How to use glOrtho in OpenGL

Question

I can t understand the usage of glOrtho  Can someone explain what it is used for   Is it used to set the range of x y and z coordinates limit   glOrtho -1 0  1 0  -1 0  1 0  -1 0  1 0     It means that the x  y and z range is from -1 to 1

User · Answer

glOrtho describes a transformation that produces a parallel projection. The current matrix (see glMatrixMode) is multiplied by this matrix and the result replaces the current matrix, as if glMultMatrix were called with the following matrix as its argument:

OpenGL documentation (my bold)

The numbers define the locations of the clipping planes (left, right, bottom, top, near and far).

The "normal" projection is a perspective projection that provides the illusion of depth. Wikipedia defines a parallel projection as:

Parallel projections have lines of projection that are parallel both in reality and in the projection plane.

Parallel projection corresponds to a perspective projection with a hypothetical viewpoint—e.g., one where the camera lies an infinite distance away from the object and has an infinite focal length, or "zoom".

User · Answer

Minimal runnable example glOrtho  2D games  objects close and far appear the same size   glFrustrum  more real-life like 3D  identical objects further away appear smaller   main c  include  lt stdlib h gt    include  lt GL gl h gt   include  lt GL glu h gt   include  lt GL glut h gt   static int ortho   0   static void display void        glClear GL COLOR BUFFER BIT       glLoadIdentity        if  ortho          else              This only rotates and translates the world around to look like the camera moved             gluLookAt 0 0  0 0  -3 0  0 0  0 0  0 0  0 0  1 0  0 0             glColor3f 1 0f  1 0f  1 0f       glutWireCube 2       glFlush       static void reshape int w  int h        glViewport 0  0  w  h       glMatrixMode GL PROJECTION       glLoadIdentity        if  ortho            glOrtho -2 0  2 0  -2 0  2 0  -1 5  1 5         else           glFrustum -1 0  1 0  -1 0  1 0  1 5  20 0             glMatrixMode GL MODELVIEW      int main int argc  char   argv        glutInit  amp argc  argv       if  argc  gt  1            ortho   1            glutInitDisplayMode GLUT SINGLE   GLUT RGB       glutInitWindowSize 500  500       glutInitWindowPosition 100  100       glutCreateWindow argv 0        glClearColor 0 0  0 0  0 0  0 0       glShadeModel GL FLAT       glutDisplayFunc display       glutReshapeFunc reshape       glutMainLoop        return EXIT SUCCESS     GitHub upstream  Compile  gcc -ggdb3 -O0 -o main -std c99 -Wall -Wextra -pedantic main c -lGL -lGLU -lglut  Run with glOrtho    main 1  Run with glFrustrum    main  Tested on Ubuntu 18 10  Schema Ortho  camera is a plane  visible volume a rectangle   Frustrum  camera is a point visible volume a slice of a pyramid   Image source  Parameters We are always looking from  z to -z with  y upwards  glOrtho left  right  bottom  top  near  far    left  minimum x we see right  maximum x we  see bottom  minimum y we see top  maximum y we  see -near  minimum z we see  Yes  this is -1 times near  So a negative input means positive z  -far  maximum z we see  Also negative   Schema   Image source  How it works under the hood In the end  OpenGL always  quot uses quot   glOrtho -1 0  1 0  -1 0  1 0  -1 0  1 0    If we use neither glOrtho nor glFrustrum  that is what we get  glOrtho and glFrustrum are just linear transformations  AKA matrix multiplication  such that   glOrtho  takes a given 3D rectangle into the default cube glFrustrum  takes a given pyramid section into the default cube  This transformation is then applied to all vertexes  This is what I mean in 2D   Image source  The final step after transformation is simple   remove any points outside of the cube  culling   just ensure that x  y and z are in  -1   1  ignore the z component and take only x and y  which now can be put into a 2D screen  With glOrtho  z is ignored  so you might as well always use 0  One reason you might want to use z    0 is to make sprites hide the background with the depth buffer  Deprecation glOrtho is deprecated as of OpenGL 4 5  the compatibility profile 12 1   quot FIXED-FUNCTION VERTEX