# OpenGL Size in pixel of object in screen

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Hi I want to make a GUI in OpenGL and so I want my widgets to have a specific size in pixels, but I want to have a few effects or models in it that glOrtho wouldn't look good (other than that it works fine) ... so any ideas how I get that to work? cu & thx ernii

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You could just switch between orthographic and perspective projection every
time you want to draw a different object. i.e. call glOrtho() when you want to render the GUI graphics, and gluPerspective() when you want to render the other stuff.

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If W and H are the viewport width and height, respectively, then you can measure pixels in your window with glOrtho: glOrtho2D(0,W,0,H, -1 , 1);

If you need to mix 3D with it, then size depends on the view angle. Assuming that you are using a 'regular' projection matrix (such as the type generated by gluPerspective) then what you need are:
W: viewport width in pixels
H: viewport height in pixels
Aspect: Aspect ratio of width/height.
FOV: Vertical field of view. (top to bottom of screen)

If your field of view is 90 degrees, top to bottom, then from the center of the screen to the top of the screen is 45 degrees. A point in space at X=0, Z=-1 unit (1 unit into the screen), will be just at the top of your viewport when Y = sin(45 degrees). This will give you a conversion factor:

At Z=-1 : sin(45) vertical world units = half a screen height. = H/2
This inversely proportional to Z depth units; an object twice as far away will be half the size.

Height in pixels = height of object * sin(45) * height of screen /2 /Z

If your projection matrix scales X and Y by the same amount (which is normally the case) , then the same formula should work for width.

Note, all of this is from memory, so there might be some bugs in my math. I can check it against my implementation when I get home, since I have done this same thing before.

[Edited by - DracoLacertae on September 27, 2006 5:15:28 PM]

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Hi

@Redien:
Its more like that I want to do like little animations with the Widgets so I need the pixel size of them anyway. But other than that I think it would very good :)

@DracoLacertae:
Okay, sorry, I just don't get it ;)
I use gluPerspective with an angle of 90 deg and I get the part that now the half screen height should have the size sin(45 deg) in world units.

Since half of the screen should have the size H/2 in world units (since it has the same amount of pixels), so the factor should be 1/(H / 2 / sin(45)).

I tried to scale everything with that, but it didn't work ... so I tried with only H/2 and it seemed to work, I also tried to move everything H/2 away and that worked to .... but sometimes it is wrong by like a pixel.
But

So I'm a little bit confused right now ;) ... what didn't I get right?

cu & thx
ernii

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whats wrong with glOrtho()?

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Are you using GLU? I just found out about gluProject:
http://www.mevis.de/opengl/gluProject.html

Just gluProject the corners of your object, and you'll get exactly where it is on the screen.

The method I gave may be off by a few pixels, because this method is not the method GL uses; instead it is a rather crude geometric construction. (When I find the code I last used it in, I'll post it.) To get the true pixel coordinates for a point in space, you need to transform using the projection and modelview matrices; much like how gluProject does.

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@zedzeek:
I want to transform the widget in 3D and that looks kind of odd with orthogonal projection, like a rotation in depth. ANd yes this is nearly only for eyecandy .. but it is still cool :)

@DracoLacertae:
Wow, that looks like the "solve problem" function for me :)
I will try it out. Thank you for the tip

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