• Advertisement
Sign in to follow this  

OpenGL 2D rotations

This topic is 4622 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

Hi all. Just registered! I'm starting out with GLUT and OpenGL and have been playing around. Wrote my own little version of Pong and now I'm moving on to 'Asteroids' But I've got a problem. I'm using the 2D view (gluOrtho2D) but don't know how to rotate something (ie. the spaceship) in 2D. I've rotated things in 3D before whilst learning some of the stuff, but glRotate doesn't seem to work in 2D. Is there a specific command to do this? And if not, how can I go about it? Is there some forumla I can use to calculate new x,y co-ords of a shape based on how much I want to rotate it by? Thanks. [Edited by - BigDogUK on May 27, 2005 3:10:44 AM]

Share this post


Link to post
Share on other sites
Advertisement
You could certainly do the calculations yourself. However, AFAIK working with 2d graphics in OpenGL ortho mode is basically just like drawing graphics to the xy plane in 3d. All rotations take place about the z axis, so your rotate calls look like this:

glRotate(angle, 0, 0, 1);

I believe there's a glVertex2(), but no glTranslate2(). So your translation calls will just be:

glTranslate(x, y, 0);

Share this post


Link to post
Share on other sites
Ahh, ok. My ship rotates now, but I'm not sure how to set the origin of rotation. It seems to be rotating around 0,0 on my 2D view and fiddling with the:

glRotatef(angle, x, y, 1)

x and y doesn't seem to help. One question. How do the float values correspond to places on a say 800x600 2D view. I'm having trouble working out where things are going to appear which is why I switched to 2DOrtho since it seemed to let me use real co-ords within the window. Ie. 400, 300 is the centre.

Here's my draw code, bit of a mess at the moment since I don't fully understand the viewports, matrix modes etc. Any guidance would be welcome :)

void renderGame(void) {

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 760, 480);
glLoadIdentity();
gluOrtho2D(0, 760, 0, 480);
glPushMatrix();
glColor3f(0.0,0.0,0.0); // set bg colour
glBegin(GL_QUADS); // background
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour
glRotatef(craftangle, 0, 0, 1);
glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(260+craftx, 200+crafty); // bottom l
glVertex2i(267+craftx, 224+crafty); // middle
glVertex2i(274+craftx, 200+crafty); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

}

Share this post


Link to post
Share on other sites
This:

glRotatef(angle, x, y, 1);

Isn't what you want, but you're doing it correctly in the code below. A couple of other things... You probably need to add glMatrixMode(MODELVIEW) somewhere. And you shouldn't be including your ship's position in the glVertex() calls - use glTranslate() instead. Also, your ship triangle (I assume that's what it is) is currently not centered at the origin. For both those reasons the ship will not behave onscreen as you expect.

Here's your code with a few edits:

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 760, 480);
glLoadIdentity();
gluOrtho2D(0, 760, 0, 480);
glPushMatrix(); // Is this doing anything?
glMatrixMode(GL_MODELVIEW); // Added
glLoadIdentity(); // Added
glColor3f(0.0,0.0,0.0); // set bg colour
glBegin(GL_QUADS); // background
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour
glTranslatef(ship.x, ship.y, 0); // Added
glRotatef(craftangle, 0, 0, 1);

// You can make this basic shape whatever you want - I
// made it a triangle pointing down the +x axis.
glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(200, 0); // bottom l
glVertex2i(-200, 200); // middle
glVertex2i(-200, -200); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

Anyway, you might give that a try and see if it behaves any differently.

Share this post


Link to post
Share on other sites
Thanks! You've been a great help. I altered my code to your suggestions and it now spins properly on the spot. But I've got one last problem.

At the moment, the spacecraft spins around fine on the spot at the centre where it starts. But when I accelerate it off in a direction and try and rotate it, it still uses the same 0,0 co-ords so the rotation is off again! It's as though its attached to a peice of string connected to 0,0.

