# OpenGL OpenGL triangle winding order

This topic is 1863 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

Hello,

I'm not sure I understand how the triangle winding order works. I've been reading http://www.arcsynthesis.org/gltut/Positioning/Tutorial%2004.html but I don't quite understand how it works.

Specifically these lines, where the bolded I do not understand:

[quote]No matter what size or shape the triangle is, you can classify the ordering of a triangle in two ways: clockwise or counter-clockwise. That is, if the order of the vertices from 1 to 2 to 3 moves clockwise in a circle, relative to the triangle's center, then the triangle is [b]facing clockwise relative to the viewer[/b]. Otherwise, the triangle is counter-clockwise relative to the viewer.[/quote]

For reference, I'm trying to make a cube using glDrawElements, and I want to know how I should order them and why. Below is a naive attempt:

[CODE]
const float vertexPositions[] =
{
// front vertices
-0.5f, 0.5f, 0.0f,
0.5f, 0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
// back vertices
-0.5f, 0.5f, 0.5f,
0.5f, 0.5f, 0.5f,
0.5f, -0.5f, 0.5f,
-0.5f, -0.5f, 0.5f
};
const GLshort indexData[] =
{
// back
4, 5, 6,
7, 4, 5,
// right
1, 5, 6,
2, 1, 6,
// left
4, 0, 3,
7, 4, 3,
// top
4, 5, 1,
0, 4, 1,
// bottom
7, 6, 2,
3, 7, 2,
// front
0, 1, 2,
3, 0, 1
};
[/CODE] Edited by KaiserJohan

##### Share on other sites
Imagine you paint a triangle on a piece of paper, labeling the vertices A, B, C. Pick a thin paper and pen which is clearly visible from both sides. Looking at it from one side you would see something like this:
[code]
A
|\
| \
B--C
[/code]
Like this, the triangle ABC is in counterclockwise order. Now move your head around the paper so you see it from the other side (alternatively, just flip it over).
What you see from this position (minus the flipped BC) is:
[code]
A
/|
/ |
C--B
[/code]
The triangle ABC is now in clockwise order (and would be culled normally).

##### Share on other sites
Also take into account this deals with 2D space. Regarding 3D models, you'd determine whether a triangle is clockwise or not depending from where you're looking at. Generally you choose whether triangles when seen from the "front" (e.g. outside of the mesh) are clockwise or not and arrange vertices accordingly.

The main reason this is done is backface culling (to get rid of most triangles you'll never see anyway) and stuff like normals (which need to point towards one side of the triangle)

##### Share on other sites
Which one is prefered, CW or CCW?

##### Share on other sites
[quote name='BitMaster' timestamp='1355321446' post='5009825']
Imagine you paint a triangle on a piece of paper, labeling the vertices A, B, C. Pick a thin paper and pen which is clearly visible from both sides. Looking at it from one side you would see something like this:
[code]
A
|\
| \
B--C
[/code]
Like this, the triangle ABC is in counterclockwise order. Now move your head around the paper so you see it from the other side (alternatively, just flip it over).
What you see from this position (minus the flipped BC) is:
[code]
A
/|
/ |
C--B
[/code]
The triangle ABC is now in clockwise order (and would be culled normally).
[/quote]

What effect does this have when I have to specify in what order to draw the vertices?
For example, if I use CW and I draw vertices (0, 1, 2), and then try the same with (2, 1, 0), it wont be shown?

##### Share on other sites
The triangle ABC is in CCW order in my first example (and would be rendered). The same triangle ABC is in CW order in my second example (and would be culled). The second example is the same situation as the first example, just viewed from the opposite direction. So the same triangle is visible or invisible depending on the view direction and that decision is made using the winding order of the triangle vertices projected into 2D.
On the other hand, the triangle CBA is in CW order in the first example and in CCW order in the second example.

Most renderers (that includes DirectX and OpenGL) expect a CCW face to be the front face and will simply cull backfaces. At least in OpenGL you can specify which winding is treated as the front face and how exactly to handle front- and backfaces.

• 11
• 9
• 16
• 18
• 10
• ### Similar Content

• i got error 1282 in my code.
#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?
• 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);