Sign in to follow this  
Danicco

OpenGL Mapping OpenGL Coordinates to Screen Pixels

Recommended Posts

How can I map the coordinates from an OpenGL window to a window's pixels?

 

For example, say I've created a 1920x1080 window, and I want to draw a certain object exactly at the pixel 360, how can I get the OpenGL coordinate value?

 

Also, is this even a feasible scenario? I'm coding the coordinate positioning of a bunch of objects and I want to add this option so I can, for example, do something like positioning a player's menu bar at the pixel 10 of the screen, and his inventory icon at the (width - 10).

 

Edit: Modern OpenGL, so I don't want to use glOrtho().

Edited by Danicco

Share this post


Link to post
Share on other sites

You can construct your own ortho matrix and load it, then it's just a simple matter of multiplying input position by ortho matrix in your vertex shader.  The documentation page for glOrtho tells you how to construct the matrix (scroll down) or you can use a matrix library to do it for you.

Share this post


Link to post
Share on other sites

If you don't want to switch from a perspective projection to a orthographic projection for the drawing of 2d elements you'll need to crunch the numbers in your projection matrix to generate a transform that converts screen coordinates to view space coordinates.  Note that it doesn't make a lot of sense to use a perspective transform to render 2d screen elements with pixel perfect alignment.  If you are dead set on a perspective transform...

// Transforming a view space point (vx, vy, vz, 1) into clip space looks like this:
|sx  0  0  0||vx|   |sx * vx     |
| 0 sy  0  0||vy| = |sx * vy     |
| 0  0 sz tz||vz|   |sz * vz + tz|
| 0  0 -1  0|| 1|   |-vz         |

// The corresponding NCD values are computed as follows:
|nx|   | (sx * vx)      / -vz |
|nx| = | (sy * vy)      / -vz |
|nz|   | (sz * vz + tz) / -vz |

// You want to go from NDC (nx, ny, nz) to view space (vx, vy, vz)
// Fortunately this is pretty simple...
vz = -tz / (nz - sz)  <-- Evaluate this first!
vx = (-vz * nx) / sx
vy = (-vz * ny) / sy
// Note that (sx, sy, sz, tz) were all pulled from your perspective transform.

Converting a pixel coordinate to NDC is a simple matter scaling and biasing one interval into another...

Range of x values in NDC:  [-1, 1]  (1 is the right of the screen and -1 is the left)

Range of y values in NDC:  [-1, 1]  (1 is the top of the screen and -1 is the bottom)

Range of x values in pixels [0, ScreenWidth-1]

Range of y values in pixels: [0, ScreenHeight-1]

 

I'm sure you can figure that transform out yourself. smile.png

Edited by nonoptimalrobot

Share this post


Link to post
Share on other sites

...forgot two things.

 

1) Here is how a perspective transform is constructed for OpenGL:  http://www.songho.ca/opengl/gl_projectionmatrix.html.  You will want to consult this while diagnosing bugs in your math.

 

2) The way UV coordinates are used to address pixels in a texture is not always the same as the way NDC values are used to address pixels on the screen.  I believe DirectX 11 finally made these two addressing modes consistent so the interval [-1, 1] addresses pixels on screen in the exact same way the interval [0, 1] addresses pixels in an identically sized texture.  I'm not sure what iteration of OpenGL fixed this inconsistency if it happened at all.  Search for "mapping texels to pixels" to figure out how to rectify the different addressing modes for whatever version of OpenGL you are using.

Share this post


Link to post
Share on other sites


The way UV coordinates are used to address pixels in a texture is not always the same as the way NDC values are used to address pixels on the screen.  I believe DirectX 11 finally made these two addressing modes consistent so the interval [-1, 1] addresses pixels on screen in the exact same way the interval [0, 1] addresses pixels in an identically sized texture.  I'm not sure what iteration of OpenGL fixed this inconsistency if it happened at all. 

In a sensible API (DX10 / DX11 / GL), texture coordinates and screen coordinates should work the same way, except that NDC is from [-1,1] and textures from [0,1]

uv = ndc * 0.5 + 0.5;

pixel_Index = clamp( round(uv * num_Pixels - 0.5), 0, num_Pixels-1 );

 

On D3D9, the definition of pixel coordinates is stupidly shifted so that the centre of the top-left pixel lines up perfectly with the top-left edge of the screen. i.e. all the pixels are shifted by half a pixel in that direction, so you need:

uv = ndc * 0.5 + 0.5 + 0.5/num_Pixels;

 

GL's only stupidity in this regard is that z also ranges from -1 to +1, instead of from 0 to 1, which has no impact in this situation wink.png


Also, is this even a feasible scenario? I'm coding the coordinate positioning of a bunch of objects and I want to add this option so I can, for example, do something like positioning a player's menu bar at the pixel 10 of the screen, and his inventory icon at the (width - 10).
Ignoring projection matrices, the screen is addressed in NDC (normalized device coordinates), which range from -1 to 1.

i.e. a vertex at x=-1 will be on the left hand edge of the screen, and a vertex at x=1 will be on the right hand edge of the screen. 

