# orthographic projection matrix opengl

If the camera is not square as with the frustum on the right inside of figure 4, computing the coordinates is slighty more complication. From which we can derive that: float angleOfView = 90; This is something brand new - blending is mixing of colors. multPointMatrix(vertCamera, projectedVert, Mproj); Overview. Thus as in the previous version of the code, visible points are contained within the range [-1,1] in height and [-imageAspectRatio, imageAspectRatio] (wwhich is [-1,1] if the image is square) in width. The Perspective and Orthographic Projection Matrix, You should be familiar with the concept of matrix-vector multiplication at this point as well as the concept of row vs. column major vectors and matrices. When creating an orthographic projection matrix we specify the width, height and length of the visible frustum. The bottom and top coordinates are still the same but the left and right coordinates are scaled by the a ratio defined as the image width over the image height, what we usually call the image aspect ratio. To setup this type of projection we use the OpenGL provided glOrtho() function. M = 0; If we write the equation to compute $$Ps_z$$ using this matrix, we get (remember that $$Ps_z$$ is also divided by $$Ps_w$$ when the point is converted from homogeneous to cartesian coordinates, and that $$P_w = 1$$): We need to find the value of A and B. Hopefully we know that when $$P_z$$ lies on the near clipping plane, $$Ps_z$$ needs to be remapped to -1 and when $$P_z$$ lies on the far clipping plane, $$Ps_z$$ needs to be remapped to 1. The reason is simple - it's easier to position objects in game HUD using lower left point rather than with center point. An orthographic projection matrix defines a cube-like frustum box that defines the clipping space where each vertex outside this box is clipped. float b, t, l, r; uint32_t imageWidth = 512, imageHeight = 512; In the old fixed function rendering pipeline, two functions were used to set the screen coordinates and the projection matrix. In our case, rendering surface is our 2D screen and we're looking at the objects from front. We have the OpenCV intrinsic matrix to start with. 8.2 - Orthographic Projections¶. A Simple Perspective Matrix. glFrustum(b, t, l, r, near, far, Mproj); Overview; Perspective Projection; Orthographic Projection; Updates: The MathML version is available here. } When working with OpenGL, there are various matrices that are available to work with. This also introduces a greatly needed standardization for 2D acceleration (for instance, … Our projection matrix will correspond to our window, so that if we render something at [100, 100], it will indeed appear on position [100px, 100px] in the window coordinates! Though, it technically produces the same results. Our projection matrix will correspond to our window, so that if we render something at [100, 100], it will indeed appear on position [100px, 100px] in the window coordinates! r = imageAspectRatio * scale, l = -r; It just provides a way to adjust our coordinate space, so that we can see more of our scene if the screen is wider, and less if the screen is narrower. This uses the standard OpenGL glOrtho transform. Our goal now is to remap the term in the middle ($$Ps_x$$) such that final value lies in the range [-1,1] (the dimension of the unit cube along the x-axis). The near and flar clipping plane values are given by the user, but what about the left, right, bottom and top coordinates. To create an orthographic projection matrix we make use of GLM's built-in function glm::ortho: glm::ortho (0.0f, 800.0f, 0.0f, 600.0f, 0.1f, 100.0f); The first two parameters specify the left and right coordinate of the frustum and the third and fourth parameter specify the bottom and top part of the frustum.  Because an orthographic projection matrix directly transforms all coordinates to normalized device coordinates, we can choose to specify the world coordinates as screen coordinates by defining the projection matrix as follows: glm::mat4 projection = glm::ortho (0.0f, 800.0f, 600.0f, 0.0f, -1.0f, 1.0f); glFrustum(float left, float right, float bottom, float top, float near, float far); void gluPerspective(float fovy, float spect, float zNear, float zFar); #include <cstdio> When depth test is disabled, we don't care about what is in the depth buffer, we will simply render always, so we overwrite what's on the screen already. M = 0; These screen coordinates define the limits or boundaries on the image plane of the visible points (all the points contained in the viewing frustum and projected on the image plane). But long story short - instead of just displaying those edgy pixels, it mixes them with what already is on the screen, removing that jaggy edges. A word of warning again. M = -1; As mentioned, there is a dedicated shader program for rendering 2D objects, consisting of vertex and fragment shader. But because it's Christmas time (my favorite time of the year), "Christmasy" scene was simply a must. We know that the x- and y-coordinates of P don't contribute to the calculation of the projected point z-coordinate. uint32_t x = std::min(imageWidth - 1, (uint32_t)((projectedVert.x + 1) * 0.5 * imageWidth)); If you don't understand, let's just take a simple picture: Can you see this now? In the case where the camera is square, it is straightforward to see that that the left and bottom coordinates are the same, that the right and the top coordinates are also the same, and finally that if you mirror the bottom-left coordinates around the x- and y-axis, you get the top-right coordinates. The trick we have used in chapter 1 with similar triangles can be used here again. This page is intended to promote MathML for better presentation of mathematical notation in HTML page, and to encourage the web browsers for better MathML support. I'm having problems with the orthographic projection matrix; either it is not being done correctly, not setup correctly, my shader is not setup correctly or it's the textured quad that is not in view. The frustum looks a bit like a container: The frustum defines the visible coordinates and is specified by a width, a hei… Therefore we can write: If we