TRANSFORMATIONS quot  is in red  So don t use it for production   In any case  understanding it is a good way to get some OpenGL insight  Modern OpenGL 4 programs calculate the transformation matrix  which is small  on the CPU  and then give the matrix and all points to be transformed to OpenGL  which can do the thousands of matrix multiplications for different points really fast in parallel  Manually written vertex shaders then do the multiplication explicitly  usually with the convenient vector data types of the OpenGL Shading Language  Since you write the shader explicitly  this allows you to tweak the algorithm to your needs  Such flexibility is a major feature of more modern GPUs  which unlike the old ones that did a fixed algorithm with some input parameters  can now do arbitrary computations  See also  https   stackoverflow com a 36211337 895245 With an explicit GLfloat transform   it would look something like this  glfw transform c  include  lt math h gt   include  lt stdio h gt   include  lt stdlib h gt    define GLEW STATIC  include  lt GL glew h gt    include  lt GLFW glfw3 h gt   static const GLuint WIDTH   800  static const GLuint HEIGHT   600     ourColor is passed on to the fragment shader     static const GLchar  vertex shader source        quot  version 330 core n quot       quot layout  location   0  in vec3 position  n quot       quot layout  location   1  in vec3 color  n quot       quot out vec3 ourColor  n quot       quot uniform mat4 transform  n quot       quot void main     n quot       quot     gl Position   transform   vec4 position  1 0f   n quot       quot     ourColor   color  n quot       quot   n quot   static const GLchar  fragment shader source        quot  version 330 core n quot       quot in vec3 ourColor  n quot       quot out vec4 color  n quot       quot void main     n quot       quot     color   vec4 ourColor  1 0f   n quot       quot   n quot   static GLfloat vertices            Positions          Colors         0 5f  -0 5f  0 0f  1 0f  0 0f  0 0f      -0 5f  -0 5f  0 0f  0 0f  1 0f  0 0f       0 0f   0 5f  0 0f  0 0f  0 0f  1 0f        Build and compile shader program  return its ID     GLuint common get shader program      const char  vertex shader source      const char  fragment shader source         GLchar  log   NULL      GLint log length  success      GLuint fragment shader  program  vertex shader          Vertex shader        vertex shader   glCreateShader GL VERTEX SHADER       glShaderSource vertex shader  1   amp vertex shader source  NULL       glCompileShader vertex shader       glGetShaderiv vertex shader  GL COMPILE STATUS   amp success       glGetShaderiv vertex shader  GL INFO LOG LENGTH   amp log length       log   malloc log length       if  log length  gt  0            glGetShaderInfoLog vertex shader  log length  NULL  log           printf  quot vertex shader log  n n s n quot   log             if   success            printf  quot vertex shader compile error n quot            exit EXIT FAILURE                 Fragment shader        fragment shader   glCreateShader GL FRAGMENT SHADER       glShaderSource fragment shader  1   amp fragment shader source  NULL       glCompileShader fragment shader       glGetShaderiv fragment shader  GL COMPILE STATUS   amp success       glGetShaderiv fragment shader  GL INFO LOG LENGTH   amp log length       if  log length  gt  0            log   realloc log  log length           glGetShaderInfoLog fragment shader  log length  NULL  log           printf  quot fragment shader log  n n s n quot   log             if   success            printf  quot fragment shader compile error n quot            exit EXIT FAILURE                 Link shaders        program   glCreateProgram        glAttachShader program  vertex shader       glAttachShader program  fragment shader       glLinkProgram program       glGetProgramiv program  GL LINK STATUS   amp success       glGetProgramiv program  GL INFO LOG LENGTH   amp log length       if  log length  gt  0            log   realloc log  log length           glGetProgramInfoLog program  log length  NULL  log           printf  quot shader link log  n n s n quot   log             if   success            printf  quot shader link error