From: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Allegro camera things (aspect)
Date: 24 May 1997 07:35:39 GMT
Organization: The National Capital FreeNet
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Message-ID: <5m65ob$hqr@freenet-news.carleton.ca>
References: <01bc66fe$da95ada0$e13663c3 AT default> <5m41fg$61r$1 AT news0 DOT xs4all DOT nl>
Reply-To: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
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 (ellman AT xs4all DOT nl) writes:
> In article <01bc66fe$da95ada0$e13663c3 AT default>,
> Thomas Harte <T DOT Harte AT btinternet DOT com DOT REMOVE_THIS_BIT> wrote:
>>	I just recompiled Allegro (under Pentium GCC, as it happens, but I don't
>>think this is related), and now I notice that when I use 3d, the correct
>>aspect ratio is no longer set. What I want to now is - how do you calculate
>>this? I know the aspect ratio of a monitor is 4:3, but I do not understand
>>what Allegro wants of me.
>>
>>		-Thomas
> 
> I don't know what effect re-compiling Allegro with PGCC has, but normaly,
> the persp_project() function distorts the view to the rectangle of the
> projection viewport (which isn't usually square). If you have some points at
> the end of the long-end of the viewport and rotate 90 degrees, the points are
> now at the short-end instead of off the viewport where they should be.
> 
> To correct this, use the following matrix to compensate for the perspective
> distortion.
> 
> get_scaling_matrix_f(&tmp_matrix_1, 1.0, ((float) SCREEN_W)/((float)SCREEN_H), 1.0);
> Or if you're using fixed numbers, use:
> get_scaling_matrix(&tmp_matrix_1, 1<<16, fdiv(itofix(SCREEN_W),itofix(SCREEN_H)), 1<<16);
> 
> Replace SCREEN_W with the viewport width and SCREEN_H with the viewport height.
> 
> This matrix must be applied after the rotations, etc, so if you're using it
> with a camera matrix, do the following.
> 
> matrix_mul_f(&orginal_camera_matrix, &tmp_matrix_1, &camera_matrix);
> 
> If you're using Allegro's get_camera_matrix to do your transformations, you can
> make use of the 'aspect' paramater (the last paramater) and pass the the
> height-to-width ratio (although this may be the width-to height ratio) as used
> with get_scaling_matrix.

What about simply scaling by 4/3? No matter what the viewport settings
are, the monitor is 4/3 as wide as high.

--
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Paul Derbyshire ao950 AT freenet DOT carleton DOT ca, http://chat.carleton.ca/~pderbysh