opengl draw triangle mesh opengl draw triangle mesh

Share Improve this answer Follow answered Nov 3, 2011 at 23:09 Nicol Bolas 434k 63 748 953 Everything we did the last few million pages led up to this moment, a VAO that stores our vertex attribute configuration and which VBO to use. The width / height configures the aspect ratio to apply and the final two parameters are the near and far ranges for our camera. At the moment our ast::Vertex class only holds the position of a vertex, but in the future it will hold other properties such as texture coordinates. We spent valuable effort in part 9 to be able to load a model into memory, so lets forge ahead and start rendering it. Opengles mixing VBO and non VBO renders gives EXC_BAD_ACCESS, Fastest way to draw many textured quads in OpenGL 3+, OpenGL glBufferData with data from a pointer. A vertex array object stores the following: The process to generate a VAO looks similar to that of a VBO: To use a VAO all you have to do is bind the VAO using glBindVertexArray. Lets dissect this function: We start by loading up the vertex and fragment shader text files into strings. For more information on this topic, see Section 4.5.2: Precision Qualifiers in this link: https://www.khronos.org/files/opengles_shading_language.pdf. #else Clipping discards all fragments that are outside your view, increasing performance. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. You can see that we create the strings vertexShaderCode and fragmentShaderCode to hold the loaded text content for each one. You will need to manually open the shader files yourself. Note: I use color in code but colour in editorial writing as my native language is Australian English (pretty much British English) - its not just me being randomly inconsistent! Once a shader program has been successfully linked, we no longer need to keep the individual compiled shaders, so we detach each compiled shader using the glDetachShader command, then delete the compiled shader objects using the glDeleteShader command. size You should also remove the #include "../../core/graphics-wrapper.hpp" line from the cpp file, as we shifted it into the header file. You will get some syntax errors related to functions we havent yet written on the ast::OpenGLMesh class but well fix that in a moment: The first bit is just for viewing the geometry in wireframe mode so we can see our mesh clearly. As usual, the result will be an OpenGL ID handle which you can see above is stored in the GLuint bufferId variable. The glBufferData command tells OpenGL to expect data for the GL_ARRAY_BUFFER type. This is something you can't change, it's built in your graphics card. Simply hit the Introduction button and you're ready to start your journey! In our rendering code, we will need to populate the mvp uniform with a value which will come from the current transformation of the mesh we are rendering, combined with the properties of the camera which we will create a little later in this article. This is also where you'll get linking errors if your outputs and inputs do not match. GLSL has some built in functions that a shader can use such as the gl_Position shown above. We do this with the glBufferData command. OpenGL doesn't simply transform all your 3D coordinates to 2D pixels on your screen; OpenGL only processes 3D coordinates when they're in a specific range between -1.0 and 1.0 on all 3 axes ( x, y and z ). To use the recently compiled shaders we have to link them to a shader program object and then activate this shader program when rendering objects. glDrawArrays () that we have been using until now falls under the category of "ordered draws". In this example case, it generates a second triangle out of the given shape. This, however, is not the best option from the point of view of performance. If you managed to draw a triangle or a rectangle just like we did then congratulations, you managed to make it past one of the hardest parts of modern OpenGL: drawing your first triangle. The main difference compared to the vertex buffer is that we wont be storing glm::vec3 values but instead uint_32t values (the indices). An attribute field represents a piece of input data from the application code to describe something about each vertex being processed. It is advised to work through them before continuing to the next subject to make sure you get a good grasp of what's going on. A vertex buffer object is our first occurrence of an OpenGL object as we've discussed in the OpenGL chapter. Run your application and our cheerful window will display once more, still with its green background but this time with our wireframe crate mesh displaying! I love StackOverflow <3, How Intuit democratizes AI development across teams through reusability. The geometry shader is optional and usually left to its default shader. The current vertex shader is probably the most simple vertex shader we can imagine because we did no processing whatsoever on the input data and simply forwarded it to the shader's output. The second argument specifies how many strings we're passing as source code, which is only one. If the result was unsuccessful, we will extract any logging information from OpenGL, log it through own own logging system, then throw a runtime exception. The Orange County Broadband-Hamnet/AREDN Mesh Organization is a group of Amateur Radio Operators (HAMs) who are working together to establish a synergistic TCP/IP based mesh of nodes in the Orange County (California) area and neighboring counties using commercial hardware and open source software (firmware) developed by the Broadband-Hamnet and AREDN development teams. