Jump to content
  • Advertisement
Sign in to follow this  

OpenGL Alpha-blending woes (it's CLOSE!)

This topic is 4276 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

The short version: I'm trying to draw a star-box (texture alpha=1), then a fade-out skybox over it (texture alpha=1 in the center, fade to 0 at the edges), then some geometry over that. Try as I may, and I've read the tutorials and the redbook and appropriate sections of the bluebook, I just can't seem to get it to come out right. :( The medium-length version: Please see 2 posts down. (Some said my long version was too long to follow, so I tried to summarize.) The long version: Here are some clips from posts I made to <news:comp.graphics.api.opengl>, they contain a URL to some screen shots, and a few code snippets. --- Here's what I've got: My source image is a .png, so the data is stored ARGB. So I tell GL that it's BGRA, but to use reversed bytes. So my calls look something like this:
// Earlier in setup:
gl.glEnable (GL.GL_BLEND);

// When I'm creating texture:
gl.glBindTexture (GL.GL_TEXTURE_2D, textureID);
gl.glTexImage2D (GL.GL_TEXTURE_2D, 0, GL_RGBA8 width, height, 0, 
GL_BGRA_EXT, GL_UNSIGNED_INT_8_8_8_8_REV, imageData);
gl.glBindTexture (GL.GL_TEXTURE_2D, 0);

The problem that I'm running into is: even though my texture.png looks fine when I display it over the top of another image, and it fades nicely from the center with alpha=1.0 to the edges where alpha=0.0, as a texture, even where the texture-alpha is very small (say, 0.1), it's still behaving as if alpha=1.0, but with very faint color. then there's a "clean break" at alpha=0. This does not give the desired "see the other objects through my texture" effect. --- Here is a .zip with a handful of screenshots. Here's what I'm trying to do: * Draw a field of stars - ...then clear the depth buffer * Draw a blue-faded sky (alpha=1 at zenith, alpha=0 near the edges, with a gradual fade between.) - ... then clear the depth buffer * Draw some geometry. Some of it is textured and/or blended. My screen shots come from 3 separate runs: * Scenario 1 (Alpha)
gl.glAlphaFunc (GL.GL_GREATER, 0.0f);
gl.glEnable (GL.GL_ALPHA_TEST);
gl.glDisable (GL.GL_BLEND);

Pict 2, 4 Flaws: Blue sky fades to white, until we get to the alpha=0 part, then there's a sudden cutoff. I'm wanting the blue to gently fade away and expose the starfield underneath. * Scenario 2 (both)
gl.glEnable (GL.GL_BLEND);

gl.glAlphaFunc (GL.GL_GREATER, 0.0f);
gl.glEnable (GL.GL_ALPHA_TEST);
gl.glDisable (GL.GL_BLEND);

Pict 1 Flaws: Same as scenario #1 * Scenario 3 (blend)
gl.glEnable (GL.GL_BLEND);

Pict 5 Flaws: I think of this as the closest to what I want, in that the blue gradually fades out, but it STILL doesn't show -- say, at the alpha=0.5 point -- the starfield behind it; there's still that sudden cutoff when we get to alpha=0. In addition, the geometry is weird. While the "deck" does, indeed show some faded tiles (as desired), the purple cube & cylindar are mysteriously see-through, even though they're supposed to be alpha=1, not-blended, not-textured, normal-old-geometry (as in the other pix.) NOTES: My geometry consists of... * A red gluSphere * 3 axis lines, red, green blue = X, Y, Z. * A white, smaller gluSphere on the end of the X-line * A white-ish emmitting gluSphere to represent where my light-source is. * A purple gluCube * A purple gluCylinder * A rectangle with a solid "grassy" texture. Also, I glClear(GL_DEPTH_BUFFER_BIT) after the starfield and the sky-box (both are skybox objects, just with different textures. Pict#6 shows just the starfield (no skybox) -- it has all alpha=1, and appears to work pretty much as I'd expect. My problems seem to come when I try to draw a variable-alpha-texture over the top of it. --- (NOTE: my skyboxes are cubes, not spheres. They're drawn to "face inward.") I guess I should take some comfort in the fact that this doesn't appear to be an obvious n00b mistake. As in, no one's piped-up with "well, it's clear that you're calling glFoo(), when you need to glBar()", or whatever... Can anyone point me to sample code that does this sort of thing? By "this sort of thing", I specifically mean... * Draw a field of stars (alpha=1 for whole texture.) - ...then clear the depth buffer * Draw a blue-faded sky (alpha=1 at zenith, alpha=0 near the edges, with a gradual fade between.) - ... then clear the depth buffer * Draw some geometry. Some of it is textured and/or blended. Thanks! [Edited by - Chum on October 5, 2006 9:36:14 AM]