 

Say the screen is 1280 pixels wide -- that's pixel #0 to pixel #1279.

The left edge of pixel #0 corresponds to an NDC value of -1. The right edge of pixel #1279 corresponds to an NDC value of -1 (this is also the left edge of imaginary pixel #1280).

 

If you want a shape to cover the pixels from #10 to #20, first calculate the size of a pixel. NDC is 2 units across, but our "pixel" coordinates are 1280 units across. Therefore one pixel is 2/1280 NDC units wide.

The left edge is -1, and we want to the coordinates to a point 10 pixels right of that, and then another 10 pixels right.

p1 = -1 + 2/1280 * 10

p2 = -1 + 2/1280 * 20

 

If you use an ortho matrix, it will just be doing this translation (by -1) and scaling (by 2/1280) for you cool.png

Share this post


Link to post
Share on other sites

On D3D9, the definition of pixel coordinates is stupidly shifted so that the centre of the top-left pixel lines up perfectly with the top-left edge of the screen. i.e. all the pixels are shifted by half a pixel in that direction, so you need:

uv = ndc * 0.5 + 0.5 + 0.5/num_Pixels;

 

GL's only stupidity in this regard is that z also ranges from -1 to +1, instead of from 0 to 1, which has no impact in this situation wink.png

 

OpenGL never had this problem!?  Sigh.  I've been using the wrong API all these years...

Share this post


Link to post
Share on other sites

If you want a shape to cover the pixels from #10 to #20, first calculate the size of a pixel. NDC is 2 units across, but our "pixel" coordinates are 1280 units across. Therefore one pixel is 2/1280 NDC units wide.

The left edge is -1, and we want to the coordinates to a point 10 pixels right of that, and then another 10 pixels right.

p1 = -1 + 2/1280 * 10

p2 = -1 + 2/1280 * 20

 

If you use an ortho matrix, it will just be doing this translation (by -1) and scaling (by 2/1280) for you cool.png

 

That's what I did, it seemed way easier than dealing with matrices again (ugh!).

 

I have to recalculate the pixel size every time the screen size changes, but that's minor.

It took me some time and tries to get it correctly though, I even had some ifs to check the region portion of the screen before I noticed I just had to subtract the value from 1...

 

What my question was about is, I'd like to know how developers deal with images and different screen ratios/sizes.

For example, if they do this sort of calculation to make the image appear exactly as the original resource (same pixel W * H) or adjust it to the screen to show, for example, between space -1 to 0.5 (25% of the screen).

 

I think I've seen some games that when I change the resolution to something unusual the images do appear distorted as well, so I wasn't sure if showing the image's exact pixel size might cause some trouble later on.

 

Anyway, thank you very much for the replies, with this my UI code is nearly finished!

