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Member Since 21 Mar 2012
Offline Last Active Jan 26 2014 01:17 AM

Topics I've Started

Why the efficiency a simple 2D program is so low?

21 March 2012 - 09:25 AM

I'm a totally new beginner to OpenGL and I'm learning it by online resource http://www.arcsynthesis.org/gltut/.

It is a 2D example that is a simply 2D spinning triangle around one point. A attach its main code below.

My problem is why a simple program run so slow. It does not run very smooth and I can a few obvious lags. This is much more frequent when I open v-sync (CPU 100% on a laptop is annoying). I also try to sleep sometime before call another glutPostRedisplay. This seems even lagger than v-sync.

My question is how to make this program run more smooth? I can never feel such lag when I play some games written in OpenGL. Also is there a convenient way to limit the FPS?

#define ARRAY_COUNT( array ) (sizeof( array ) / (sizeof( array[0] ) * (sizeof( array ) != sizeof(void*) || sizeof( array[0] ) <= sizeof(void*))))

GLuint theProgram;

void InitializeProgram()
    std::vector<GLuint> shaderList;

    shaderList.push_back(Framework::LoadShader(GL_VERTEX_SHADER, "standard.vert"));
    shaderList.push_back(Framework::LoadShader(GL_FRAGMENT_SHADER, "standard.frag"));

    theProgram = Framework::CreateProgram(shaderList);

const float vertexPositions[] = {
    0.25f, 0.25f, 0.0f, 1.0f,
    0.25f, -0.25f, 0.0f, 1.0f,
    -0.25f, -0.25f, 0.0f, 1.0f,

GLuint positionBufferObject;
GLuint vao;

void InitializeVertexBuffer()
    glGenBuffers(1, &positionBufferObject);

    glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexPositions), vertexPositions, GL_STREAM_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

//Called after the window and OpenGL are initialized. Called exactly once, before the main loop.
void init()

    glGenVertexArrays(1, &vao);

void ComputePositionOffsets(float &fXOffset, float &fYOffset)
    const float fLoopDuration = 1.0f;
    const float fScale = 3.14159f * 2.0f / fLoopDuration;

    float fElapsedTime = glutGet(GLUT_ELAPSED_TIME) / 1000.0f;

    float fCurrTimeThroughLoop = fmodf(fElapsedTime, fLoopDuration);

    fXOffset = cosf(fCurrTimeThroughLoop * fScale) * 0.5f;
    fYOffset = sinf(fCurrTimeThroughLoop * fScale) * 0.5f;

void AdjustVertexData(float fXOffset, float fYOffset)
    std::vector<float> fNewData(ARRAY_COUNT(vertexPositions));
    memcpy(&fNewData[0], vertexPositions, sizeof(vertexPositions));

    for(int iVertex = 0; iVertex < ARRAY_COUNT(vertexPositions); iVertex += 4)
        fNewData[iVertex] += fXOffset;
        fNewData[iVertex + 1] += fYOffset;

    glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
    glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vertexPositions), &fNewData[0]);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

void display()
    float fXOffset = 0.0f, fYOffset = 0.0f;
    ComputePositionOffsets(fXOffset, fYOffset);
    AdjustVertexData(fXOffset, fYOffset);

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);


    glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

    glDrawArrays(GL_TRIANGLES, 0, 3);



#version 330

layout(location = 0) in vec4 position;

void main()
    gl_Position = position;

#version 330

out vec4 outputColor;

void main()
    outputColor = vec4(1.0f, 1.0f, 1.0f, 1.0f);

BTW, Is http://www.arcsynthesis.org/gltut/ a good choice for beginner to learn opengl?