• Advertisement
Sign in to follow this  

OpenGL How to move a rectangle properly?

This topic is 1560 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

I recently started to learn OpenGL. Right now I finished the first chapter of the "OpenGL SuperBible". There were two examples. The first had the complete code and showed how to draw a simple triangle. The second example is supposed to show how to move a rectangle using SpecialKeys. The only code provided for this example was the SpecialKeys method. I still tried to implement it but I had two problems.


  1. In the previous example I declared and instaciated vVerts in the SetupRC() method. Now as it is also used in the SpecialKeys() method, I moved the declaration and instantiation to the top of the code. Is this proper c++ practice?
  2. I copied the part where vertex positions are recalculated from the book, but I had to pick the vertices for the rectangle on my own. So now every time I press a key for the first time the rectangle's upper left vertex is moved to (-0,5:-0.5). This ok because of:
  3. GLfloat blockX = vVerts[0]; //Upper left X
    GLfloat blockY = vVerts[7]; // Upper left Y

    But I also think that this is the reason why my rectangle is shifted in the beginning. After the first time a key was pressed everything works just fine. Here is my complete code I hope you can help me on those two points.

GLBatch squareBatch;
GLShaderManager shaderManager;
//Load up a triangle    
GLfloat vVerts[] = {-0.5f,0.5f,0.0f,

//Window has changed size, or has just been created.
//We need to use the window dimensions to set the viewport and the projection matrix.
void ChangeSize(int w, int h)

//Called to draw the scene.
void RenderScene(void)
    //Clear the window with the current clearing color

    GLfloat vRed[] = {1.0f,0.0f,0.0f,1.0f};


    //perform the buffer swap to display the back buffer

//This function does any needed initialization on the rendering context.
//This is the first opportunity to do any OpenGL related Tasks.
void SetupRC()
    //Blue Background




//Respond to arrow keys by moving the camera frame of reference
void SpecialKeys(int key,int x,int y)
    GLfloat stepSize = 0.025f;
    GLfloat blockSize = 0.5f;
    GLfloat blockX = vVerts[0]; //Upper left X
    GLfloat blockY = vVerts[7]; // Upper left Y

    if(key == GLUT_KEY_UP)
        blockY += stepSize;

    if(key == GLUT_KEY_DOWN){blockY -= stepSize;}

    if(key == GLUT_KEY_LEFT){blockX -= stepSize;}

    if(key == GLUT_KEY_RIGHT){blockX += stepSize;}

    //Recalculate vertex positions
    vVerts[0] = blockX;
    vVerts[1] = blockY - blockSize*2;

    vVerts[3] = blockX + blockSize * 2;
    vVerts[4] = blockY - blockSize *2;

    vVerts[6] = blockX+blockSize*2;
    vVerts[7] = blockY;

    vVerts[9] = blockX;
    vVerts[10] = blockY;



//Main entry point for GLUT based programs
int main(int argc, char** argv)
    //Sets the working directory. Not really needed
    //Passes along the command-line parameters and initializes the GLUT library.
    //Tells the GLUT library what type of display mode to use, when creating the window.
    //Double buffered window, RGBA-Color mode,depth-buffer as part of our display, stencil buffer also available
    //Window size
    //initialize GLEW library
    GLenum err = glewInit();

    //Check that nothing goes wrong with the driver initialization before we try and do any rendering.
    if(GLEW_OK != err)
        fprintf(stderr,"Glew Error: %s\n",glewGetErrorString);

        return 1;


    return 0;

Share this post

Link to post
Share on other sites

If you want to move the actual vertices, you could just set some delta:

float dx = 0, dy = 0;

if (left)  dx = -speed;
if (right) dx += speed;
// etc.

// iterate all vertices (not every float)
for (int i = 0; i < vertexCount * 3; i += 3)
    vertex[i]     += dx; // all X'es
    vertex[i + 1] += dy; // all Y's
    // not increasing any Z's which would be vertex[i + 2]
    // vertex[i + 3] belongs to any next vertex
Edited by Kaptein

Share this post

Link to post
Share on other sites

It is not performance-suave to update the vertices directly.


You should not touch the vertices and instead supply a transform matrix of some kind and transform them in the vertex shader.