Edited by Danicco

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Forum Statistics

    • Total Topics
      628275
    • Total Posts
      2981760
  • Similar Content

    • By mellinoe
      Hi all,
      First time poster here, although I've been reading posts here for quite a while. This place has been invaluable for learning graphics programming -- thanks for a great resource!
      Right now, I'm working on a graphics abstraction layer for .NET which supports D3D11, Vulkan, and OpenGL at the moment. I have implemented most of my planned features already, and things are working well. Some remaining features that I am planning are Compute Shaders, and some flavor of read-write shader resources. At the moment, my shaders can just get simple read-only access to a uniform (or constant) buffer, a texture, or a sampler. Unfortunately, I'm having a tough time grasping the distinctions between all of the different kinds of read-write resources that are available. In D3D alone, there seem to be 5 or 6 different kinds of resources with similar but different characteristics. On top of that, I get the impression that some of them are more or less "obsoleted" by the newer kinds, and don't have much of a place in modern code. There seem to be a few pivots:
      The data source/destination (buffer or texture) Read-write or read-only Structured or unstructured (?) Ordered vs unordered (?) These are just my observations based on a lot of MSDN and OpenGL doc reading. For my library, I'm not interested in exposing every possibility to the user -- just trying to find a good "middle-ground" that can be represented cleanly across API's which is good enough for common scenarios.
      Can anyone give a sort of "overview" of the different options, and perhaps compare/contrast the concepts between Direct3D, OpenGL, and Vulkan? I'd also be very interested in hearing how other folks have abstracted these concepts in their libraries.
    • By aejt
      I recently started getting into graphics programming (2nd try, first try was many years ago) and I'm working on a 3d rendering engine which I hope to be able to make a 3D game with sooner or later. I have plenty of C++ experience, but not a lot when it comes to graphics, and while it's definitely going much better this time, I'm having trouble figuring out how assets are usually handled by engines.
      I'm not having trouble with handling the GPU resources, but more so with how the resources should be defined and used in the system (materials, models, etc).
      This is my plan now, I've implemented most of it except for the XML parts and factories and those are the ones I'm not sure of at all:
      I have these classes:
      For GPU resources:
      Geometry: holds and manages everything needed to render a geometry: VAO, VBO, EBO. Texture: holds and manages a texture which is loaded into the GPU. Shader: holds and manages a shader which is loaded into the GPU. For assets relying on GPU resources:
      Material: holds a shader resource, multiple texture resources, as well as uniform settings. Mesh: holds a geometry and a material. Model: holds multiple meshes, possibly in a tree structure to more easily support skinning later on? For handling GPU resources:
      ResourceCache<T>: T can be any resource loaded into the GPU. It owns these resources and only hands out handles to them on request (currently string identifiers are used when requesting handles, but all resources are stored in a vector and each handle only contains resource's index in that vector) Resource<T>: The handles given out from ResourceCache. The handles are reference counted and to get the underlying resource you simply deference like with pointers (*handle).  
      And my plan is to define everything into these XML documents to abstract away files:
      Resources.xml for ref-counted GPU resources (geometry, shaders, textures) Resources are assigned names/ids and resource files, and possibly some attributes (what vertex attributes does this geometry have? what vertex attributes does this shader expect? what uniforms does this shader use? and so on) Are reference counted using ResourceCache<T> Assets.xml for assets using the GPU resources (materials, meshes, models) Assets are not reference counted, but they hold handles to ref-counted resources. References the resources defined in Resources.xml by names/ids. The XMLs are loaded into some structure in memory which is then used for loading the resources/assets using factory classes:
      Factory classes for resources:
      For example, a texture factory could contain the texture definitions from the XML containing data about textures in the game, as well as a cache containing all loaded textures. This means it has mappings from each name/id to a file and when asked to load a texture with a name/id, it can look up its path and use a "BinaryLoader" to either load the file and create the resource directly, or asynchronously load the file's data into a queue which then can be read from later to create the resources synchronously in the GL context. These factories only return handles.
      Factory classes for assets:
      Much like for resources, these classes contain the definitions for the assets they can load. For example, with the definition the MaterialFactory will know which shader, textures and possibly uniform a certain material has, and with the help of TextureFactory and ShaderFactory, it can retrieve handles to the resources it needs (Shader + Textures), setup itself from XML data (uniform values), and return a created instance of requested material. These factories return actual instances, not handles (but the instances contain handles).
       
       
      Is this a good or commonly used approach? Is this going to bite me in the ass later on? Are there other more preferable approaches? Is this outside of the scope of a 3d renderer and should be on the engine side? I'd love to receive and kind of advice or suggestions!
      Thanks!
    • By nedondev
      I 'm learning how to create game by using opengl with c/c++ coding, so here is my fist game. In video description also have game contain in Dropbox. May be I will make it better in future.
      Thanks.
    • By Abecederia
      So I've recently started learning some GLSL and now I'm toying with a POM shader. I'm trying to optimize it and notice that it starts having issues at high texture sizes, especially with self-shadowing.
      Now I know POM is expensive either way, but would pulling the heightmap out of the normalmap alpha channel and in it's own 8bit texture make doing all those dozens of texture fetches more cheap? Or is everything in the cache aligned to 32bit anyway? I haven't implemented texture compression yet, I think that would help? But regardless, should there be a performance boost from decoupling the heightmap? I could also keep it in a lower resolution than the normalmap if that would improve performance.
      Any help is much appreciated, please keep in mind I'm somewhat of a newbie. Thanks!
    • By test opty
      Hi,
      I'm trying to learn OpenGL through a website and have proceeded until this page of it. The output is a simple triangle. The problem is the complexity.
      I have read that page several times and tried to analyse the code but I haven't understood the code properly and completely yet. This is the code:
       