Share this post

Link to post
Share on other sites
i cant concentrate with long posts but i assume u want a skybox of stars

draw box first
then disable depthtest
draw stars around camera

see my website all starboxs were made with this method

Share this post

Link to post
Share on other sites
Ok, I'll try to restate in a shorter way:

* I am successfully drawing a skybox object containing a field of stars, my "starbox"
* The STARbox texture is alpha=1 throughout, and draws correctly.
* I have a 2nd skybox that I try to draw over the top of that one, it is my "sky" skybox
* The "SKY" box has a texture that is all blue, with alpha=1 in the center fading to alpha=0 just below the horizon, and a GRADUAL FADE between the two.
* My problem seems to be that, in the places where 0 < alpha < 1, while the colors are faded (as expected), the alpha does not blend -- that is, at the alpha=0.5 part of the picture, I'd expect to see some blue, but a few stars starting to poke through, etc., down to where alpha=0.

...Instead, it's as if everything alpha > 0 is treadted as alpha=1, with a sudden cuttoff to alpha=0.

My 1st post has code examples of what I'm doing, including a description of the code and faulty behaviour of the screenshots at this link

I hope that's easier to follow.

Thanks for your help!

Share this post

Link to post
Share on other sites
sorry i still dont understand fully (its early)
but ill try (dont use glAlphaTest
*draw the starbox
*enable blending with src_ALPHA 1-src_alpha
*draw the blue texture
*glDisable( GL_BLEND );
*draw eveything else