L. Spiro

Share this post

Link to post
Share on other sites

Suave? Like suave Ben from Blue Velvet?




Clip: (obvs NSFW or at any time)




Another warning: it's not safe for work or at any other time, that clip.


I  think you mean "savvy"? ;)

Edited by Paradigm Shifter

Share this post

Link to post
Share on other sites

Suave” means “elegant”, “sophisticated”, “refined”, “polished”, etc.  In reference to performance in this case.

Savvy” means “understanding”.



L. Spiro

Edited by L. Spiro

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • Similar Content

    • By khawk
      We've just released all of the source code for the NeHe OpenGL lessons on our Github page at https://github.com/gamedev-net/nehe-opengl. code - 43 total platforms, configurations, and languages are included.
      Now operated by GameDev.net, NeHe is located at http://nehe.gamedev.net where it has been a valuable resource for developers wanting to learn OpenGL and graphics programming.

      View full story
    • By TheChubu
      The Khronos™ Group, an open consortium of leading hardware and software companies, announces from the SIGGRAPH 2017 Conference the immediate public availability of the OpenGL® 4.6 specification. OpenGL 4.6 integrates the functionality of numerous ARB and EXT extensions created by Khronos members AMD, Intel, and NVIDIA into core, including the capability to ingest SPIR-V™ shaders.
      SPIR-V is a Khronos-defined standard intermediate language for parallel compute and graphics, which enables content creators to simplify their shader authoring and management pipelines while providing significant source shading language flexibility. OpenGL 4.6 adds support for ingesting SPIR-V shaders to the core specification, guaranteeing that SPIR-V shaders will be widely supported by OpenGL implementations.
      OpenGL 4.6 adds the functionality of these ARB extensions to OpenGL’s core specification:
      GL_ARB_gl_spirv and GL_ARB_spirv_extensions to standardize SPIR-V support for OpenGL GL_ARB_indirect_parameters and GL_ARB_shader_draw_parameters for reducing the CPU overhead associated with rendering batches of geometry GL_ARB_pipeline_statistics_query and GL_ARB_transform_feedback_overflow_querystandardize OpenGL support for features available in Direct3D GL_ARB_texture_filter_anisotropic (based on GL_EXT_texture_filter_anisotropic) brings previously IP encumbered functionality into OpenGL to improve the visual quality of textured scenes GL_ARB_polygon_offset_clamp (based on GL_EXT_polygon_offset_clamp) suppresses a common visual artifact known as a “light leak” associated with rendering shadows GL_ARB_shader_atomic_counter_ops and GL_ARB_shader_group_vote add shader intrinsics supported by all desktop vendors to improve functionality and performance GL_KHR_no_error reduces driver overhead by allowing the application to indicate that it expects error-free operation so errors need not be generated In addition to the above features being added to OpenGL 4.6, the following are being released as extensions:
      GL_KHR_parallel_shader_compile allows applications to launch multiple shader compile threads to improve shader compile throughput WGL_ARB_create_context_no_error and GXL_ARB_create_context_no_error allow no error contexts to be created with WGL or GLX that support the GL_KHR_no_error extension “I’m proud to announce OpenGL 4.6 as the most feature-rich version of OpenGL yet. We've brought together the most popular, widely-supported extensions into a new core specification to give OpenGL developers and end users an improved baseline feature set. This includes resolving previous intellectual property roadblocks to bringing anisotropic texture filtering and polygon offset clamping into the core specification to enable widespread implementation and usage,” said Piers Daniell, chair of the OpenGL Working Group at Khronos. “The OpenGL working group will continue to respond to market needs and work with GPU vendors to ensure OpenGL remains a viable and evolving graphics API for all its customers and users across many vital industries.“
      The OpenGL 4.6 specification can be found at https://khronos.org/registry/OpenGL/index_gl.php. The GLSL to SPIR-V compiler glslang has been updated with GLSL 4.60 support, and can be found at https://github.com/KhronosGroup/glslang.
      Sophisticated graphics applications will also benefit from a set of newly released extensions for both OpenGL and OpenGL ES to enable interoperability with Vulkan and Direct3D. These extensions are named:
      GL_EXT_memory_object GL_EXT_memory_object_fd GL_EXT_memory_object_win32 GL_EXT_semaphore GL_EXT_semaphore_fd GL_EXT_semaphore_win32 GL_EXT_win32_keyed_mutex They can be found at: https://khronos.org/registry/OpenGL/index_gl.php
      Industry Support for OpenGL 4.6
      “With OpenGL 4.6 our customers have an improved set of core features available on our full range of OpenGL 4.x capable GPUs. These features provide improved rendering quality, performance and functionality. As the graphics industry’s most popular API, we fully support OpenGL and will continue to work closely with the Khronos Group on the development of new OpenGL specifications and extensions for our customers. NVIDIA has released beta OpenGL 4.6 drivers today at https://developer.nvidia.com/opengl-driver so developers can use these new features right away,” said Bob Pette, vice president, Professional Graphics at NVIDIA.
      "OpenGL 4.6 will be the first OpenGL release where conformant open source implementations based on the Mesa project will be deliverable in a reasonable timeframe after release. The open sourcing of the OpenGL conformance test suite and ongoing work between Khronos and X.org will also allow for non-vendor led open source implementations to achieve conformance in the near future," said David Airlie, senior principal engineer at Red Hat, and developer on Mesa/X.org projects.

