Sign in to follow this  

OpenGL polygon edges and alpha blending

Recommended Posts

aaron_cook    122
I'm new to OpenGl so i've been doing some simple experimenting to get the hang of it. Ive setup a single multitextured quad in OpenGL using triangle strips as my primitive type. Ive got blending turned on with GL_ONE_MINUS_SRC_ALPHA as my blending function. The polygons colour is white (1.0, 1.0, 1.0, 1.0) and I see an alpha blended white diagonal line show through my texture that is the edge of the two poly triangles that make up the quad. This white line only appears on my nVidia card however and does not appear on an ati card that i tested the application on. If I turn off alpha blending the white line disappears so i'm convinced it has to do with blending. My rendering states are set up like so: glClearColor(1.0, 1.0, 1.0, 0.0); glPointSize(5.0); glEnable(GL_POINT_SMOOTH); glEnable(GL_LINE_SMOOTH); glEnable(GL_POLYGON_SMOOTH); glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); glShadeModel(GL_SMOOTH); glDisable(GL_DEPTH_TEST); glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); If anyone could shed some light on this most frustrating problem it would be most appreciated!

Share this post

Link to post
Share on other sites
stroma    218
your background is white polygons are white and the line is white? i cant understand the prolem well. but it may be a problem with fixed points.

Share this post

Link to post
Share on other sites
Solias    564
I don't fully understand what you are doing here. Maybe post some more of the drawing code and possibly a screen shot of the bug. My first thought was that you might be using GL_CLAMP, however that causes a similar problem on ati cards while "working" on nvidia cards. It also only happens at the edge of the texture.

Share this post

Link to post
Share on other sites
aaron_cook    122
Ive hacked out the meat of the code that i was experiencing the problem on and have created a program with only the necissary code
which follows below. I'll try to get a screen shot of the problem in action but for now clearer description of the problem is:
Imaging a single square polygon. The polygon is textured with a simple blue gradiant texture. The texture looks fine except there is a diagonal white line running from the top right corner of the square to the bottom left corner of the square. This diagonal is not 'pure' white - although it is VERY noticable - it appears to be blended with the gradiant so that it is darker where the gradiant is darker and lighter where the gradiant is lighter. ( whew! a picture IS worth a thousand words!)
Ive experimented a little and found that if i set the glTexEnv parameter to GL_REPLACE it still occures?! The only way i can get the diagonal line to dissappear completely is to turn off blending.

This is extremely frustrating! Any help would be greatly appreciated.
*note that in the code below the cPNGImage class is a helper class i wrote to load png images. Ive left out the window creation code and such for brevity...

void PrepareScene()

void Render()

gluLookAt(0.0, 0.0, 1.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0);

void InitRenderer()
int pixel_format;

memset(&pfd, 0, sizeof(PIXELFORMATDESCRIPTOR));

pfd.nVersion = 1;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = 24;
pfd.cDepthBits = 16;
pfd.iLayerType = PFD_MAIN_PLANE;
pixel_format = ChoosePixelFormat(hdc, &pfd);

if(!SetPixelFormat(hdc, pixel_format, &pfd)) {
OutputDebugString("unable to set pixel format\n");

// create our render context
hglrc = wglCreateContext(hdc);
if(hglrc == NULL) {
OutputDebugString("unable to create context\n");

if(!wglMakeCurrent(hdc, hglrc)) {
OutputDebugString("unable to make current\n");

// set the camera
RECT rect;
GetClientRect(hwnd, &rect);
glViewport(0, 0, rect.right, rect.bottom);
glOrtho(0.0f, rect.right - 1.0, 0.0, rect.bottom - 1.0, -1.0, 1.0);

glClearColor(1.0, 1.0, 1.0, 0.0);



void InitScene()
pvert = new VertexArray[4];
ptex = new TextureArray[4];

GLfloat x1=300.0, y1=300.0, x2=128.0, y2=128.0;

pvert[0].x = x1;
pvert[0].y = y2+y1;
pvert[0].z = 0.0;

pvert[1].x = x1;
pvert[1].y = y1;
pvert[1].z = 0.0;

pvert[2].x = x1+x2;
pvert[2].y = y1+y2;
pvert[2].z = 0.0;

pvert[3].x = x1+x2;
pvert[3].y = y1;
pvert[3].z = 0.0;

//create the texture
cPNGImage* png = new cPNGImage();


unsigned int id;
glGenTextures(1, &id);
glBindTexture(GL_TEXTURE_2D, id);


unsigned char* pixmap = png->GetPixmap();
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, png->GetWidth(), png->GetHeight(), 0, GL_RGBA,
GL_UNSIGNED_BYTE, (void*)pixmap);
GLenum e = glGetError();

ptex[0].u = 0.0;
ptex[0].v = 0.0;

ptex[1].u = 0.0;
ptex[1].v = 1.0;

ptex[2].u = 1.0;
ptex[2].v = 0.0;

ptex[3].u = 1.0;
ptex[3].v = 1.0;

glEnableClientState( GL_VERTEX_ARRAY);
glVertexPointer( 3, GL_FLOAT, 0, pvert );
glEnableClientState( GL_TEXTURE_COORD_ARRAY );
glTexCoordPointer( 2, GL_FLOAT, 0, ptex );

Share this post

Link to post
Share on other sites
Solias    564
Good catch. Been so long since I looked at polygon_smooth I missed it.

When you are using polygon antialiasing you have to do a few other things to get it to work right. From the Redbook:

To antialias polygons in RGBA mode, you use the alpha value to represent coverage values of polygon edges. You need to enable polygon antialiasing by passing GL_POLYGON_SMOOTH to glEnable(). This causes pixels on the edges of the polygon to be assigned fractional alpha values based on their coverage, as though they were lines being antialiased. Also, if you desire, you can supply a value for GL_POLYGON_SMOOTH_HINT.

This caused the pixels along they polygon edge to become partly transparent and let you see the white background through them. If you want polygon antialiasing check out chapter 6 in the redbook and follow all the instructions there. You can't use GL_POLYGON_SMOOTH with aplha blended translucent objects because it modifies the alpha values and also requires the objects to be drawn front to back. With tranlucent object you want to sort back to front.

I think I remember reading somewhere that certain ATI cards no longer support GL_POLYGON_SMOOTH, which could explain why it only happened on your Nvidia card.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Similar Content

    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
    • By Picpenguin
      I'm new to learning OpenGL and still learning C. I'm using SDL2, glew, OpenGL 3.3, linmath and stb_image.
      I started following through and got through it until I had to load models. The problem is, it uses Assimp for loading models. Assimp is C++ and uses things I don't want in my program (boost for example) and C support doesn't seem that good.
      Things like glVertexAttribPointer and shaders are still confusing to me, but I have to start somewhere right?
      I can't seem to find any good loading/rendering tutorials or source code that is simple to use and easy to understand.
      I have tried this for over a week by myself, searching for solutions but so far no luck. With tinyobjloader-c and project that uses it, FantasyGolfSimulator, I was able to actually load the model with plain color (always the same color no matter what I do) on screen and move it around, but cannot figure out how to use textures or use its multiple textures with it.
      I don't ask much: I just want to load models with textures in them, maybe have lights affect them (directional spotlight etc). Also, some models have multiple parts and multiple textures in them, how can I handle those?
      Are there solutions anywhere?
      Thank you for your time. Sorry if this is a bit confusing, English isn't my native language
  • Popular Now