Change camera position and direction in OpenGL? - c++

Question

I have a code (game) with a fixed camera in an ortho projection. It runs smoothly until I change the camera position from (0,0,1) to (0,0,-1).
In a nutshell, I have 2 textures:
{ //texture 1
960.0f, 0.0f, -5.0f, 0.0f, 0.0f,
960.0f, 1080.0f, -5.0f, 1.0f, 0.0f,
1920.0f, 0.0f, -5.0f, 0.0f, 1.0f,
1920.0f, 1080.0f, -5.0f, 1.0f, 1.0f
}
{ // texture 2
1290.0f, 390.0f, -7.0f, 0.0f, 0.0f,
1290.0f, 690.0f, -7.0f, 1.0f, 0.0f,
1590.0f, 390.0f, -7.0f, 0.0f, 1.0f,
1590.0f, 690.0f, -7.0f, 1.0f, 1.0f
}
the transformation matrices:
view = glm::lookAt
(
glm::vec3( 0.0f, 0.0f, 1.0f ),
glm::vec3( 0.0f, 0.0f, 0.0f ),
glm::vec3( 0.0f, 1.0f, 0.0f )
);
projection = glm::ortho
(
0.0f,
1920.0f,
0.0f,
1080.0f,
1.0f, // zNear
10.0f // zFar
);
the vertex shader:
#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec2 aTexCoord;
out vec2 TexCoord;
uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;
void main()
{
gl_Position = projection * view * model * vec4( aPos, 1.0 );
TexCoord = vec2( aTexCoord.x, aTexCoord.y );
}
If I run this code, it properly displays both textures, does depth testing,...
However, if I change the camera position to (0, 0, -1) and textures' Z-coordinate to their inverse +5 and +7, and keep the same direction (0, 0, 0), no texture is displayed (rendered). Shouldn't it display the same as before the changes ?

Answer 1

The issue is related to the orthographic projection matrix, because it is not centered. When the z axis of the view is inverted, then the x axis is inverted, too. Note the Right-hand rule has to be still fulfilled and the x.axis is the cross product of the y-axis and z-axis.
When the geometry is at z-5and the view and projection matrix is as follows
view = glm::lookAt(
glm::vec3(0.0f, 0.0f, 1.0f),
glm::vec3(0.0f, 0.0f, 0.0f),
glm::vec3(0.0f, 1.0f, 0.0f);
projection = glm::ortho(0.0f, 1920.0f, 0.0f, 1080.0f, 1.0f, 10.0f);
then the object is projected to the viewport:
But if you switch the z position of the geometry and the view, then you get the following situation:
view = glm::lookAt(
glm::vec3(0.0f, 0.0f, -1.0f),
glm::vec3(0.0f, 0.0f, 0.0f),
glm::vec3(0.0f, 1.0f, 0.0f);
then the object is beside the viewport:
Shift the the orthographic projection along the X-axis, to solve your issue:
projection = glm::ortho(-1920.0f, 0.0f, 0.0f, 1080.0f, 1.0f, 10.0f);