEND);

[edited by - zealouselixir on September 8, 2002 10:17:58 PM]

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Ok, if i''ve accidentally enabled some sort of antialiasing, how do i disable it? I don''t want smoothed lines, just single pixel, so i get nice crisp, *non shimering* lines on compass points. I know this has to be possible, it seems to be the most common form of wireframe mode avalible in other apps..

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No, it's quite frankly not possible.
EDIT : Okay, it's possible, but I don't know how unless you just do something like glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);

Think about it like this: your monitor has a certain number of pixels (from whence your 'resolution' is derived). When you draw a line from point A to point B, a piece of software called a rasterizer is invoked, and its job is to determine which pixels on your monitor accurately represent your line.

If you had infinite resolution, the artifacts you're experiencing wouldn't be an issue. However, there's something called the Diamond Pixel Principle (or something like that); basically, each pixel on your screen is surrounded by a virtual diamond, and during rasterization, if the calculated value of the line at a given point is within the diamond, the pixel gets turned on.

That's why with lines that aren't perfectly horizontal or vertical, you sometimes get 'overlapping' pixels. It would actually look *worse* if you rasterized lines so that they were *always* exactly 1 pixel thick.
EDIT : That's mostly false, I suppose.

For this reason, antialiasing was introduced. Basically, antialiasing uses a kind of blurring effect, similar to font kerning, to make the lines look as though they're very thin, but that they also don't have ugly visual glitches.

Why is it so necessary that you have lines that are precisely one pixel thick and lack antialiasing?

Peace,
ZE.

//email me.//zealouselixir software.//msdn.//n00biez.//

[edited by - zealouselixir on September 9, 2002 7:09:03 PM]

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This is silly, i''ve *seen* lines as i''m trying to get when learning Java3D, and thats just built on OpenGL as a core. And its always been like that with games in wireframe mode etc.

More to the point, even lines that should be dead vertical/horizontal still flicker and look crap.

Yes I know what aliasing is and how smoothing/averaging stops it, but thats not what i''m getting at - if i want nice lines i''ll turn that on, for now i''m just trying to get basic lines drawn fast - no blending, just a line!

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Can you post a screenshot, or better yet, the line drawing code? It seems rather dodgy that your vertical lines are still flickering...

Sorry for assuming your ignorance, it just seems like the gl isn''t the problem here.

Peace,
ZE.

//email me.//zealouselixir software.//msdn.//n00biez.//

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Er, ok, i kinda messed up and kinda not...

I just ran npr Quake just to check that i''m not going slightly funny in the head, and it seemed all good as i was expecting/aiming for. Then I tinkered with my code, and lo! it worked.. except that I didnt actually change anything.

I think that there was some odd driver anti-aliasing going on (despite me manually disabling it) that npr quake seems to have forced to off. So my code now runs as expected

Trying to learn openGL with the lightweight java game library is not making things especially easy on me :o

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do u have fullscreen antialiasing tued on (in the drivers)

http://uk.geocities.com/sloppyturds/gotterdammerung.html

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