values for glPolygonOffset

Member
Posts: 40
Joined: 2009.05
Post: #1
Hi All,

I'm using GL_POLYGON_OFFSET_FILL to get an effect in my game. I'm drawing lines on top of polygons and want the lines to show up properly.

It seems that this behaves differently on various devices...

On the iPhone devices 3GS and 4.0 and some Android devices using

glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(1,1);

Works fine.

On Palm and the Motorola droid using

glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(-0.1,0);

Works.

Are there any OpengGL attributes I can query that will give me a hint as to what numbers to use in the glPolygonOffset call?

I've checked the values coming back from GL_DEPTH_RANGE and they are always 0,1.

Is there anything else I can check? Is glPolygonOffset a device specific thing?

Thanks
Chris.
Looks like as a workaround I can use this:

http://blog.thedailyanvil.com/?p=335

Code:
#define EDGE_OFFSET 0.00001

glDepthRange(EDGE_OFFSET, 1.0);
/* draw all non-edge geometry */

glDepthRange(0.0, 1.0 - EDGE_OFFSET);
/* draw all edges */
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Sage
Posts: 1,482
Joined: 2002.09
Post: #2
I seem to remember polygon offsets are one of those features that people tend to shy away from because the values are implementation defined.

Scott Lembcke - Howling Moon Software
Author of Chipmunk Physics - A fast and simple rigid body physics library in C.
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