      View full story
    • By _OskaR
      I have an OpenGL application but without possibility to wite own shaders.
      I need to perform small VS modification - is possible to do it in an alternative way? Do we have apps or driver modifictions which will catch the shader sent to GPU and override it?
    • By xhcao
      Does sync be needed to read texture content after access texture image in compute shader?
      My simple code is as below,
      glBindImageTexture(0, texture[0], 0, GL_FALSE, 3, GL_READ_ONLY, GL_R32UI);
      glBindImageTexture(1, texture[1], 0, GL_FALSE, 4, GL_WRITE_ONLY, GL_R32UI);
      glDispatchCompute(1, 1, 1);
      // Does sync be needed here?
      glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
                                     GL_TEXTURE_CUBE_MAP_POSITIVE_X + face, texture[1], 0);
      glReadPixels(0, 0, kWidth, kHeight, GL_RED_INTEGER, GL_UNSIGNED_INT, outputValues);
      Compute shader is very simple, imageLoad content from texture[0], and imageStore content to texture[1]. Does need to sync after dispatchCompute?
    • By Jonathan2006
      My question: is it possible to transform multiple angular velocities so that they can be reinserted as one? My research is below:
      // This works quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); quat quaternion2 = GEMultiplyQuaternions(quaternion1, GEQuaternionFromAngleRadians(angleRadiansVector2)); quat quaternion3 = GEMultiplyQuaternions(quaternion2, GEQuaternionFromAngleRadians(angleRadiansVector3)); glMultMatrixf(GEMat4FromQuaternion(quaternion3).array); // The first two work fine but not the third. Why? quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); vec3 vector1 = GETransformQuaternionAndVector(quaternion1, angularVelocity1); quat quaternion2 = GEQuaternionFromAngleRadians(angleRadiansVector2); vec3 vector2 = GETransformQuaternionAndVector(quaternion2, angularVelocity2); // This doesn't work //quat quaternion3 = GEQuaternionFromAngleRadians(angleRadiansVector3); //vec3 vector3 = GETransformQuaternionAndVector(quaternion3, angularVelocity3); vec3 angleVelocity = GEAddVectors(vector1, vector2); // Does not work: vec3 angleVelocity = GEAddVectors(vector1, GEAddVectors(vector2, vector3)); static vec3 angleRadiansVector; vec3 angularAcceleration = GESetVector(0.0, 0.0, 0.0); // Sending it through one angular velocity later in my motion engine angleVelocity = GEAddVectors(angleVelocity, GEMultiplyVectorAndScalar(angularAcceleration, timeStep)); angleRadiansVector = GEAddVectors(angleRadiansVector, GEMultiplyVectorAndScalar(angleVelocity, timeStep)); glMultMatrixf(GEMat4FromEulerAngle(angleRadiansVector).array); Also how do I combine multiple angularAcceleration variables? Is there an easier way to transform the angular values?
  • Advertisement