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PODrawing a single line in glsl
 

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  1. PODrawing a single line in glsl
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    <p>I am trying to draw a single line as a part of learning OpenGL 4.x and above. Here are the shaders that I am using.</p> <p>Vertex Shader</p> <pre><code> "#version 430 core \n" " \n" "void main(void) \n" "{ \n" " const vec4 vertices[2] = vec4[2](vec4(0.25,-0.25,0.5,1.0),vec4(-0.25,-0.25,0.5,1.0)); \n" " gl_Position = vertices[gl_VertexID]; n" "} \n" </code></pre> <p>and Fragment Shader</p> <pre><code> "#version 430 core \n" " \n" "out vec4 color; \n" " \n" "void main(void) \n" "{ \n" " color = vec4(0.0, 0.8, 0.0, 1.0); \n" "} \n" </code></pre> <p>And here is what I do in my <code>display()</code> function.</p> <pre><code>void display() { static const GLfloat red[] = { 1.0f, 0.0f, 0.0f, 1.0f }; glClearBufferfv(GL_COLOR, 0, red); glUseProgram(program); glLineWidth(8.0f); glDrawArrays(GL_LINES, 0, 2); glFlush(); } </code></pre> <p>However, nothing gets drawn in the window except red color. I have read <a href="https://stackoverflow.com/questions/5458185/drawing-lines-with-glsl">this</a>,<a href="https://stackoverflow.com/questions/11594247/opengl-lines-with-shaders">this</a> and <a href="https://stackoverflow.com/questions/11594804/opengl-shader-two-object-and-one-line-to-connect-them">this</a> questions. Two of them refer me to use Vertex Buffers and the third one tells me to draw a triangle with two vertices in the same position. </p> <p>My question is, why does <code>glDrawArrays(GL_LINES, 0, 2)</code> in my code fail to draw a single line? Or is it essential that I use Vertex Array Buffers when I need to draw a single line?</p> <p>UPDATE:</p> <p>Here is the code from the <code>main()</code> function.</p> <pre><code>startShaders(); while (!glfwWindowShouldClose(window)) { RenderFunction(); glfwSwapBuffers(window); glfwPollEvents(); } shutdown(); cout &lt;&lt; "INFO: OpenGL Version: " &lt;&lt; glGetString(GL_VERSION) &lt;&lt; endl; glfwTerminate(); exit(EXIT_SUCCESS); </code></pre> <p>And here is my <code>RenderFunction()</code> that calls <code>Display()</code> function.</p> <pre><code>void RenderFunction() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); display(); } </code></pre> <p>UPDATE 2: Here is the pic of console window displaying <code>stderr</code> output. <img src="https://i.stack.imgur.com/hXj4z.jpg" alt="enter image description here"></p>
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    1. USCommunity
    1. USRecker
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    1. COwhere do you define the contents of the enabled arrays? you might also test the return value on the `glDrawArrays` call.
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      1. PODrawing a single line in glsl
      1. USwoolstar
    2. COInside the vertex shader code.I have included it in the question.
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      1. USRecker
    3. COYou technically do not need any arrays enabled to draw vertices. If you have none then the only two things you will get unique to each vertex in the vertex shader are the pre-declared variables: `gl_VertexID` and `gl_InstanceID`. My concern is actually that you are using single-buffered rendering in an OpenGL 4.3 app. It is unheard of to use single-buffered drawing these days, in modern window systems you need double-buffering because they only update the contents of a window when you swap a back buffer to the front. Some hybrid GPU solutions also require double-buffering to work correctly.
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      1. PODrawing a single line in glsl
      1. USAndon M. Coleman
 

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