Modern equivalent of `gluOrtho2d ` - opengl

Question

What is the modern equivalent of the OpenGL function gluOrtho2d? clang is giving me deprecation warnings. I believe I need to write some kind of vertex shader? What should it look like?

Answer 1

I started off this answer thinking "It's not that different, you just have to...".
I started writing some code to prove myself right, and ended up not really doing so. Anyway, here are the fruits of my efforts: a minimal annotated example of "modern" OpenGL.
There's a good bit of code you'll need before modern OpenGL will start to act like old-school OpenGL. I'm not going to get into the reasons why you might like to do it the new way (or not) -- there are countless other answers that give a pretty good rundown. Instead I'll post some minimal code that can get you running if you're so inclined.
You should end up with this stunning piece of art:
Basic Render Process
Part 1: Vertex buffers
void TestDraw(){
// create a vertex buffer (This is a buffer in video memory)
GLuint my_vertex_buffer;
glGenBuffers(1 /*ask for one buffer*/, &my_vertex_buffer);
const float a_2d_triangle[] =
{
200.0f, 10.0f,
10.0f, 200.0f,
400.0f, 200.0f
};
// GL_ARRAY_BUFFER indicates we're using this for
// vertex data (as opposed to things like feedback, index, or texture data)
// so this call says use my_vertex_data as the vertex data source
// this will become relevant as we make draw calls later
glBindBuffer(GL_ARRAY_BUFFER, my_vertex_buffer);
// allocate some space for our buffer
glBufferData(GL_ARRAY_BUFFER, 4096, NULL, GL_DYNAMIC_DRAW);
// we've been a bit optimistic, asking for 4k of space even
// though there is only one triangle.
// the NULL source indicates that we don't have any data
// to fill the buffer quite yet.
// GL_DYNAMIC_DRAW indicates that we intend to change the buffer
// data from frame-to-frame.
// the idea is that we can place more than 3(!) vertices in the
// buffer later as part of normal drawing activity
// now we actually put the vertices into the buffer.
glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(a_2d_triangle), a_2d_triangle);
Part 2: Vertex Array Object:
We need to define how the data contained in my_vertex_array is structured. This state is contained in a vertex array object (VAO). In modern OpenGL there needs to be at least one of these
GLuint my_vao;
glGenVertexArrays(1, &my_vao);
//lets use the VAO we created
glBindVertexArray(my_vao);
// now we need to tell the VAO how the vertices in my_vertex_buffer
// are structured
// our vertices are really simple: each one has 2 floats of position data
// they could have been more complicated (texture coordinates, color --
// whatever you want)
// enable the first attribute in our VAO
glEnableVertexAttribArray(0);
// describe what the data for this attribute is like
glVertexAttribPointer(0, // the index we just enabled
2, // the number of components (our two position floats)
GL_FLOAT, // the type of each component
false, // should the GL normalize this for us?
2 * sizeof(float), // number of bytes until the next component like this
(void*)0); // the offset into our vertex buffer where this element starts
Part 3: Shaders
OK, we have our source data all set up, now we can set up the shader which will transform it into pixels
// first create some ids
GLuint my_shader_program = glCreateProgram();
GLuint my_fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
GLuint my_vertex_shader = glCreateShader(GL_VERTEX_SHADER);
// we'll need to compile the vertex shader and fragment shader
// and then link them into a full "shader program"
// load one string from &my_fragment_source
// the NULL indicates that the string is null-terminated
