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Depth Buffering Question

Started by Roach Jan 6, 2003 at 1:19 PM 9 replies 800+ views
Original Post
Roach
Roach
Hi Gang, Need some clarification on how Depth Buffering works in the case where the Z values for two objects being drawn are the same value. Does the object being drawn econd go on top of the object drawn first? When I tried to test this situation, it looked like the two objects "blended" together, with the second object intersecting the first object in some spots. Is there w way that I can get the second object to lay directly on top of the first (without using a offset for the Z axis)? For instance my first object it a Sphere (actually a globe) and I am trying to draw a lat/lon grid on top of the globe, but I don''t want any space between the globe and the lat/lon grid. so this is what I do (in theory - not real code!) 
glEnable(GL_DEPTH_TEST);
gluSphere(globe_, globeRadius, 48, 48);
                                     
// Draw Lat/Lon takes in a radius value
drawLatLon(globeRadius);

[/CODE]

	Like I said earlier - keeping the radius of both objects the same seems to make the objects blend together, with the lat/lon grid intersecting through the globe in some areas. 

	If I do the following, I solve the intersecting problem, but the lat/lon grid seems to "float" above the globe (i.e. you can see a space between the grid and the globe).

[CODE]

glEnable(GL_DEPTH_TEST);                                     gluSphere(globe_, globeRadius, 48, 48);
                                     
// Draw Lat/Lon takes in a radius value
drawLatLon(globeRadius + 0.1);

[/CODE]

Any ideas? Comments?

Thanks
Roach
vincoof
vincoof
polygon offset is what you need
LockePick
LockePick
>>drawLatLon(globeRadius + 0.1);
Make the value smaller until you don''t notice the space anymore. 0.000001?

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vincoof
vincoof
LockePick: it should not work for two reasons :
1st- that offset is more or less efficient depending of the distance between the camera and the spheres, thus it''s hard to rule.
2nd- if solid model and wireframe model do not share the same vertices, you will more than likely see interections between both models.

Though in a (very) special case it''d be a better solution, I don''t think it''s good for that case. Nice idea to keep for further cases indeed !
Roming22
Roming22
Where do you use the function glEnable(GL_POLYGON_OFFSET_FILL)?


glEnable(GL_POLYGON_OFFSET_FILL);
draw_solid
glDisable(GL_POLYGON_OFFSET_FILL);
draw_wireframe


or


draw_solid
glEnable(GL_POLYGON_OFFSET_FILL);
draw_wireframe
glDisable(GL_POLYGON_OFFSET_FILL);


or maybe both are wrong???
vincoof
vincoof
The second will not work because you''re more than likely rendering your wireframe model with lines... so you need to enable GL_POLYGON_OFFSET_LINE instead of FILL ! Apart from that, both should work, except the fact that arguments to glPolygonOffset should be different of course.

I recommend you to read that FAQ at opengl.org
Roach
Roach
Hi Gang,
Thnaks for the responces, I do want a clarification on polyogon offset though if I could. I believe that I will not be able to use poygon offset because I am rendering the lat long grid, (which is a display list made up of Line Strips) over a gluSphere(). Can I still use polygon offset?

quote:

from OpenGL.org FAQ:

Polygon offset, as its name implies, only works with polygonal primitives. It affects only the filled primitives: GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_QUADS, GL_QUAD_STRIP, and GL_POLYGON. Polygon offset will work when you render them with glPolygonMode setto GL_FILL, GL_LINE, or GL_POINT.

Polygon offset doesn''t affect non-polygonal primitives. The GL_POINTS, GL_LINES, GL_LINE_STRIP, and GL_LINE_LOOP primitives can''t be offset with glPolygonOffset().


Current update - I actually have the lat/lon grid looking correct (with no visual space) when I use an offset of 0.1. The problem is that I do not want to use an offset if I do not have to. (Cause if I cange the viewport, or field of view - that offset will probibly have to change as well, making the code less maintainable overall). If I need the offset, well so be it - but if I can get around it - great!

Roach
vincoof
vincoof
If you''re rendering your grid with LINES, then you can''t offset the grid : you have to offset the sphere.
But if you''re rendering your grid with POLYGONS or QUADS (or strips, triangles... any filled primitive) with glPolygonMode set to GL_LINE, then yes you can use polygon offset for your grid.

quote:

The problem is that I do not want to use an offset if I do not have to. (Cause if I cange the viewport, or field of view - that offset will probibly have to change as well, making the code less maintainable overall).

You''re absolutely right, and that''s why I do not recommend an offset *but* I do recommend a "polygon offset" which is really different for OpenGL''s depth test point of view. With a polygon offset, you can push or pull the depth of 3D objects without bothering the object''s size or distance to the viewer.
snisarenko
snisarenko
Do this

glDisable(DEPTH_TEST);
//Draw your sphere
glEnable(DEPTH_TEST);
//Draw your polygon

that should work !!!
vincoof
vincoof
quote:

that should work !!!

Not really, and to say the least not at all
Roach
Roach
The Polygon Offset did the trick!

Thanks guys!

Roach

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