Do I need to technically keep it at 0,0 and move view or something? Or is there someway to update the origin it rotates around? Here's my current code:

void renderGame(void) {

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 550, 400);
glLoadIdentity();
gluOrtho2D(-225, 225, -200, 200);
glMatrixMode(GL_MODELVIEW); // Added
glLoadIdentity(); // Added
glColor3f(0.0,0.0,0.0); // set bg colour
glBegin(GL_QUADS); // background
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour

glRotatef(craftangle, 0, 0, 1);
glTranslatef(craftx, crafty, 0); // Added

glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(7, -12); // bottom l
glVertex2i(0, 12); // middle
glVertex2i(-7, -12); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

}

Thanks again, Oh and just for the record. I'm not actually altering the craft-x. Does this matter? I'm just altering craft-y to accelerate the craft in the direction it is pointing.

[Edited by - BigDogUK on May 26, 2005 3:14:33 AM]

Share this post


Link to post
Share on other sites
Have to leave for work in a moment, but very quickly, you need to put the glTranslate() line before glRotate(), not after. Also, you need to adjust both your ship x and y coordinates, i.e. in world space. You probably need something like this:

ship.y += cos(angle) * speed;
ship.x += sin(angle) * speed; // Or maybe -sin(angle)

Share this post


Link to post
Share on other sites
Ok. Thanks!

Been playing around with that and can't seem to get it to move in any logical direction!!! I've tried a few things and I've locked it so the angle is always between 0-360 and the angle of the ship does correspond to the correct rotation.

But it's behaving REALLY erratically. For one, it seems to completetly change direction just by adding or taking one degree from the angle. And it sure as hell isn't moving in the right direction :(

Can anyone have a quick look at my source?

http://www.zen77439.zen.co.uk/other/CurrentAsteroids.cpp

There it is! I don't seem to be doing anything strange and I've tried cleaning up the mix of floats and ints but it doesn't seem to help...

Thanks

Share this post


Link to post
Share on other sites
Quote:
But it's behaving REALLY erratically. For one, it seems to completetly change direction just by adding or taking one degree from the angle. And it sure as hell isn't moving in the right direction :(
C++ trig functions take their arguments in radians, not degrees - I suspect that's where your problem lies. You can convert your angles from degrees to radians before submitting them to trig functions by multiplying by pi/180.

Share this post


Link to post
Share on other sites
Ahh, I suspected something was off with the angles because I was checking the output of the formula through cout and they were really weird. Sin 90 giving 0.44545 or something. But my knowledge of Maths isn't quite at that level. Getting on to that at A-level hopefully.

Just reworked it and the spaceship moves perfectly! Thanks for all your help :)

Share this post


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

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By reenigne
      For those that don't know me. I am the individual who's two videos are listed here under setup for https://wiki.libsdl.org/Tutorials
      I also run grhmedia.com where I host the projects and code for the tutorials I have online.
      Recently, I received a notice from youtube they will be implementing their new policy in protecting video content as of which I won't be monetized till I meat there required number of viewers and views each month.

      Frankly, I'm pretty sick of youtube. I put up a video and someone else learns from it and puts up another video and because of the way youtube does their placement they end up with more views.
      Even guys that clearly post false information such as one individual who said GLEW 2.0 was broken because he didn't know how to compile it. He in short didn't know how to modify the script he used because he didn't understand make files and how the requirements of the compiler and library changes needed some different flags.

      At the end of the month when they implement this I will take down the content and host on my own server purely and it will be a paid system and or patreon. 

      I get my videos may be a bit dry, I generally figure people are there to learn how to do something and I rather not waste their time. 
      I used to also help people for free even those coming from the other videos. That won't be the case any more. I used to just take anyone emails and work with them my email is posted on the site.

      I don't expect to get the required number of subscribers in that time or increased views. Even if I did well it wouldn't take care of each reoccurring month.
      I figure this is simpler and I don't plan on putting some sort of exorbitant fee for a monthly subscription or the like.
      I was thinking on the lines of a few dollars 1,2, and 3 and the larger subscription gets you assistance with the content in the tutorials if needed that month.
      Maybe another fee if it is related but not directly in the content. 
      The fees would serve to cut down on the number of people who ask for help and maybe encourage some of the people to actually pay attention to what is said rather than do their own thing. That actually turns out to be 90% of the issues. I spent 6 hours helping one individual last week I must have asked him 20 times did you do exactly like I said in the video even pointed directly to the section. When he finally sent me a copy of the what he entered I knew then and there he had not. I circled it and I pointed out that wasn't what I said to do in the video. I didn't tell him what was wrong and how I knew that way he would go back and actually follow what it said to do. He then reported it worked. Yea, no kidding following directions works. But hey isn't alone and well its part of the learning process.

