Sign in to follow this  

OpenGL What makes OpenGL right handed?

This topic is 2840 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

Quote:
:(
Hehe :)

The image at the top of that page shows the view frustum in view space. The coordinate system as shown is right-handed, but that's just by convention; you could just as easily set it up to be left-handed (e.g. by building your own projection matrix and uploading it via glLoadMatrix(), or by performing the transform yourself in a shader).

After the division by w, the geometry is in normalized device coordinates, which are described here. Note that it says that the 'near clip plane will map to -1' and 'the far clip plane will map to 1'. In other words, normalized device coordinates are left-handed in OpenGL.

See also here, in the section titled 'The Projection Transform and Perspective Division'. The second diagram in that section shows the canonical view volume. Note the arrangement of the axes (left-handed). It also states that NDC are left-handed in the next paragraph.

As for clip space, unless I'm missing something very obvious, it is also left-handed. z values are clipped to the range -w (near) to w (far), which after division by w is transformed to the range [-1, 1].

So in short, I don't see anything there that indicates that clip space is right-handed in OpenGL.

Share this post


Link to post
Share on other sites
Quote:
Original post by jyk
So in short, I don't see anything there that indicates that clip space is right-handed in OpenGL.


Here's a test for you jyk - try running your right handed OpenGL/Direct3D engine through PIX and see what your post vertex shader values are. I'm willing to bet that they are represented in this space by a left handed system. If we could somehow use PIX with OpenGL, then I would guess they would be represented by a right handed system. It's precisely this space that I'm *guessing* makes DirectX/OpenGL left/right handed.

I've just tried this using two Direct3D right handed engines and both are left handed when I look at the vertex shader output.

Thoughts?

Share this post


Link to post
Share on other sites
Quote:
Here's a test for you jyk - try running your right handed OpenGL/Direct3D engine through PIX and see what your post vertex shader values are. I'm willing to bet that they are represented in this space by a left handed system. If we could somehow use PIX with OpenGL, then I would guess they would be represented by a right handed system. It's precisely this space that I'm *guessing* makes DirectX/OpenGL left/right handed.

I've just tried this using two Direct3D right handed engines and both are left handed when I look at the vertex shader output.

Thoughts?
Honestly, I don't think that idea makes any sense. I haven't busted out PIX yet, but I will if I have to (I'm committed to this thread now - this is going to get resolved one way or another ;).

First, let me ask you this. Do you agree that normalized device coordinates are left-handed in OpenGL? For what it's worth, I've provided a couple of links to substantiate it, and I think it would probably be useful to establish this as a 'fact', at least for the purpose of this discussion.

I think we can also agree that prior to being transformed in the vertex shader, geometry is in whatever space we want it to be (left-handed, right-handed, etc.).

So that leaves what I understand to be called 'homogenous clip space'; the space that geometry is in *after* transformation by the vertex shader and *before* division by w. This is the space that you're arguing is somehow left-handed in Direct3D, but right-handed in OpenGL. Assuming we agree about NDC, this would mean that along with the division by w, coordinates are somehow transformed from a RH system to a LH system between clip space and NDC space. How does this happen exactly?

There are several lines of argument that could be pursued here, so I'll just choose a couple. First, I'll refer you again to the second diagram on this page. The NDCS shown there is left-handed, as we've established. Note how the axes are labeled: x'/w', y'/w', and z'/w'. In other words, normalized device coordinates are simply the 'projection' of homogenous clip coordinates into 3-d space. So how would it be exactly that this projection would also effect a change of handedness?

To make this a little more concrete, let's work through an example. Consider a right-handed perspective projection transform with a field of view of 90 degrees, a square aspect ratio, and near and far values of 1 and 2, respectively. The matrix generated by gluPerspective() for this transform is:
1 0  0  0
0 1 0 0
0 0 -3 -4
0 0 -1 0
Now consider the two points:
A = [0 0 -1 1]T
B = [0 0 -2 1]T
Since the system is right-handed and the modelview transform is identity, these points lie directly in front of the viewer, with B farther away than A. Ok so far?

Now, we apply the projection transform to yield homogenous clip coordinates:
[1 0  0  0][ 0]   [ 0]
[0 1 0 0][ 0] = [ 0]
[0 0 -3 -4][-1] [-1]
[0 0 -1 0][ 1] [ 1]

[1 0 0 0][ 0] [ 0]
[0 1 0 0][ 0] = [ 0]
[0 0 -3 -4][-2] [ 2]
[0 0 -1 0][ 1] [ 2]
We already know that B is farther from the viewer than A, and we can see here that if we examine the values z/w, Bz is farther along the positive z axis than is Az. Of course these are just the values of the corresponding normalized device coordinates, so it's no surprise that the results would appear to be in a left-handed coordinate system.

Anyway, I guess my question for you would be, what in the above example indicates that clip coordinates are in a right-handed space? What should I be looking for exactly?

Disclaimer time. There are folks around here who know everything there is to know about the 3-d graphics pipeline (both Direct3D and OpenGL). I'm not one of them, so I can't claim 100% certainty or authority on this matter. There have certainly been times that I really thought I understood something and then found out I was wrong, and who knows - maybe this'll be one of those times. Maybe there's a 'magical right-handed fairy' hiding in the OpenGL pipeline somewhere that I just happen to be blissfully unaware of :)

I don't think that's the case though. I can understand the confusion, but I really think that you (and others) have just heard 'OpenGL is right-handed' so many times that you're determined to find 'right-handedness' where there is none :)

I could trot out some more references and examples, but I think what might be useful at this point would be for you to explain why you think clip space is right-handed in OpenGL. What's the evidence? Can you find any references that state this? How do you reconcile it with the example shown above? And what does it even mean for a homogenous space to be right-handed, or for the space in which the homogenous coordinates reside to have a different handedness than the space in which their 3-d projections reside?

