Archived

This topic is now archived and is closed to further replies.

OpenGL Sprites and transparency in OpenGL...

This topic is 6016 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, Can anyone explain to me how to go about billboarding in OpenGL? The openGL User''s Guide says to use AlphaFunc(), but my images are 24-bits and don''t contain any alpha values. Thanks in advance.

Share this post


Link to post
Share on other sites
change the blending function to glBlendFunc(GL_SRC_ALPHA, GL_ONE); and enable blending when
you draw the sprite... Of course, it makes the sprite more transparent all over which is rather annoying
I figured this out the hard way the other day... Alpha channels are a really nice thing to have in
your sprites.

------------------------------
Trent (ShiningKnight)
E-mail me
OpenGL Game Programming Tutorials

Share this post


Link to post
Share on other sites
alphafunc has nothing to do with billboarding. billboard is when u make a polygon face the screen, i believe theres a recent thread in nehe that has a bt of info about possible methods

Share this post


Link to post
Share on other sites
tga is easy to load
png is difficult to load (use a library) but handles all formats and does good lossless compression

Share this post


Link to post
Share on other sites
It depends. If you have multiple levels of transparency in that alpha channel, you''ll need GL_RGBA (or something specific like GL_RGBA8). You might also be able to get away with a format like GL_RGB5_A1 (if you do this, chances are that the display adapter will store that texture in its native format, which would probably be GL_RGBA8) So, again, it depends on the image.

If you want to use the standard sort of sprites where a certain color value represents transparency, you can create the alpha channel at runtime. In the function you use to load images into a buffer (right before using that buffer to make a texture), analyze the RGB value to determine if it is. If so, make the alpha value 0, otherwise, make it 255 (assuming that you''re using an 8-bit byte per component). Then make the texture as in the paragraph above.

To make the alpha channel useful, you have to enable blending and use some blending such as glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) (that''s how I''ve always done it, though GL_SRC_ALPHA, GL_ONE might work as well).

So, then you have to draw the billboard. Since it needs to face the camera always, you just use the camera''s angle to determine the angle necessary to make the billboard parallel to it. Then just draw the billboard using the sprite texture and blending mode.

Share this post


Link to post
Share on other sites

  • Similar Content

    • 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?
    • By dpadam450
      I have this code below in both my vertex and fragment shader, however when I request glGetUniformLocation("Lights[0].diffuse") or "Lights[0].attenuation", it returns -1. It will only give me a valid uniform location if I actually use the diffuse/attenuation variables in the VERTEX shader. Because I use position in the vertex shader, it always returns a valid uniform location. I've read that I can share uniforms across both vertex and fragment, but I'm confused what this is even compiling to if this is the case.
       
      #define NUM_LIGHTS 2
      struct Light
      {
          vec3 position;
          vec3 diffuse;
          float attenuation;
      };
      uniform Light Lights[NUM_LIGHTS];
       
       
    • By pr033r
      Hello,
      I have a Bachelor project on topic "Implenet 3D Boid's algorithm in OpenGL". All OpenGL issues works fine for me, all rendering etc. But when I started implement the boid's algorithm it was getting worse and worse. I read article (http://natureofcode.com/book/chapter-6-autonomous-agents/) inspirate from another code (here: https://github.com/jyanar/Boids/tree/master/src) but it still doesn't work like in tutorials and videos. For example the main problem: when I apply Cohesion (one of three main laws of boids) it makes some "cycling knot". Second, when some flock touch to another it scary change the coordination or respawn in origin (x: 0, y:0. z:0). Just some streng things. 
      I followed many tutorials, change a try everything but it isn't so smooth, without lags like in another videos. I really need your help. 
      My code (optimalizing branch): https://github.com/pr033r/BachelorProject/tree/Optimalizing
      Exe file (if you want to look) and models folder (for those who will download the sources):
      http://leteckaposta.cz/367190436
      Thanks for any help...

  • Popular Now