Consequently, glDrawElements works with only one index array, and this is the index into all enabled vertex attribute arrays at the same time. If it still works as intended, your normal data happens to be organized in the correct way. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 6 years, 3 months ago. Active 6 years, 3 months ago. Viewed 2k times. 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.
To draw our objects of choice, OpenGL provides us with the glDrawArrays function that draws primitives using the currently active shader, the previously defined vertex attribute configuration and with the VBO's vertex data indirectly bound via the VAO. The glDrawArrays function takes as its first argument the OpenGL primitive type we would like to draw. The second argument specifies the starting index of the vertex array we'd like to draw; we just leave this at 0.
The last argument specifies how many vertices we want to draw, which is 3 we only render 1 triangle from our data, which is exactly 3 vertices long.
Now try to compile the code and work your way backwards if any errors popped up. As soon as your application compiles, you should see the following result:. The source code for the complete program can be found here. If your output does not look the same you probably did something wrong along the way so check the complete source code and see if you missed anything.
There is one last thing we'd like to discuss when rendering vertices and that is element buffer objects abbreviated to EBO. To explain how element buffer objects work it's best to give an example: suppose we want to draw a rectangle instead of a triangle. We can draw a rectangle using two triangles OpenGL mainly works with triangles. This will generate the following set of vertices:. As you can see, there is some overlap on the vertices specified.
We specify bottom right and top left twice! This will only get worse as soon as we have more complex models that have over s of triangles where there will be large chunks that overlap. What would be a better solution is to store only the unique vertices and then specify the order at which we want to draw these vertices in. In that case we would only have to store 4 vertices for the rectangle, and then just specify at which order we'd like to draw them. Wouldn't it be great if OpenGL provided us with a feature like that?
Thankfully, element buffer objects work exactly like that. An EBO is a buffer, just like a vertex buffer object, that stores indices that OpenGL uses to decide what vertices to draw. This so called indexed drawing is exactly the solution to our problem. To get started we first have to specify the unique vertices and the indices to draw them as a rectangle:. You can see that, when using indices, we only need 4 vertices instead of 6.
Next we need to create the element buffer object:. 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. When using glDrawElements we're going to draw using indices provided in the element buffer object currently bound:. The first argument specifies the mode we want to draw in, similar to glDrawArrays.
The second argument is the count or number of elements we'd like to draw. We specified 6 indices so we want to draw 6 vertices in total.
The last argument allows us to specify an offset in the EBO or pass in an index array, but that is when you're not using element buffer objects , but we're just going to leave this at 0. This means we have to bind the corresponding EBO each time we want to render an object with indices which again is a bit cumbersome. It just so happens that a vertex array object also keeps track of element buffer object bindings. Running the program should give an image as depicted below.
The left image should look familiar and the right image is the rectangle drawn in wireframe mode. The wireframe rectangle shows that the rectangle indeed consists of two triangles. If you have any errors, work your way backwards and see if you missed anything. You can find the complete source code here. 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.
This is a difficult part since there is a large chunk of knowledge required before being able to draw your first triangle. Thankfully, we now made it past that barrier and the upcoming chapters will hopefully be much easier to understand. To really get a good grasp of the concepts discussed a few exercises were set up.
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. If you're running AdBlock, please consider whitelisting this site if you'd like to support LearnOpenGL; and no worries, I won't be mad if you don't :.
In order for OpenGL to know what to make of your collection of coordinates and color values OpenGL requires you to hint what kind of render types you want to form with the data.
Do we want the data rendered as a collection of points, a collection of triangles or perhaps just one long line? Those hints are called primitives and are given to OpenGL while calling any of the drawing commands. Vertex input To start drawing something we have to first give OpenGL some input vertex data.
Normalized Device Coordinates NDC Once your vertex coordinates have been processed in the vertex shader, they should be in normalized device coordinates which is a small space where the x , y and z values vary from Below you can see the triangle we specified within normalized device coordinates ignoring the z axis : Unlike usual screen coordinates the positive y-axis points in the up-direction and the 0,0 coordinates are at the center of the graph, instead of top-left.
Once the data is in the graphics card's memory the vertex shader has almost instant access to the vertices making it extremely fast A vertex buffer object is our first occurrence of an OpenGL object as we've discussed in the OpenGL chapter.
Vertex shader The vertex shader is one of the shaders that are programmable by people like us. A vector in GLSL has a maximum size of 4 and each of its values can be retrieved via vec.
Note that the vec. We'll discuss vectors in much greater depth in a later chapter. Fragment shader The fragment shader is the second and final shader we're going to create for rendering a triangle. Colors in computer graphics are represented as an array of 4 values: the red, green, blue and alpha opacity component, commonly abbreviated to RGBA. Because the array pointers and enables are client-side state variables, their values affect display lists when the lists are created, not when the lists are executed.
Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. The remaining three faces must be done differently since the vertices are not in the order they would need to be in order to draw those faces with glDrawArrays. This is where glDrawElements comes in.
Assuming the VAO is still bound e. The code for the y min and y max faces is analogous and is left as an exercise.
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