quot            exit EXIT FAILURE                 Cleanup         free log       glDeleteShader vertex shader       glDeleteShader fragment shader       return program     int main void        GLint shader program      GLint transform location      GLuint vbo      GLuint vao      GLFWwindow  window      double time       glfwInit        window   glfwCreateWindow WIDTH  HEIGHT    FILE    NULL  NULL       glfwMakeContextCurrent window       glewExperimental   GL TRUE      glewInit        glClearColor 0 0f  0 0f  0 0f  1 0f       glViewport 0  0  WIDTH  HEIGHT        shader program   common get shader program vertex shader source  fragment shader source        glGenVertexArrays 1   amp vao       glGenBuffers 1   amp vbo       glBindVertexArray vao       glBindBuffer GL ARRAY BUFFER  vbo       glBufferData GL ARRAY BUFFER  sizeof vertices   vertices  GL STATIC DRAW          Position attribute        glVertexAttribPointer 0  3  GL FLOAT  GL FALSE  6   sizeof GLfloat    GLvoid  0       glEnableVertexAttribArray 0          Color attribute        glVertexAttribPointer 1  3  GL FLOAT  GL FALSE  6   sizeof GLfloat    GLvoid   3   sizeof GLfloat         glEnableVertexAttribArray 1       glBindVertexArray 0        while   glfwWindowShouldClose window             glfwPollEvents            glClear GL COLOR BUFFER BIT            glUseProgram shader program           transform location   glGetUniformLocation shader program   quot transform quot               THIS is just a dummy transform             GLfloat transform                   0 0f  0 0f  0 0f  0 0f              0 0f  0 0f  0 0f  0 0f              0 0f  0 0f  1 0f  0 0f              0 0f  0 0f  0 0f  1 0f                     time   glfwGetTime            transform 0    2 0f   sin time           transform 5    2 0f   cos time           glUniformMatrix4fv transform location  1  GL FALSE  transform            glBindVertexArray vao           glDrawArrays GL TRIANGLES  0  3           glBindVertexArray 0           glfwSwapBuffers window             glDeleteVertexArrays 1   amp vao       glDeleteBuffers 1   amp vbo       glfwTerminate        return EXIT SUCCESS     GitHub upstream  Compile and run  gcc -ggdb3 -O0 -o glfw transform out -std c99 -Wall -Wextra -pedantic glfw transform c -lGL -lGLU -lglut -lGLEW -lglfw -lm   glfw transform out  Output   The matrix for glOrtho is really simple  composed only of scaling and translation  scalex  0       0       translatex  0       scaley  0       translatey  0       0       scalez  translatez  0       0       0       1  as mentioned in the OpenGL 2 docs  The glFrustum matrix is not too hard to calculate by hand either  but starts getting annoying  Note how frustum cannot be made up with only scaling and translations like glOrtho  more info at  https   gamedev stackexchange com a 118848 25171 The GLM OpenGL C   math library is a popular choice for calculating such matrices  http   glm g-truc net 0 9 2 api a00245 html documents both an ortho and frustum operations

User · Answer

Have a look at this picture  Graphical Projections   The glOrtho command produces an  Oblique  projection that you see in the bottom row  No matter how far away vertexes are in the z direction  they will not recede into the distance   I use glOrtho every time I need to do 2D graphics in OpenGL  such as health bars  menus etc  using the following code every time the window is resized   glMatrixMode GL PROJECTION   glLoadIdentity    glOrtho 0 0f  windowWidth  windowHeight  0 0f  0 0f  1 0f     This will remap the OpenGL coordinates into the equivalent pixel values  X going from 0 to windowWidth and Y going from 0 to windowHeight   Note that I ve flipped the Y values because OpenGL coordinates start from the bottom left corner of the window  So by flipping  I get a more conventional  0 0  starting at the top left corner of the window rather   Note that the Z values are clipped from 0 to 1  So be careful when you specify a Z value for your vertex s position  it will be clipped if it falls outside that range  Otherwise if it s inside that range  it will appear to have no effect on the position except for Z tests

[c++] How to use glOrtho() in OpenGL?

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