const char* my_fragment_source = FragmentSourceFromSomewhere();
glShaderSource(my_fragment_shader, 1, &my_fragment_source, NULL);
// now compile it:
glCompileShader(my_fragment_shader);
// then check the result
GLint compiled_ok;
glGetShaderiv(my_fragment_shader, GL_COMPILE_STATUS, &compiled_ok);
if (!compiled_ok){ printf("Oh Noes, fragment shader didn't compile!\n"); }
else{
glAttachShader(my_shader_program, my_fragment_shader);
}
// and again for the vertex shader
const char* my_vertex_source = VertexSourceFromSomewhere();
glShaderSource(my_vertex_shader, 1, &my_vertex_source, NULL);
glCompileShader(my_vertex_shader);
glGetShaderiv(my_vertex_shader, GL_COMPILE_STATUS, &compiled_ok);
if (!compiled_ok){ printf("Oh Noes, vertex shader didn't compile!\n"); }
else{
glAttachShader(my_shader_program, my_vertex_shader);
}
//finally, link the program, and set it active
glLinkProgram(my_shader_program);
glUseProgram(my_shader_program);
Part 4: Drawing things on the screen
//get the screen size
float my_viewport[4];
glGetFloatv(GL_VIEWPORT, my_viewport);
//now create a projection matrix
float my_proj_matrix[16];
MyOrtho2D(my_proj_matrix, 0.0f, my_viewport[2], my_viewport[3], 0.0f);
//"uProjectionMatrix" refers directly to the variable of that name in
// shader source
GLuint my_projection_ref =
glGetUniformLocation(my_shader_program, "uProjectionMatrix");
// send our projection matrix to the shader
glUniformMatrix4fv(my_projection_ref, 1, GL_FALSE, my_proj_matrix );
//clear the background
glClearColor(0.3, 0.4, 0.4, 1.0);
glClear(GL_COLOR_BUFFER_BIT| GL_DEPTH_BUFFER_BIT);
// *now* after that tiny setup, we're ready to draw the best 24 bytes of
// vertex data ever.
// draw the 3 vertices starting at index 0, interpreting them as triangles
glDrawArrays(GL_TRIANGLES, 0, 3);
// now just swap buffers however your window manager lets you
}
And That's it!
... except for the actual
Shaders
I started to get a little tired at this point, so the comments are a bit lacking. Let me know if you'd like anything clarified.
const char* VertexSourceFromSomewhere()
{
return
"#version 330\n"
"layout(location = 0) in vec2 inCoord;\n"
"uniform mat4 uProjectionMatrix;\n"
"void main()\n"
"{\n"
" gl_Position = uProjectionMatrix*(vec4(inCoord, 0, 1.0));\n"
"}\n";
}
const char* FragmentSourceFromSomewhere()
{
return
"#version 330 \n"
"out vec4 outFragColor;\n"
"vec4 DebugMagenta(){ return vec4(1.0, 0.0, 1.0, 1.0); }\n"
"void main() \n"
"{\n"
" outFragColor = DebugMagenta();\n"
"}\n";
}
The Actual Question you asked: Orthographic Projection
As noted, the actual math is just directly from Wikipedia.
void MyOrtho2D(float* mat, float left, float right, float bottom, float top)
{
// this is basically from
// http://en.wikipedia.org/wiki/Orthographic_projection_(geometry)
const float zNear = -1.0f;
const float zFar = 1.0f;
const float inv_z = 1.0f / (zFar - zNear);
const float inv_y = 1.0f / (top - bottom);
const float inv_x = 1.0f / (right - left);
//first column
*mat++ = (2.0f*inv_x);
*mat++ = (0.0f);
*mat++ = (0.0f);
*mat++ = (0.0f);
//second
*mat++ = (0.0f);
*mat++ = (2.0*inv_y);
*mat++ = (0.0f);
*mat++ = (0.0f);
//third
*mat++ = (0.0f);
*mat++ = (0.0f);
*mat++ = (-2.0f*inv_z);
*mat++ = (0.0f);
//fourth
*mat++ = (-(right + left)*inv_x);
*mat++ = (-(top + bottom)*inv_y);
*mat++ = (-(zFar + zNear)*inv_z);
*mat++ = (1.0f);
}

Answer 2

Modern OpenGL is significantly different. You won't be able to just drop in a new function. Read up...
http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Chapter-1:-The-Graphics-Pipeline.html
http://www.arcsynthesis.org/gltut/index.html
http://www.opengl-tutorial.org/beginners-tutorials/tutorial-2-the-first-triangle/