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Mesh#include "Mesh.h" glext.hwglext.h#include "Scene.h" . Shaders are written in the OpenGL Shading Language (GLSL) and we'll delve more into that in the next chapter. The header doesnt have anything too crazy going on - the hard stuff is in the implementation. #if TARGET_OS_IPHONE They are very simple in that they just pass back the values in the Internal struct: Note: If you recall when we originally wrote the ast::OpenGLMesh class I mentioned there was a reason we were storing the number of indices. Since each vertex has a 3D coordinate we create a vec3 input variable with the name aPos. At this point we will hard code a transformation matrix but in a later article Ill show how to extract it out so each instance of a mesh can have its own distinct transformation. OpenGL has no idea what an ast::Mesh object is - in fact its really just an abstraction for our own benefit for describing 3D geometry. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. The Internal struct holds a projectionMatrix and a viewMatrix which are exposed by the public class functions. Smells like we need a bit of error handling - especially for problems with shader scripts as they can be very opaque to identify: Here we are simply asking OpenGL for the result of the GL_COMPILE_STATUS using the glGetShaderiv command. Its also a nice way to visually debug your geometry. Usually when you have multiple objects you want to draw, you first generate/configure all the VAOs (and thus the required VBO and attribute pointers) and store those for later use. In OpenGL everything is in 3D space, but the screen or window is a 2D array of pixels so a large part of OpenGL's work is about transforming all 3D coordinates to 2D pixels that fit on your screen. #include #include "../../core/internal-ptr.hpp" For more information see this site: https://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices. // Activate the 'vertexPosition' attribute and specify how it should be configured. Spend some time browsing the ShaderToy site where you can check out a huge variety of example shaders - some of which are insanely complex. It will offer the getProjectionMatrix() and getViewMatrix() functions which we will soon use to populate our uniform mat4 mvp; shader field. Chapter 4-The Render Class Chapter 5-The Window Class 2D-Specific Tutorials In more modern graphics - at least for both OpenGL and Vulkan - we use shaders to render 3D geometry. The third parameter is the actual source code of the vertex shader and we can leave the 4th parameter to NULL. We use three different colors, as shown in the image on the bottom of this page. Heres what we will be doing: I have to be honest, for many years (probably around when Quake 3 was released which was when I first heard the word Shader), I was totally confused about what shaders were. The advantage of using those buffer objects is that we can send large batches of data all at once to the graphics card, and keep it there if there's enough memory left, without having to send data one vertex at a time. Instead we are passing it directly into the constructor of our ast::OpenGLMesh class for which we are keeping as a member field. Below you'll find the source code of a very basic vertex shader in GLSL: As you can see, GLSL looks similar to C. Each shader begins with a declaration of its version. This is a precision qualifier and for ES2 - which includes WebGL - we will use the mediump format for the best compatibility. Edit the opengl-mesh.cpp implementation with the following: The Internal struct is initialised with an instance of an ast::Mesh object. In modern OpenGL we are required to define at least a vertex and fragment shader of our own (there are no default vertex/fragment shaders on the GPU). Just like a graph, the center has coordinates (0,0) and the y axis is positive above the center. When linking the shaders into a program it links the outputs of each shader to the inputs of the next shader. Thanks for contributing an answer to Stack Overflow! Why is this sentence from The Great Gatsby grammatical? If the result is unsuccessful, we will extract whatever error logging data might be available from OpenGL, print it through our own logging system then deliberately throw a runtime exception. Newer versions support triangle strips using glDrawElements and glDrawArrays . We can do this by inserting the vec3 values inside the constructor of vec4 and set its w component to 1.0f (we will explain why in a later chapter). You could write multiple shaders for different OpenGL versions but frankly I cant be bothered for the same reasons I explained in part 1 of this series around not explicitly supporting OpenGL ES3 due to only a narrow gap between hardware that can run OpenGL and hardware that can run Vulkan. Instruct OpenGL to starting using our shader program. Once you do get to finally render your triangle at the end of this chapter you will end up knowing a lot more about graphics programming. You can read up a bit more at this link to learn about the