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.\" Automatically generated by Pandoc 1.19.2.4
.\"
.TH "al_perspective_transform" "3" "" "Allegro reference manual" ""
.hy
.SH NAME
.PP
al_perspective_transform \- Allegro 5 API
.SH SYNOPSIS
.IP
.nf
\f[C]
#include\ <allegro5/allegro.h>
void\ al_perspective_transform(ALLEGRO_TRANSFORM\ *trans,
\ \ \ float\ left,\ float\ top,\ float\ n,
\ \ \ float\ right,\ float\ bottom,\ float\ f)
\f[]
.fi
.SH DESCRIPTION
.PP
Like al_orthographic_transform(3) but honors perspective.
If everything is at a z\-position of \-near it will look the same as
with an orthographic transformation.
.PP
To use a specific horizontal field of view you can use the relation:
.IP
.nf
\f[C]
tan(hfov\ /\ 2)\ =\ (right\ \-\ left)\ /\ 2\ /\ near
\f[]
.fi
.PP
and therefore
.IP
.nf
\f[C]
near\ =\ (right\ \-\ left)\ /\ 2\ /\ tan(hfov\ /\ 2)
\f[]
.fi
.PP
Example 1:
.IP
.nf
\f[C]
\ \ \ float\ w\ =\ 800,\ h\ =\ 450;\ //\ assume\ our\ display\ is\ 800\ x\ 450
\ \ \ float\ fov\ =\ tan(90\ *\ ALLEGRO_PI\ /\ 180\ /\ 2);\ //\ 90\ degree\ field\ of\ view
\ \ \ //\ Our\ projection\ goes\ from\ 0/0\ to\ w/h\ with\ the\ near\ parameter\ set
\ \ \ //\ for\ a\ 90\ degree\ horizontal\ viewing\ angle.
\ \ \ ALLEGRO_TRANSFORM\ projection;
\ \ \ al_identity_transform(&projection);
\ \ \ al_perspective_transform(&projection,\ 0,\ 0,
\ \ \ \ w\ /\ 2\ /\ fov,
\ \ \ \ w,\ h,
\ \ \ \ 2000);
\ \ \ al_use_projection_transform(&projection);
\ \ \ //\ Set\ the\ camera\ z\ to\ +400\ (which\ is\ exactly\ the\ near\ distance)
\ \ \ ALLEGRO_TRANSFORM\ camera;
\ \ \ al_build_camera_transform(&camera,\ 0,\ 0,\ 400,\ 0,\ 0,\ 0,\ 0,\ 1,\ 0);
\ \ \ al_use_transform(&camera);
\ \ \ //\ This\ will\ draw\ two\ rectangles\ at\ the\ left\ and\ right\ edge\ of\ the
\ \ \ //\ screen\ and\ vertically\ centered.\ The\ x\ distance\ between\ them\ is\ 800
\ \ \ //\ units,\ but\ the\ camera\ transform\ moves\ them\ 400\ along\ z,\ so\ with
\ \ \ //\ a\ 90°\ viewing\ angle\ both\ are\ visible.
\ \ \ al_draw_filled_rectangle(0,\ 200,\ 50,\ 250,\ red;
\ \ \ al_draw_filled_rectangle(750,\ 200,\ 800,\ 250,\ red);
\f[]
.fi
.PP
Example 2:
.IP
.nf
\f[C]
\ \ \ float\ w\ =\ 800,\ h\ =\ 450;\ //\ assume\ our\ display\ is\ 800\ x\ 450
\ \ \ float\ fov\ =\ tan(90\ *\ ALLEGRO_PI\ /\ 180\ /\ 2);\ //\ 90\ degree\ field\ of\ view
\ \ \ float\ aspect\ =\ h\ /\ w;
\ \ \ float\ zoom\ =\ 2;\ //\ enlarge\ x\ 2
\ \ \ //\ This\ projection\ is\ very\ different\ from\ the\ one\ in\ the\ first\ example.
\ \ \ //\ Here\ we\ map\ the\ left\ and\ right\ edge\ of\ the\ screen\ to\ \-1\ and\ +1.\ And
\ \ \ //\ the\ y\ axis\ goes\ from\ \-1\ at\ the\ bottom\ to\ +1\ at\ the\ top,\ scaled\ by
\ \ \ //\ the\ aspect\ ratio.\ We\ also\ add\ a\ zoom\ parameter\ so\ we\ can\ control
\ \ \ //\ the\ visible\ portion\ of\ the\ scene\ independent\ of\ the\ field\ of\ view.
\ \ \ ALLEGRO_TRANSFORM\ projection;
\ \ \ al_identity_transform(&projection);
\ \ \ al_perspective_transform(&projection,
\ \ \ \ \ \ \-1\ /\ zoom,\ aspect\ /\ zoom,
\ \ \ \ \ \ 1\ /\ fov,
\ \ \ \ \ \ 1\ /\ zoom,\ \-aspect\ /\ zoom,
\ \ \ \ \ \ 2000);
\ \ \ al_use_projection_transform(&projection);
\ \ \ //\ Moves\ everything\ by\ \-4\ in\ the\ z\ direction.
\ \ \ ALLEGRO_TRANSFORM\ camera;
\ \ \ al_build_camera_transform(&camera,\ 0,\ 0,\ 4,\ 0,\ 0,\ 0,\ 0,\ 1,\ 0);
\ \ \ al_use_transform(&camera);
\ \ \ //\ At\ a\ z\ distance\ of\ 4\ with\ a\ 90°\ hfov\ everything\ would\ be\ scaled
\ \ \ //\ down\ to\ 25%.\ However\ we\ also\ zoom\ 2\-fold,\ so\ the\ final\ scaling\ is
\ \ \ //\ 50%.\ This\ rectangle\ therefore\ will\ appear\ at\ a\ size\ of\ 400\ x\ 225
\ \ \ //\ pixel\ (assuming\ the\ display\ is\ 800\ x\ 450).
\ \ \ al_draw_filled_rectangle(\-1,\ \-1,\ 1,\ 1,\ red);
\f[]
.fi
.SH SINCE
.PP
5.1.3
.SH SEE ALSO
.PP
al_use_projection_transform(3), al_orthographic_transform(3)
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