[Ouch - that was a long post :-|]

Share this post


Link to post
Share on other sites
szecs, your prediction about the length of this thread seems to have been well founded ;)

Anyway, I dug around in the specifications a little in the hopes of perhaps putting this to rest. We'll start with this section of the 1.1 spec, item 15.

Item 15 is very clear. Among other things, it states that 'OpenGL does not force left- or right-handedness on any of its coordinate systems'.

It then goes on to give an example of a typical setup, where the view and projection transforms are right-handed, no mirroring or reflection is performed, and near < far for the depth range. It then states that given this setup, the eye coordinate system is right-handed and the clip, normalized device, and window coordinate systems are left-handed [emphasis added]. To repeat, assuming a typical setup, the clip coordinate system is left-handed according to the 1.1 spec.

Now, I don't think any of that has changed since 1.1, but just to be sure, I checked the 4.0 spec as well.

In 'Corollaries', item 7, it states that 'OpenGL does not force left- or right-handedness on any of its coordinate systems'. This is of course the same as the corresponding entry in the 1.1 spec, except that the example has been removed (presumably because it refers to functionality that has been deprecated).

Although the example (in which it is explicitly stated that clip space is left-handed) has been removed, I don't see anything in the section on clipping (2.20) to suggest that anything has changed as far as clipping is concerned.

It seems to me that we can conclude that:

1. The OpenGL API is not 'right-handed', nor is it 'left-handed'. As far as the developer is concerned, it has no inherent handedness.

2. Assuming a typical setup, clip space is left-handed.

So, I hope that settles it :) We'll see though...

Share this post


Link to post
Share on other sites
Quote:
Original post by jyk
1. The OpenGL API is not 'right-handed', nor is it 'left-handed'. As far as the developer is concerned, it has no inherent handedness.

What a surprise... [rolleyes]

Quote:
Original post by jyk
2. Assuming a typical setup, clip space is left-handed.

OpenGL clip space is a totally symmetric cube. As such, it doesn't have any inherent handedness by itself. As you quoted, it's apparent handedness is entirely dependent on whatever data you feed it with, and however you use this data afterwards.

Share this post


Link to post
Share on other sites
Quote:
What a surprise...
Yeah, I know - I was just trying to convince the holdouts :)

Share this post


Link to post
Share on other sites
Quote:
Original post by jyk
Anyway, I guess my question for you would be, what in the above example indicates that clip coordinates are in a right-handed space? What should I be looking for exactly?


Thanks for the great example jyk. Going through your calculations, it's clear that this confirms what you found regarding OpenGL clip space being left handed. The -1 (3,2) entry in the projection matrix is what turns the right handed system into a left handed one.

To me, this thread has been a great help in simplifying OpenGL/Direct3D interoperability. To me, the only fundamental difference between the two is the z-axis of NDC space going from [-1,1] in OGL and [0,1] in D3D, and an engine that uses both renderers only has to take this difference into consideration.

Share this post


Link to post
Share on other sites
Quote:
To me, the only fundamental difference between the two is the z-axis of NDC space going from [-1,1] in OGL and [0,1] in D3D, and an engine that uses both renderers only has to take this difference into consideration.
Yup, exactly - that's pretty much the only thing that differs (mathematically) between my OpenGL and D3D renderers as well.

Share this post


Link to post
Share on other sites
Quote:
Original post by GaryNas
Quote:
Original post by jyk
Anyway, I guess my question for you would be, what in the above example indicates that clip coordinates are in a right-handed space? What should I be looking for exactly?


Thanks for the great example jyk. Going through your calculations, it's clear that this confirms what you found regarding OpenGL clip space being left handed. The -1 (3,2) entry in the projection matrix is what turns the right handed system into a left handed one.

To me, this thread has been a great help in simplifying OpenGL/Direct3D interoperability. To me, the only fundamental difference between the two is the z-axis of NDC space going from [-1,1] in OGL and [0,1] in D3D, and an engine that uses both renderers only has to take this difference into consideration.


How would you take that into consideration? Is your projection matrix slightly different between OpenGL and Direct3D?

Share this post


Link to post
Share on other sites
Ha - the thread is back! :)
Quote:
Is your projection matrix slightly different between OpenGL and Direct3D?
Yup, that's exactly right. The projection matrices (both the right-handed and left-handed versions) are the same for the two APIs except for a couple of elements that are computed differently due to the differing canonical view volumes.

Share this post


Link to post
Share on other sites
Opengl fixed pipeline is dead, now all is made on vertex/fragment programs, also Directx, right handed or left handed depends on your projection matrix ONLY, you could use the same in both APIs.

Share this post


Link to post
Share on other sites
Quote:
Opengl fixed pipeline is dead, now all is made on vertex/fragment programs, also Directx, right handed or left handed depends on your projection matrix ONLY, you could use the same in both APIs.
Well, yes, that was kind of the whole point of the thread :)

Share this post


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

  • 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
      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?
    • By xhcao
      Does sync be needed to read texture content after access texture image in compute shader?
      My simple code is as below,
      glUseProgram(program.get());
      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?
      glUseProgram(0);
      glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
      glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                                     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?
  • Popular Now