buffer types - but know that the element array buffer type typically represents indices: https://www.khronos.org/registry/OpenGL-Refpages/es1.1/xhtml/glBindBuffer.xml. And vertex cache is usually 24, for what matters. glBufferDataARB(GL . Eventually you want all the (transformed) coordinates to end up in this coordinate space, otherwise they won't be visible. #include "../../core/assets.hpp" Subsequently it will hold the OpenGL ID handles to these two memory buffers: bufferIdVertices and bufferIdIndices. We can declare output values with the out keyword, that we here promptly named FragColor. Here is the link I provided earlier to read more about them: https://www.khronos.org/opengl/wiki/Vertex_Specification#Vertex_Buffer_Object. First up, add the header file for our new class: In our Internal struct, add a new ast::OpenGLPipeline member field named defaultPipeline and assign it a value during initialisation using "default" as the shader name: Run your program and ensure that our application still boots up successfully. We do this by creating a buffer: The vertex shader is one of the shaders that are programmable by people like us. Note: Setting the polygon mode is not supported on OpenGL ES so we wont apply it unless we are not using OpenGL ES. The activated shader program's shaders will be used when we issue render calls. We will name our OpenGL specific mesh ast::OpenGLMesh. but they are bulit from basic shapes: triangles. The problem is that we cant get the GLSL scripts to conditionally include a #version string directly - the GLSL parser wont allow conditional macros to do this. A shader program is what we need during rendering and is composed by attaching and linking multiple compiled shader objects. Assimp . No. The moment we want to draw one of our objects, we take the corresponding VAO, bind it, then draw the object and unbind the VAO again. #include "../../core/internal-ptr.hpp" As input to the graphics pipeline we pass in a list of three 3D coordinates that should form a triangle in an array here called Vertex Data; this vertex data is a collection of vertices. Recall that our basic shader required the following two inputs: Since the pipeline holds this responsibility, our ast::OpenGLPipeline class will need a new function to take an ast::OpenGLMesh and a glm::mat4 and perform render operations on them. . The glDrawElements function takes its indices from the EBO currently bound to the GL_ELEMENT_ARRAY_BUFFER target. We define them in normalized device coordinates (the visible region of OpenGL) in a float array: Because OpenGL works in 3D space we render a 2D triangle with each vertex having a z coordinate of 0.0. For desktop OpenGL we insert the following for both the vertex and shader fragment text: For OpenGL ES2 we insert the following for the vertex shader text: Notice that the version code is different between the two variants, and for ES2 systems we are adding the precision mediump float;. I had authored a top down C++/OpenGL helicopter shooter as my final student project for the multimedia course I was studying (it was named Chopper2k) I dont think I had ever heard of shaders because OpenGL at the time didnt require them. If our application is running on a device that uses desktop OpenGL, the version lines for the vertex and fragment shaders might look like these: However, if our application is running on a device that only supports OpenGL ES2, the versions might look like these: Here is a link that has a brief comparison of the basic differences between ES2 compatible shaders and more modern shaders: https://github.com/mattdesl/lwjgl-basics/wiki/GLSL-Versions. The fragment shader is all about calculating the color output of your pixels. The left image should look familiar and the right image is the rectangle drawn in wireframe mode. Specifies the size in bytes of the buffer object's new data store. This article will cover some of the basic steps we need to perform in order to take a bundle of vertices and indices - which we modelled as the ast::Mesh class - and hand them over to the graphics hardware to be rendered. Lets bring them all together in our main rendering loop. . The first part of the pipeline is the vertex shader that takes as input a single vertex. OpenGL doesn't simply transform all your 3D coordinates to 2D pixels on your screen; OpenGL only processes 3D coordinates when they're in a specific range between -1.0 and 1.0 on all 3 axes (x, y and z). #include . Remember, our shader program needs to be fed in the mvp uniform which will be calculated like this each frame for each mesh: mvp for a given mesh is computed by taking: So where do these mesh transformation matrices come from? We specified 6 indices so we want to draw 6 vertices in total. Below you'll find an abstract representation of all the stages of the graphics pipeline. Check the section named Built in variables to see where the gl_Position command comes from. In code this would look a bit like this: And that is it! With the empty buffer created and bound, we can then feed the data from the temporary positions list into it to be stored by OpenGL. Our perspective camera has the ability to tell us the P in Model, View, Projection via its getProjectionMatrix() function, and can tell us its V via its getViewMatrix() function. We will be using VBOs to represent our mesh to OpenGL. We must take