Question

I’m learning OpenGL 3.3, using some tutorials (http://opengl-tutorial.org). In the tutorial I’m using, there is a vertex shader which does the following:

Tutorial Shader source

#version 330 core

// Input vertex data, different for all executions of this shader.
layout(location = 0) in vec3 vertexPosition_modelspace;

// Values that stay constant for the whole mesh.
uniform mat4 MVP;

void main(){

    // Output position of the vertex, in clip space : MVP * position
    gl_Position =  MVP * vec4(vertexPosition_modelspace,1);

}

Yet, when I try to emulate the same behavior in my application, I get the following:

error: implicit cast from "vec4" to "vec3".

After seeing this, I wasn’t sure if it was because I was using 4.2 version shaders as opposed to 3.3, so changed everything to match what the author had been using, still receiving the same error afterward.

So, I changed my shader to do this:

My (latest) Source

#version 330 core

layout(location = 0) in vec3 vertexPosition_modelspace;

uniform mat4 MVP;

void main()
{
    vec4 a = vec4(vertexPosition_modelspace, 1);    

    gl_Position.xyz = MVP * a;
}

Which, of course, still produces the same error.

Does anyone know why this is the case, as well as what a solution might be to this? I’m not sure if it could be my calling code (which I’ve posted, just in case).

---

Calling Code

static const GLfloat T_VERTEX_BUF_DATA[] = 
{
    // x, y z
    -1.0f, -1.0f, 0.0f,
     1.0f, -1.0f, 0.0f,
     0.0f, 1.0f, 0.0f
};

static const GLushort T_ELEMENT_BUF_DATA[] = 
{ 0, 1, 2 };

void TriangleDemo::Run(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    GLuint matrixID = glGetUniformLocation(mProgramID, "MVP");

    glUseProgram(mProgramID);

    glUniformMatrix4fv(matrixID, 1, GL_FALSE, &mMVP[0][0]); // This sends our transformation to the MVP uniform matrix, in the currently bound vertex shader 

    const GLuint vertexShaderID = 0;

    glEnableVertexAttribArray(vertexShaderID);
    glBindBuffer(GL_ARRAY_BUFFER, mVertexBuffer);
    glVertexAttribPointer(
        vertexShaderID, // Specify the ID of the shader to point to (in this case, the shader is built in to GL, which will just produce a white triangle)
        3,              // Specify the number of indices per vertex in the vertex buffer  
        GL_FLOAT,       // Type of value the vertex buffer is holding as data
        GL_FALSE,       // Normalized? 
        0,              // Amount of stride
        (void*)0 );     // Offset within the array buffer

    glDrawArrays(GL_TRIANGLES, 0, 3); //0 => start index of the buffer, 3 => number of vertices

    glDisableVertexAttribArray(vertexShaderID);
}

void TriangleDemo::Initialize(void)
{
    glGenVertexArrays(1, &mVertexArrayID);

    glBindVertexArray(mVertexArrayID);

    glGenBuffers(1, &mVertexBuffer);

    glBindBuffer(GL_ARRAY_BUFFER, mVertexBuffer);

    glBufferData(GL_ARRAY_BUFFER, sizeof(T_VERTEX_BUF_DATA), T_VERTEX_BUF_DATA, GL_STATIC_DRAW );

    mProgramID = LoadShaders("v_Triangle", "f_Triangle");

    glm::mat4 projection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f); // field of view, aspect ratio (4:3), 0.1 units near, to 100 units far

    glm::mat4 view  = glm::lookAt(
                                glm::vec3(4, 3, 3), // Camera is at (4, 3, 3) in world space
                                glm::vec3(0, 0, 0), // and looks at the origin
                                glm::vec3(0, 1, 0)  // this is the up vector - the head of the camera is facing upwards. We'd use (0, -1, 0) to look upside down
                            );

    glm::mat4 model = glm::mat4(1.0f); // set model matrix to identity matrix, meaning the model will be at the origin

    mMVP            = projection * view * model;

}

Notes

• I’m in Visual Studio 2012
• I’m using Shader Maker for the GLSL editing

Answer 1

I can’t say what’s wrong with the tutorial code.

In “My latest source” though, there’s

gl_Position.xyz = MVP * a;

which looks weird because you’re assigning a vec4 to a vec3.

---

EDIT

I can’t reproduce your problem.

I have used a trivial fragment shader for testing…

#version 330 core

void main()
{
}

Testing “Tutorial Shader source”:

3.3.11762 Core Profile Context
Log: Vertex shader was successfully compiled to run on hardware.

Log: Fragment shader was successfully compiled to run on hardware.

Log: Vertex shader(s) linked, fragment shader(s) linked.

Testing “My latest source”:

3.3.11762 Core Profile Context
Log: Vertex shader was successfully compiled to run on hardware.
 WARNING: 0:11: warning(#402) Implicit truncation of vector from size 4 to size 3.

Log: Fragment shader was successfully compiled to run on hardware.

Log: Vertex shader(s) linked, fragment shader(s) linked.

And the warning goes away after replacing gl_Position.xyz with gl_Position.

---

What’s your setup? Do you have a correct version of OpenGL context? Is glGetError() silent?

Finally, are your GPU drivers up-to-date?