      So the point of this isn't to be a gripe session. I'm just looking for a bit of feed back. Do you think the fees are unreasonable?
      Should I keep the youtube channel and do just the fees with patreon or do you think locking the content to my site and require a subscription is an idea.

      I'm just looking at the fact it is unrealistic to think youtube/google will actually get stuff right or that youtube viewers will actually bother to start looking for more accurate videos. 
    • By Balma Alparisi
      i got error 1282 in my code.
      sf::ContextSettings settings; settings.majorVersion = 4; settings.minorVersion = 5; settings.attributeFlags = settings.Core; sf::Window window; window.create(sf::VideoMode(1600, 900), "Texture Unit Rectangle", sf::Style::Close, settings); window.setActive(true); window.setVerticalSyncEnabled(true); glewInit(); GLuint shaderProgram = createShaderProgram("FX/Rectangle.vss", "FX/Rectangle.fss"); float vertex[] = { -0.5f,0.5f,0.0f, 0.0f,0.0f, -0.5f,-0.5f,0.0f, 0.0f,1.0f, 0.5f,0.5f,0.0f, 1.0f,0.0f, 0.5,-0.5f,0.0f, 1.0f,1.0f, }; GLuint indices[] = { 0,1,2, 1,2,3, }; GLuint vao; glGenVertexArrays(1, &vao); glBindVertexArray(vao); GLuint vbo; glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); glBufferData(GL_ARRAY_BUFFER, sizeof(vertex), vertex, GL_STATIC_DRAW); GLuint ebo; glGenBuffers(1, &ebo); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices,GL_STATIC_DRAW); glVertexAttribPointer(0, 3, GL_FLOAT, false, sizeof(float) * 5, (void*)0); glEnableVertexAttribArray(0); glVertexAttribPointer(1, 2, GL_FLOAT, false, sizeof(float) * 5, (void*)(sizeof(float) * 3)); glEnableVertexAttribArray(1); GLuint texture[2]; glGenTextures(2, texture); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageOne = new sf::Image; bool isImageOneLoaded = imageOne->loadFromFile("Texture/container.jpg"); if (isImageOneLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageOne->getSize().x, imageOne->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageOne->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageOne; glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageTwo = new sf::Image; bool isImageTwoLoaded = imageTwo->loadFromFile("Texture/awesomeface.png"); if (isImageTwoLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageTwo->getSize().x, imageTwo->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageTwo->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageTwo; glUniform1i(glGetUniformLocation(shaderProgram, "inTextureOne"), 0); glUniform1i(glGetUniformLocation(shaderProgram, "inTextureTwo"), 1); GLenum error = glGetError(); std::cout << error << std::endl; sf::Event event; bool isRunning = true; while (isRunning) { while (window.pollEvent(event)) { if (event.type == event.Closed) { isRunning = false; } } glClear(GL_COLOR_BUFFER_BIT); if (isImageOneLoaded && isImageTwoLoaded) { glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glUseProgram(shaderProgram); } glBindVertexArray(vao); glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr); glBindVertexArray(0); window.display(); } glDeleteVertexArrays(1, &vao); glDeleteBuffers(1, &vbo); glDeleteBuffers(1, &ebo); glDeleteProgram(shaderProgram); glDeleteTextures(2,texture); return 0; } and this is the vertex shader
      #version 450 core layout(location=0) in vec3 inPos; layout(location=1) in vec2 inTexCoord; out vec2 TexCoord; void main() { gl_Position=vec4(inPos,1.0); TexCoord=inTexCoord; } and the fragment shader
      #version 450 core in vec2 TexCoord; uniform sampler2D inTextureOne; uniform sampler2D inTextureTwo; out vec4 FragmentColor; void main() { FragmentColor=mix(texture(inTextureOne,TexCoord),texture(inTextureTwo,TexCoord),0.2); } I was expecting awesomeface.png on top of container.jpg

    • 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
      Hi,
      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?
  • Advertisement