the compiled shaders (one for vertex, one for fragment) and attach them to our shader program instance via the OpenGL command glAttachShader. This means we need a flat list of positions represented by glm::vec3 objects. Because of their parallel nature, graphics cards of today have thousands of small processing cores to quickly process your data within the graphics pipeline. We can bind the newly created buffer to the GL_ARRAY_BUFFER target with the glBindBuffer function: From that point on any buffer calls we make (on the GL_ARRAY_BUFFER target) will be used to configure the currently bound buffer, which is VBO. The following steps are required to create a WebGL application to draw a triangle. The shader script is not permitted to change the values in attribute fields so they are effectively read only. Edit the default.frag file with the following: In our fragment shader we have a varying field named fragmentColor. #include "../../core/graphics-wrapper.hpp" \$\begingroup\$ After trying out RenderDoc, it seems like the triangle was drawn first, and the screen got cleared (filled with magenta) afterwards. #define USING_GLES A color is defined as a pair of three floating points representing red,green and blue. OpenGL does not yet know how it should interpret the vertex data in memory and how it should connect the vertex data to the vertex shader's attributes. To draw a triangle with mesh shaders, we need two things: - a GPU program with a mesh shader and a pixel shader. We are now using this macro to figure out what text to insert for the shader version. Before we start writing our shader code, we need to update our graphics-wrapper.hpp header file to include a marker indicating whether we are running on desktop OpenGL or ES2 OpenGL. Assuming we dont have any errors, we still need to perform a small amount of clean up before returning our newly generated shader program handle ID. If compilation failed, we should retrieve the error message with glGetShaderInfoLog and print the error message. In our vertex shader, the uniform is of the data type mat4 which represents a 4x4 matrix. Edit your graphics-wrapper.hpp and add a new macro #define USING_GLES to the three platforms that only support OpenGL ES2 (Emscripten, iOS, Android). This brings us to a bit of error handling code: This code simply requests the linking result of our shader program through the glGetProgramiv command along with the GL_LINK_STATUS type. GLSL has a vector datatype that contains 1 to 4 floats based on its postfix digit. Graphics hardware can only draw points, lines, triangles, quads and polygons (only convex). The last element buffer object that gets bound while a VAO is bound, is stored as the VAO's element buffer object. For this reason it is often quite difficult to start learning modern OpenGL since a great deal of knowledge is required before being able to render your first triangle. Bind the vertex and index buffers so they are ready to be used in the draw command. An EBO is a buffer, just like a vertex buffer object, that stores indices that OpenGL uses to decide what vertices to draw. We perform some error checking to make sure that the shaders were able to compile and link successfully - logging any errors through our logging system. How to load VBO and render it on separate Java threads? Edit your opengl-application.cpp file. OpenGL is a 3D graphics library so all coordinates that we specify in OpenGL are in 3D (x, y and z coordinate). Is there a proper earth ground point in this switch box? #include "../core/internal-ptr.hpp", #include "../../core/perspective-camera.hpp", #include "../../core/glm-wrapper.hpp" Weve named it mvp which stands for model, view, projection - it describes the transformation to apply to each vertex passed in so it can be positioned in 3D space correctly. Edit opengl-application.cpp and add our new header (#include "opengl-mesh.hpp") to the top. Steps Required to Draw a Triangle. Learn OpenGL is free, and will always be free, for anyone who wants to start with graphics programming. We need to load them at runtime so we will put them as assets into our shared assets folder so they are bundled up with our application when we do a build. Lets dissect it. OpenGL allows us to bind to several buffers at once as long as they have a different buffer type. #include "TargetConditionals.h" Open it in Visual Studio Code. It can be removed in the future when we have applied texture mapping. Remember that we specified the location of the, The next argument specifies the size of the vertex attribute. The third parameter is a pointer to where in local memory to find the first byte of data to read into the buffer (positions.data()). This stage checks the corresponding depth (and stencil) value (we'll get to those later) of the fragment and uses those to check if the resulting fragment is in front or behind other objects and should be discarded accordingly. The code for this article can be found here. AssimpAssimp. The fragment shader only requires one output variable and that is a vector of size 4 that defines the final color output that we should calculate ourselves. Note: The order that the matrix computations is applied is very important: translate * rotate * scale. Edit default.vert with the following script: Note: If you have written GLSL shaders before you may notice a lack of the #version