• Advertisement
Sign in to follow this  

OpenGL Matrices and the pipeline

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, Well I'm after making some decent progress since I first took up learning OpenGL a few months ago. I can now use vertex arrays, vertex buffer objects, indexed geometry, texture mapping, fog, mipmapping, vertex/pixel shaders etc. etc.. However I feel as if I've skipped some of the basics, which I should really know inside out at this stage... My first question relates to the two matrices: the 'modelview' matrix and 'projection matrix'. In my 3D engine I only use the projection matrix to translate and rotate the map and all the game objects. This works fine, however I am unsure as why there is a second matrix (modelview) and what its purpose is. I have looked for some good explanations on the differences between the two matrices, but couldn't find anything decent to help explain them. So why do we need two transformation matrices ? What typical scenarios would you use each of the two matrices? Is the modelview matrix a transformation matrix used for transformations local to each object in a game (eg: monster / map item) and the projection matrix used for transforming the world (entire map geometry) ? Or is the modelview matrix simply an additional matrix which can recalled and used if needed ? Why use it? The second question regards the pipeline and viewing transforms. When I render each frame I first setup the projection matrix / rotations etc and then draw the map. I then push this matrix to save its status. The matrix is then transformed again by whatever position/rotation that each monster has, and the monster is then drawn. After drawing each monster I then pop the matrix to reload the previously saved state. Now.. If i remove the pop matrix line the entire world gets translated by whatever I translate the monster by. This brings me to some alarming conclusions.. When I am translating the monster, am I really translating everything else too? I thought once something was sent down the pipeline it stays in whatever position it was when it was first sent? Does this mean everytime I move a monster a whole bunch of unnecessary calculations are being peformed on the rest of the map? If so, how can I avoid such calculations? Thanks in advance for your help. I know this post is a bit of a pain [smile] but these are things I really need to get clear in my head before I go any further.

Share this post


Link to post
Share on other sites
Advertisement
Hi,

Whether you know it or not, you are using the modelview matrix. Search through your code - somewhere I'm sure you'll find the line glMatrixMode(GL_MODELVIEW).

Ok, so on to what the two matrices do. The modelview matrix performs an affine transformation; more practically, it sets the position and orientation of your camera and of objects in the scene in 3d space, and applies other linear transformations such as scale, reflection, and shear.

The projection matrix performs a different function: it converts the 3d world into 2d information that can be displayed on the screen. Variables include field of view, aspect ratio, and near and far clipping planes.

I'm not exactly sure how OpenGL implements this internally, but in any case you don't have to worry about doing 'too much transformation'. That's why matrix concatenation is useful; no matter how many transformations you combine, it still comes down to a single matrix-vector multiplication per vertex.

Share this post


Link to post
Share on other sites
projection matirx for the frustum and ortho things.
modelview matrix for translating/rotatting/scaling for the objects
texture matrix same as modelview matrix but for the textures GL_TEXTURE_1D/2D/3D
for more details read the red book

Share this post


Link to post
Share on other sites
mmm i am not sure about your second question but every push you should pop it or you'll face problem, because the matrix stack has a limit and every loop in your main function you'll push till it gets full .
maybe i am wrong about this ..
bye

Share this post


Link to post
Share on other sites
Ah yes- now it makes more sense.. I can see the relationship between the two more clearly now. Good reply..

Quote:
Original post by jyk
Whether you know it or not, you are using the modelview matrix. Search through your code - somewhere I'm sure you'll find the line glMatrixMode(GL_MODELVIEW).


Nope. All throughout my code the matrix used is GL_PROJECTION - even in my vertex shaders. I can see how it can be used to achieve the same effect though, since ultimately it controls how points are projected from world space onto the screen.

Ok, so that's one issue cleared up I think. Any more ideas on the other question?

Thanks again for your post.

Share this post


Link to post
Share on other sites
The answer to the second question is that, for rigid meshes, every vertex is transformed exactly once, with whatever matrix was loaded at the time the draw function is called. Basically, when you call glBegin() the driver makes a copy of the matrix at the top of the stack and sends that along with the vertices you specify, and whatever happens afterwards it preserves the illusion that the vertices are completely transformed & rendered with the current GL state before you go and muck with it.

So, the transformations are just numerical manipulations on the matrix only, and don't actually *move* anything until you draw something. It looks like a little crash-course in linear algebra would help you out a lot in understanding these concepts.

Tom

Share this post


Link to post
Share on other sites
Also, it is perfectly plausible to use the projection matrix only, as the two matrices are always concatenated prior to transformation. They are only separate because it can be handy to manipulate one and not the other. For instance:

You have a Projection matrix with the following transforms:
1. Frustum transform
2. Viewing (camera) transform

And a Model matrix with the following transforms:
3. World transform
4. Model transform
5. Sub-model transform (objects moving relative to parent objects, etc)

This is more or less the same as one Projection matrix with transforms 1-5, but say you wanted to draw mountains twice, once regularly and once reflected in water. To do the reflection, you want to flip the camera.

If you're using one matrix, you need to pop 5, 4, 3, and 2, specify a new camera transform, and then specify 3-5 again.

If you're using two matrices, you just pop 2 off the Projection and specify your new camera, and the Model matrix stays the same.

Tom

Share this post


Link to post
Share on other sites
Quote:
Original post by ParadigmShift
The answer to the second question is that, for rigid meshes, every vertex is transformed exactly once, with whatever matrix was loaded at the time the draw function is called. Basically, when you call glBegin() the driver makes a copy of the matrix at the top of the stack and sends that along with the vertices you specify, and whatever happens afterwards it preserves the illusion that the vertices are completely transformed & rendered with the current GL state before you go and muck with it.


Excellent. That was just what I needed to know.. At least I can be sure now that everything is being transformed once, and once only.

Quote:
Original post by ParadigmShift
So, the transformations are just numerical manipulations on the matrix only, and don't actually *move* anything until you draw something. It looks like a little crash-course in linear algebra would help you out a lot in understanding these concepts.
Tom


I do a lot of stuff on algebra and matrices in college- just had my final year maths exam today in fact! [smile] Its not that I don't understand the maths involved in the rotations, I'm just unsure exactly what the hell OpenGL / display drivers are doing underneath the API to my geometry. However, you're post has helped clear that up so I am thankful for that.

Well thats great guys, I feel much more enlightened now! [wink] You have been most helpful so I think extra ratings are in order.

Cheers
-Darragh

Share this post


Link to post
Share on other sites
Hm, guess I was wrong about having to use the modelview matrix :-| However, see here (section 8.030) and here for discussion of the way OpenGL intends for the matrices to be used and the reasons for following these guidelines.

Share this post


Link to post
Share on other sites
Thanks for that. Those two articles are very informative- i'll keep them for future reference.

You've also managed to solve another long outstanding problem i had...

In the article about 'GL_PROJECTION' abuse (of which I am guilty [smile]), it also mentions the problems of using the projection matrix with fog. Now cast you're eye back to the thread which I started on that very subject.. I was unable to solve the problem (until now) and subsequently had to resort to learning GLSL and using pixel shaders in order to do my fogging. If only I had known about this back then!... [smile]

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