      #include <glad/glad.h> #include <GLFW/glfw3.h> #include <C:\Users\Abbasi\Desktop\std_lib_facilities_4.h> using namespace std; //****************************************************************************** void framebuffer_size_callback(GLFWwindow* window, int width, int height); void processInput(GLFWwindow *window); // settings const unsigned int SCR_WIDTH = 800; const unsigned int SCR_HEIGHT = 600; const char *vertexShaderSource = "#version 330 core\n" "layout (location = 0) in vec3 aPos;\n" "void main()\n" "{\n" " gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n" "}\0"; const char *fragmentShaderSource = "#version 330 core\n" "out vec4 FragColor;\n" "void main()\n" "{\n" " FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n" "}\n\0"; //******************************* int main() { // glfw: initialize and configure // ------------------------------ glfwInit(); glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3); glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3); glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE); // glfw window creation GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "My First Triangle", nullptr, nullptr); if (window == nullptr) { cout << "Failed to create GLFW window" << endl; glfwTerminate(); return -1; } glfwMakeContextCurrent(window); glfwSetFramebufferSizeCallback(window, framebuffer_size_callback); // glad: load all OpenGL function pointers if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress)) { cout << "Failed to initialize GLAD" << endl; return -1; } // build and compile our shader program // vertex shader int vertexShader = glCreateShader(GL_VERTEX_SHADER); glShaderSource(vertexShader, 1, &vertexShaderSource, nullptr); glCompileShader(vertexShader); // check for shader compile errors int success; char infoLog[512]; glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success); if (!success) { glGetShaderInfoLog(vertexShader, 512, nullptr, infoLog); cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << endl; } // fragment shader int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER); glShaderSource(fragmentShader, 1, &fragmentShaderSource, nullptr); glCompileShader(fragmentShader); // check for shader compile errors glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success); if (!success) { glGetShaderInfoLog(fragmentShader, 512, nullptr, infoLog); cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << endl; } // link shaders int shaderProgram = glCreateProgram(); glAttachShader(shaderProgram, vertexShader); glAttachShader(shaderProgram, fragmentShader); glLinkProgram(shaderProgram); // check for linking errors glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success); if (!success) { glGetProgramInfoLog(shaderProgram, 512, nullptr, infoLog); cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << endl; } glDeleteShader(vertexShader); glDeleteShader(fragmentShader); // set up vertex data (and buffer(s)) and configure vertex attributes float vertices[] = { -0.5f, -0.5f, 0.0f, // left 0.5f, -0.5f, 0.0f, // right 0.0f, 0.5f, 0.0f // top }; unsigned int VBO, VAO; glGenVertexArrays(1, &VAO); glGenBuffers(1, &VBO); // bind the Vertex Array Object first, then bind and set vertex buffer(s), //and then configure vertex attributes(s). glBindVertexArray(VAO); glBindBuffer(GL_ARRAY_BUFFER, VBO); glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW); glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0); glEnableVertexAttribArray(0); // note that this is allowed, the call to glVertexAttribPointer registered VBO // as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind glBindBuffer(GL_ARRAY_BUFFER, 0); // You can unbind the VAO afterwards so other VAO calls won't accidentally // modify this VAO, but this rarely happens. Modifying other // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind // VAOs (nor VBOs) when it's not directly necessary. glBindVertexArray(0); // uncomment this call to draw in wireframe polygons. //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE); // render loop while (!glfwWindowShouldClose(window)) { // input // ----- processInput(window); // render // ------ glClearColor(0.2f, 0.3f, 0.3f, 1.0f); glClear(GL_COLOR_BUFFER_BIT); // draw our first triangle glUseProgram(shaderProgram); glBindVertexArray(VAO); // seeing as we only have a single VAO there's no need to // bind it every time, but we'll do so to keep things a bit more organized glDrawArrays(GL_TRIANGLES, 0, 3); // glBindVertexArray(0); // no need to unbind it every time // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.) glfwSwapBuffers(window); glfwPollEvents(); } // optional: de-allocate all resources once they've outlived their purpose: glDeleteVertexArrays(1, &VAO); glDeleteBuffers(1, &VBO); // glfw: terminate, clearing all previously allocated GLFW resources. glfwTerminate(); return 0; } //************************************************** // process all input: query GLFW whether relevant keys are pressed/released // this frame and react accordingly void processInput(GLFWwindow *window) { if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS) glfwSetWindowShouldClose(window, true); } //******************************************************************** // glfw: whenever the window size changed (by OS or user resize) this callback function executes void framebuffer_size_callback(GLFWwindow* window, int width, int height) { // make sure the viewport matches the new window dimensions; note that width and // height will be significantly larger than specified on retina displays. glViewport(0, 0, width, height); } As you see, about 200 lines of complicated code only for a simple triangle. 
      I don't know what parts are necessary for that output. And also, what the correct order of instructions for such an output or programs is, generally. That start point is too complex for a beginner of OpenGL like me and I don't know how to make the issue solved. What are your ideas please? What is the way to figure both the code and the whole program out correctly please?
      I wish I'd read a reference that would teach me OpenGL through a step-by-step method. 
  • Popular Now