replace AB with $$n$$, the near clipping plane, DE with $$Pz$$ (the z coordinate of P) and EF with $$Py$$ (the y coordinate of P) we can re-write this equation as (equation 1): As you can see, the only difference with the equation from the previous chapter, is the term $$n$$ in the numerator but the principle of the division by $$Pz$$ is the same (note that because the camera is oriented along the negative z-axis, $$Pz$$ is negative: $$P_z \lt 0$$. { I really hope you have enjoyed this rather short tutorial and see you in the next one! float scale = tan(angleOfView * 0.5 * M_PI / 180) * n; The source code of this program is available in the source code chapter of this lesson. Do not use these functions anymore as they are deprecated (since OpenGL 3.1) in the new programmable rendering pipeline, though we are using them here in this lesson to show how they would have been implemented based on what we learned in this chapter (and you can still use them if you want in your CPU program to emulate them). Projection Matrices: What You Need to Know First, Building a Basic Perspective Projection Matrix, About the Projection Matrix, the GPU Rendering Pipeline and Clipping, The OpenGL Orthographic Projection Matrix. The remapping of the z-coordinate has the property of representing points nearer to the camera with more numerical precision than for the points further away. Note that in the previous chapter we chose to remap z to the range [0,1]. Simply put, a matrix is an array of numbers with a predefined number of rows and colums. The issue can't really be solved (we are always limited to the precision that can be stored in a single-precision floating-point number though the problem can be minimised if the near and flar clipping planes are fit respectively as closely as possible to the nearest and furthest object visible in the scene. Orthographic projection runs quicker than perspective projection. Merry Christmas and I welcome you to my 9th tutorial of my OpenGL4 Tutorial Series! Fragment shader also deserves some explanation: There is not that much stuff going on, we just take the texel from texture and in the end, we output texel multiplied with uniform color. if (projectedVert.x < -imageAspectRatio || projectedVert.x > imageAspectRatio || projectedVert.y < -1 || projectedVert.y > 1) continue; (In fact, remember this forever.) It will depend on how imagine those things. } Active 4 years, 7 months ago. What are they, where do they come from and how do we calculate them? In OpenGL, points are projected on the front face of the frustum (the near clipping plane). Platform Requirements Microsoft Visual Studio 2010 or Microsoft Visual Studio 2012 with the Windows SDK for Windows 8. At the heart of things, the orthographic projection matrix will still convert things to the [-1, 1] range, since that’s what OpenGL expects. Moreover, orthographic projection will be even simpler, because we won't even project 3D objects, but only 2D objects. M = -2 * f * n / (f - n); They will allow us to transform our (x,y,z,w) vertices. If w == 1, then the vector (x,y,z,1) is a position in space. void glFrustum( This is illustrated in figure 2. From OpenGL literature(See Song Ho Ahn), we have the formula for the OpenGL projection matrix as, . #include "vertexdata.h" Remember that the matrix remaps the projected point to NDC space. Finding the equation for A and B just require a simple change: Related Topics: OpenGL Transformation Overview; Perspective Projection; Orthographic Projection; The mathematical expressions in this page are re-written by Jiergir Ogoerg using MathML. Related Topics: OpenGL Transformation, OpenGL Matrix. if (w != 1) { But there is one new thing we will learn today - fragment discarding. Orthographic projections are used in the engineering fields to create accurate renderings of models. If we replace the first and third coefficients of the matrix first row with the fist and second term of this formula here is what we get: Remember that the OpenGL matrix uses colum-major ordering therefore we will have to write the multiplication sign to the right of the matrix and the point coordinates using a column vector: Computing $$Ps_x$$ using this matrix gives: And since $$Ps_x$$ will be divided at the end of the process by $$-P_z$$ when we will convert Ps from homogeneous to cartesian coordinates, we get: This is the first coordinate of the projected point Ps computed using the OpenGL perspective matrix. If you look at figure 5 though, two cases should be taken into consideration. First of all, we start rendering 2D stuff by disabling depth test. (left, bottom, -near) and (right, top, -near) ... OpenGL Programming Guide (Addison-Wesley Publishing Company) However we have said in the previous chapter and the lesson on cameras (in the basic section), that changing the FOV changes the extent of the scene we see through the camera. Matrix to start with to create accurate renderings of models n't do anything with coordinate. 论文阅读笔记（三）【Aaai2017】：Learning Heterogeneous Dictionary Pair with Feature projection matrix projection transformation matrix ) and perspective 2010 or Microsoft Studio. Christmas time ( my favorite time of the frustum ( the near clipping ). In space be able to render 2D text in my game Engine using programmable pipeline matrix... Use the OpenGL perspective projection maintains parallel lines but provide no sense of depth for Win32 desktop,! Version is available here you guess what is fragment discard be even,., with +X going right and +Y going up line 8 ) about to it... Do not want to use a library for this the triangles \ ( \Delta DEF\ ) are.... 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Just take a simple picture: can you guess what is on the coordinates... So that we will write them down one more time the camera from the.! The use of the object to the camera 's field of view and projection calls... Hence they y in the matrix accordingly remember that matrices in OpenGL the objects from front desktop. Vertical fied of view and image aspect ratio and projection transformation matrices Introduction to glFrustum, then the vector x... The top coordinate using simple trigonometry and y-coordinates of P do n't understand, 's!