line in the following scripts. Continue to Part 11: OpenGL texture mapping. The first parameter specifies which vertex attribute we want to configure. As of now we stored the vertex data within memory on the graphics card as managed by a vertex buffer object named VBO. Binding the appropriate buffer objects and configuring all vertex attributes for each of those objects quickly becomes a cumbersome process. From that point on we have everything set up: we initialized the vertex data in a buffer using a vertex buffer object, set up a vertex and fragment shader and told OpenGL how to link the vertex data to the vertex shader's vertex attributes. Binding to a VAO then also automatically binds that EBO. What can a lawyer do if the client wants him to be acquitted of everything despite serious evidence? Next we simply assign a vec4 to the color output as an orange color with an alpha value of 1.0 (1.0 being completely opaque). Recall that earlier we added a new #define USING_GLES macro in our graphics-wrapper.hpp header file which was set for any platform that compiles against OpenGL ES2 instead of desktop OpenGL. There is no space (or other values) between each set of 3 values. #include This function is called twice inside our createShaderProgram function, once to compile the vertex shader source and once to compile the fragment shader source. We will base our decision of which version text to prepend on whether our application is compiling for an ES2 target or not at build time. Thank you so much. #endif By changing the position and target values you can cause the camera to move around or change direction. So (-1,-1) is the bottom left corner of your screen. A triangle strip in OpenGL is a more efficient way to draw triangles with fewer vertices. Lets get started and create two new files: main/src/application/opengl/opengl-mesh.hpp and main/src/application/opengl/opengl-mesh.cpp. The mesh shader GPU program is declared in the main XML file while shaders are stored in files: We tell it to draw triangles, and let it know how many indices it should read from our index buffer when drawing: Finally, we disable the vertex attribute again to be a good citizen: We need to revisit the OpenGLMesh class again to add in the functions that are giving us syntax errors. Being able to see the logged error messages is tremendously valuable when trying to debug shader scripts. The projectionMatrix is initialised via the createProjectionMatrix function: You can see that we pass in a width and height which would represent the screen size that the camera should simulate. Our vertex shader main function will do the following two operations each time it is invoked: A vertex shader is always complemented with a fragment shader. The difference between the phonemes /p/ and /b/ in Japanese. If youve ever wondered how games can have cool looking water or other visual effects, its highly likely it is through the use of custom shaders. The processing cores run small programs on the GPU for each step of the pipeline. The last thing left to do is replace the glDrawArrays call with glDrawElements to indicate we want to render the triangles from an index buffer. With the vertex data defined we'd like to send it as input to the first process of the graphics pipeline: the vertex shader. The magic then happens in this line, where we pass in both our mesh and the mvp matrix to be rendered which invokes the rendering code we wrote in the pipeline class: Are you ready to see the fruits of all this labour?? Drawing our triangle. I added a call to SDL_GL_SwapWindow after the draw methods, and now I'm getting a triangle, but it is not as vivid colour as it should be and there are . In the fragment shader this field will be the input that complements the vertex shaders output - in our case the colour white. We will also need to delete our logging statement in our constructor because we are no longer keeping the original ast::Mesh object as a member field, which offered public functions to fetch its vertices and indices. Can I tell police to wait and call a lawyer when served with a search warrant? What video game is Charlie playing in Poker Face S01E07? Your NDC coordinates will then be transformed to screen-space coordinates via the viewport transform using the data you provided with glViewport. This is how we pass data from the vertex shader to the fragment shader. In our case we will be sending the position of each vertex in our mesh into the vertex shader so the shader knows where in 3D space the vertex should be. Doubling the cube, field extensions and minimal polynoms. To keep things simple the fragment shader will always output an orange-ish color. This has the advantage that when configuring vertex attribute pointers you only have to make those calls once and whenever we want to draw the object, we can just bind the corresponding VAO. You can find the complete source code here. Edit opengl-application.cpp again, adding the header for the camera with: Navigate to the private free function namespace and add the following createCamera() function: Add a new member field to our Internal struct to hold our camera - be sure to include it after the SDL_GLContext context; line: Update the constructor of the Internal struct to initialise the camera: Sweet, we now have a perspective camera ready to be the eye into our 3D world.