gdalgorithms-list

[Algorithms] Keeping an object's label on the screen

[Monteleone, N. <nat...@lm...>]

I have to draw some billboard text over some shapes we've "painted" onto
our terrain via multitexturing.  Normally this is easy enough - just
plop the label in an auto-rotated billboard and float it above the
center of the geometry.  But I need to keep the label on screen at all
times if any part of the painted geometry is visible.  I.e. if I have a
square painted on the terrain, but I can only see a small corner of it,
I still need to show the whole label.

 

What's a good algorithm to figure out where to draw the label?

 

The idea I had in mind was to do something in screen space...  I'd write
out an object ID version of the projected texture, apply it, render the
terrain geometry into a low-res buffer, then compute min/max screen
coords for each object.  Then I could just draw my text label in screen
space based on that information.  Is this a sane approach?

 

Otherwise I'd have to create a geometric representation of our shapes
that conforms to the terrain and choose a 3d location for my text based
on the view frustum.  That seems painful to implement efficiently.

 

Thanks,

Nathan

Re: [Algorithms] Keeping an object's label on the screen

[Jeff R. <je...@8m...>]

*The idea I had in mind was to do something in screen space...  I’d write
out an object ID version of the projected texture, apply it, render the
terrain geometry into a low-res buffer, then compute min/max screen coords
for each object.  Then I could just draw my text label in screen space based
on that information.  Is this a sane approach?*

Could be, if your object counts are reasonably small. If this is just
sections of the terrain we're talking about, and your terrain is at least
sort of flat, you could just project the bounds of these shapes into screen
space and center your label accordingly.


-- 
--------------------------------------------
Jeff Russell
Engineer, 8monkey Labs
www.8monkeylabs.com
--------------------------------------------

Re: [Algorithms] Keeping an object's label on the screen

[Jon W. <jw...@gm...>]

Monteleone, Nathan wrote:
>
> What’s a good algorithm to figure out where to draw the label?
>

In general, this is easy, by projecting the desired location to screen 
space. However, you can get into situations where a corner is visible on 
the screen, but the center of the object is behind the camera, which may 
lead to poorly behaved labels.

Just project the corners of the bounding box of the object to screen 
space; take the average of the corners that are in front of the camera, 
and project that to screen space. Form the rectangle around that, and 
clamp that rectangle to screen space to force it onto the screen.

If you also want multiple labels to sort themselves out without 
overlapping, it gets slightly more twisty.

Sincerely,

jw

Re: [Algorithms] Keeping an object's label on the screen

[Monteleone, N. <nat...@lm...>]

Unfortunately the terrain isn't very flat :-(

 

________________________________

From: Jeff Russell [mailto:je...@8m...] 
Sent: Wednesday, April 29, 2009 1:43 PM
To: Game Development Algorithms
Subject: Re: [Algorithms] Keeping an object's label on the screen

 

The idea I had in mind was to do something in screen space...  I'd write
out an object ID version of the projected texture, apply it, render the
terrain geometry into a low-res buffer, then compute min/max screen
coords for each object.  Then I could just draw my text label in screen
space based on that information.  Is this a sane approach?

Could be, if your object counts are reasonably small. If this is just
sections of the terrain we're talking about, and your terrain is at
least sort of flat, you could just project the bounds of these shapes
into screen space and center your label accordingly.


-- 
--------------------------------------------
Jeff Russell
Engineer, 8monkey Labs
www.8monkeylabs.com
--------------------------------------------

Re: [Algorithms] Keeping an object's label on the screen

[Monteleone, N. <nat...@lm...>]

Ok thanks.  Using the bounding box like that seems a lot faster than
trying to analyze an ID buffer.

Not worried about overlapping for now :)

-----Original Message-----
From: Jon Watte [mailto:jw...@gm...] 
Sent: Wednesday, April 29, 2009 1:56 PM
To: Game Development Algorithms
Subject: Re: [Algorithms] Keeping an object's label on the screen

Monteleone, Nathan wrote:
>
> What's a good algorithm to figure out where to draw the label?
>

In general, this is easy, by projecting the desired location to screen 
space. However, you can get into situations where a corner is visible on

the screen, but the center of the object is behind the camera, which may

lead to poorly behaved labels.

Just project the corners of the bounding box of the object to screen 
space; take the average of the corners that are in front of the camera, 
and project that to screen space. Form the rectangle around that, and 
clamp that rectangle to screen space to force it onto the screen.

If you also want multiple labels to sort themselves out without 
overlapping, it gets slightly more twisty.

Sincerely,

jw


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Re: [Algorithms] Keeping an object's label on the screen

[Tom P. <ga...@fa...>]

> I have to draw some billboard text over some shapes we've "painted" onto
> our terrain via multitexturing.  Normally this is easy enough - just
> plop the label in an auto-rotated billboard and float it above the
> center of the geometry.  But I need to keep the label on screen at all
> times if any part of the painted geometry is visible.  I.e. if I have a
> square painted on the terrain, but I can only see a small corner of it,
> I still need to show the whole label.

Draw the text if the object isn't culled.

You know where you want the text drawn (if you were to draw onto the
center of the object), and if you've got the text rect you can just move
it so it's within the screen area you want to allow it to live in.

-tom!

-- 

Re: [Algorithms] Keeping an object's label on the screen

[Monteleone, N. <nat...@lm...>]

The hard part is figuring out the screen area I want to allow it to live
in - the overlay in question is rendered to a texture and then pasted on
the terrain.

I think I know how to do it though - I know the 2d bound of my overlay.
I can combine that with the min/max height of the terrain underneath to
get a pretty good bounding box, then project that to screen space like
Mr. Watte said.

-Nathan


-----Original Message-----
From: Tom Plunket [mailto:ga...@fa...] 
Sent: Sunday, May 03, 2009 8:26 PM
To: Game Development Algorithms
Subject: Re: [Algorithms] Keeping an object's label on the screen


> I have to draw some billboard text over some shapes we've "painted"
onto
> our terrain via multitexturing.  Normally this is easy enough - just
> plop the label in an auto-rotated billboard and float it above the
> center of the geometry.  But I need to keep the label on screen at all
> times if any part of the painted geometry is visible.  I.e. if I have
a
> square painted on the terrain, but I can only see a small corner of
it,
> I still need to show the whole label.

Draw the text if the object isn't culled.

You know where you want the text drawn (if you were to draw onto the
center of the object), and if you've got the text rect you can just move
it so it's within the screen area you want to allow it to live in.

-tom!

-- 



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Conference from O'Reilly Media. Velocity features a full day of 
expert-led, hands-on workshops and two days of sessions from industry 
leaders in dedicated Performance & Operations tracks. Use code vel09scf 
and Save an extra 15% before 5/3. http://p.sf.net/sfu/velocityconf
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