• Advertisement
Sign in to follow this  

OpenGL dark regions in some parts of the mesh

This topic is 4605 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 friends, I'm new in OpenGL. I loaded a .3ds model in my OpenGL code. There is no texture mapped on the model (only material color). The model contains thin triangles in some regions. The problem is that these regions are appearing darker... (as if I had applied a super smooth on the surface). In 3ds max the model doesn't show these dark regions. It seems a small shading problem... but I'm sure the normals were correctly calculated. Is there some OpenGL setting I should do to avoid that? Thanks in advance.

Share this post


Link to post
Share on other sites
Advertisement
OK friends, here is an image about the problem. The left model was exported as a 3ds file and it was rendered using OpenGL (notice the dark regions). The right model was exported as an x-file and it was rendered using Direct3D (no shading problem here).

models.jpg

Share this post


Link to post
Share on other sites
Most likely its a case of shoddy normals. Are you using normals supplied by the 3ds model? If not, make sure you're generating "good" normals. and finnally, show us your rendering code (more importantly your setup before you render)

cheers
-Dan

Share this post


Link to post
Share on other sites
Hi Ademan555, the rendering code is pratically the same than 3DS Loader shown in Game Tutorials site.


LFace face;

for (int k=0; k < GetMesh->GetFaceCount(); k++)
{
face = GetMesh->GetFace(k);

glBegin(GL_TRIANGLES);
glNormal3f(face.normals[0].x, face.normals[0].y, face.normals[0].z);
glVertex3f(face.vertices[0].x, face.vertices[0].y, face.vertices[0].z);

glNormal3f(face.normals[1].x, face.normals[1].y, face.normals[1].z);
glVertex3f(face.vertices[1].x, face.vertices[1].y, face.vertices[1].z);

glNormal3f(face.normals[2].x, face.normals[2].y, face.normals[2].z);
glVertex3f(face.vertices[2].x, face.vertices[2].y, face.vertices[2].z);
glEnd();
}


LFace is a structure containing vertices and normals data:

struct LVector3
{
float x;
float y;
float z;
};

struct LFace
{
LVector3 vertices[3];
LVector3 normals[3];
};


The function to calculate the normals is:

void LMesh::CalcNormals()
{
LVector3 vertex;
LVector3 normal;
int i, k, j;
if (m_vertexCount <= 0)
return;
m_normalCount = m_vertexCount;
m_normals = (LVector3*) malloc(m_vertexCount*sizeof(LVector3));

for (i=0; i<m_vertexCount; i++)
{
normal.x = 0.0f;
normal.y = 0.0f;
normal.z = 0.0f;
vertex = m_vertices[i];
// find all vertices with the same coords
for (k=0; k<m_vertexCount; k++)
{
if ((fabs(vertex.x - m_vertices[k].x) < 0.0000001f) &&
(fabs(vertex.y - m_vertices[k].y) < 0.0000001f) &&
(fabs(vertex.z - m_vertices[k].z) < 0.0000001f))
{
for (j=0; j<m_triangleCount; j++)
{
if ((m_triangles[j].a == (unsigned int)k) ||
(m_triangles[j].b == (unsigned int)k) ||
(m_triangles[j].c == (unsigned int)k))
{
LVector3 a, b, n;
a = SubtractVectors(m_vertices[m_triangles[j].b], m_vertices[m_triangles[j].a]);
b = SubtractVectors(m_vertices[m_triangles[j].b], m_vertices[m_triangles[j].c]);
n = CrossProduct(b, a);
n = NormalizeVector(n);
normal = AddVectors(normal, n);
}
}
}
}
m_normals[i] = NormalizeVector(normal);

}
}

Share this post


Link to post
Share on other sites
It looks like you're doing a basic vertex normal by averaging all incident face normals. This is fine for smooth meshes (eg. heightmaps) but for an object like this you actually want two discrete normals for certain verts (like those around the edges of the holes). Best option is to actually export and load those normals from your editor.

Alternativly, you need to add the concept of a 'crease angle'. This basically means if two incident normals are too dissimilar (ie. you think theres a crease in the mesh) you duplicate the vert and only use the normals that are similar. This is fairly standard so google should give you a suitable algorithm.

Share this post


Link to post
Share on other sites
Quote:
Original post by OrangyTang
Alternativly, you need to add the concept of a 'crease angle'. This basically means if two incident normals are too dissimilar (ie. you think theres a crease in the mesh) you duplicate the vert and only use the normals that are similar. This is fairly standard so google should give you a suitable algorithm.

This seems to be the case to me on the screenshot, the shading goes weird because the normals are smoothing faces which are pretty much at straight angle to one another, and consequently the renderer is trying to interpolate between 'in light' and 'in shadow' along what's really a single, flat polygon.

checking acos( dot_product( face_1_normal, face_2_normal )) if it's below defined threshold angle (in radians) would generally work from what i can see.

On side note, testing every vertex against every vertex to see if they share position isn't very fast... implementing some kind of hash_multimap which keeps references to the vertices, with vertex position being the map key... might speed things up a lot, the testing for each vertex would be then done against greatly reduced sub-range of the whole set.

Share this post


Link to post
Share on other sites
It looks like you used a boolean object in creating that model in 3d studio's, that can have some weird effects on meshes.

Besides that, try just calculating normals per face and rendering them that way instead of calculating per vertex.

It will give you more of the look you have on the .x file rather than the smoothed look you have in the 3ds file.

Share this post


Link to post
Share on other sites
Thanks folks,

I solved the problem by importing the normals from an x-file to my OpenGL code (I wrote a code to parse the x-file to OpenGL functions). I decided to use an x-file (instead of a .3ds file) because I am already familiarized with this file format.

Another reason why I decided to import normals instead of calculating them in the code is: Usually, it is more flexible to change the smooth level in the mesh editor (3ds max) than in the code.

Thank you very much again for the help.

Share this post


Link to post
Share on other sites
Quote:
Original post by Mari_p
Another reason why I decided to import normals instead of calculating them in the code is: Usually, it is more flexible to change the smooth level in the mesh editor (3ds max) than in the code.

True about the flexibility... that's why ideally for 3ds format you'd be reading the smoothing groups information stored in the file:
** Subchunks of 0x4000 - Object description Block
* Subchunks of 0x4100 - Triangular Polygon List
(..)
0x4150: Face Smoothing Group chunk.
stores: unsigned int * number of faces.
bits of the int indicate enabled smooth groups for this particular face.

... then during the check of vertices, instead of dot product of face normals, you do bitwise AND check: face_1_smoothgroup & face_2_smoothgroup ... if the result is non-zero it means faces share at least one smoothing group, and the normals for points they share should be smoothed.

The angle test on the other hand is good for other formats which make use of it, Lightwave files for example.

[edit] just for the heck of it, this thread gave me excuse to do small test how many vertex checks are done with different methods... the results for classic teapot with 5894 vertices (after unwelding):

* straight comparison (each vertex with each vertex): 34,739,236 tests
* 'smart' straight comparison (tested vertex and the target share their normals at once, vertices which already did it are skipped): 17,366,671 tests
* hash map, straight comparison: 118,354 tests
* 'smart' hash map, similar to method 2: 56,230 tests

from nearly 35 mil to 56 k... wish it's always possible to reduce workload like that :s

[Edited by - tolaris on June 12, 2005 10:22:35 AM]

Share this post


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

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • 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