Share this post

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

  • Advertisement
  • Advertisement
  • Popular Tags

  • Similar Content

    • By nOoNEE
      hello guys , i have some questions  what does glLinkProgram  and  glBindAttribLocation do?  i searched but there wasnt any good resource 
    • By owenjr
      Hi, I'm a Multimedia Engineering student. I am about to finish my dergree and I'm already thinking about what topic to cover in my final college project.
      I'm interested in the procedural animation with c++ and OpenGL of creatures, something like a spider for example. Can someone tell me what are the issues I should investigate to carry it out? I understand that it has some dependence on artificial intelligence but I do not know to what extent. Can someone help me to find information about it? Thank you very much.
      - Procedural multi-legged walking animation
      - Procedural Locomotion of Multi-Legged Characters in Dynamic Environments
    • By Lewa
      So, i'm still on my quest to unterstanding the intricacies of HDR and implementing this into my engine. Currently i'm at the step to implementing tonemapping. I stumbled upon this blogposts:
      and tried to implement some of those mentioned tonemapping methods into my postprocessing shader.
      The issue is that none of them creates the same results as shown in the blogpost which definitely has to do with the initial range in which the values are stored in the HDR buffer. For simplicity sake i store the values between 0 and 1 in the HDR buffer (ambient light is 0.3, directional light is 0.7)
      This is the tonemapping code:
      vec3 Uncharted2Tonemap(vec3 x) { float A = 0.15; float B = 0.50; float C = 0.10; float D = 0.20; float E = 0.02; float F = 0.30; return ((x*(A*x+C*B)+D*E)/(x*(A*x+B)+D*F))-E/F; } This is without the uncharted tonemapping:
      This is with the uncharted tonemapping:
      Which makes the image a lot darker.
      The shader code looks like this:
      void main() { vec3 color = texture2D(texture_diffuse, vTexcoord).rgb; color = Uncharted2Tonemap(color); //gamma correction (use only if not done in tonemapping code) color = gammaCorrection(color); outputF = vec4(color,1.0f); } Now, from my understanding is that tonemapping should bring the range down from HDR to 0-1.
      But the output of the tonemapping function heavily depends on the initial range of the values in the HDR buffer. (You can't expect to set the sun intensity the first time to 10 and the second time to 1000 and excpect the same result if you feed that into the tonemapper.) So i suppose that this also depends on the exposure which i have to implement?
      To check this i plotted the tonemapping curve:
      You can see that the curve goes only up to around to a value of 0.21 (while being fed a value of 1) and then basically flattens out. (which would explain why the image got darker.)
      My guestion is: In what range should the values in the HDR buffer be which then get tonemapped? Do i have to bring them down to a range of 0-1 by multiplying with the exposure?
      For example, if i increase the values of the light by 10 (directional light would be 7 and ambient light 3) then i would need to divide HDR values by 10 in order to get a value range of 0-1 which then could be fed into the tonemapping curve. Is that correct?
    • By nOoNEE
      i am reading this book : link
      in the OpenGL Rendering Pipeline section there is a picture like this: link
      but the question is this i dont really understand why it is necessary to turn pixel data in to fragment and then fragment into pixel could please give me a source or a clear Explanation that why it is necessary ? thank you so mu
    • By Inbar_xz
      I'm using the OPENGL with eclipse+JOGL.
      My goal is to create movement of the camera and the player.
      I create main class, which create some box in 3D and hold 
      an object of PlayerAxis.
      I create PlayerAxis class which hold the axis of the player.
      If we want to move the camera, then in the main class I call to 
      the func "cameraMove"(from PlayerAxis) and it update the player axis.
      That's work good.
      The problem start if I move the camera on 2 axis, 
      for example if I move with the camera right(that's on the y axis)
      and then down(on the x axis) -
      in some point the move front is not to the front anymore..
      In order to move to the front, I do
      player.playerMoving(0, 0, 1);
      And I learn that in order to keep the front move, 
      I need to convert (0, 0, 1) to the player axis, and then add this.
      I think I dont do the convert right.. 
      I will be glad for help!

      Here is part of my PlayerAxis class:
      //player coordinate float x[] = new float[3]; float y[] = new float[3]; float z[] = new float[3]; public PlayerAxis(float move_step, float angle_move) { x[0] = 1; y[1] = 1; z[2] = -1; step = move_step; angle = angle_move; setTransMatrix(); } public void cameraMoving(float angle_step, String axis) { float[] new_x = x; float[] new_y = y; float[] new_z = z; float alfa = angle_step * angle; switch(axis) { case "x": new_z = addVectors(multScalar(z, COS(alfa)), multScalar(y, SIN(alfa))); new_y = subVectors(multScalar(y, COS(alfa)), multScalar(z, SIN(alfa))); break; case "y": new_x = addVectors(multScalar(x, COS(alfa)), multScalar(z, SIN(alfa))); new_z = subVectors(multScalar(z, COS(alfa)), multScalar(x, SIN(alfa))); break; case "z": new_x = addVectors(multScalar(x, COS(alfa)), multScalar(y, SIN(alfa))); new_y = subVectors(multScalar(y, COS(alfa)), multScalar(x, SIN(alfa))); } x = new_x; y = new_y; z = new_z; normalization(); } public void playerMoving(float x_move, float y_move, float z_move) { float[] move = new float[3]; move[0] = x_move; move[1] = y_move; move[2] = z_move; setTransMatrix(); float[] trans_move = transVector(move); position[0] = position[0] + step*trans_move[0]; position[1] = position[1] + step*trans_move[1]; position[2] = position[2] + step*trans_move[2]; } public void setTransMatrix() { for (int i = 0; i < 3; i++) { coordiTrans[0][i] = x[i]; coordiTrans[1][i] = y[i]; coordiTrans[2][i] = z[i]; } } public float[] transVector(float[] v) { return multiplyMatrixInVector(coordiTrans, v); }  
      and in the main class i have this:
      public void keyPressed(KeyEvent e) { if (e.getKeyCode()== KeyEvent.VK_ESCAPE) { System.exit(0); //player move } else if (e.getKeyCode()== KeyEvent.VK_W) { //front //moveAmount[2] += -0.1f; player.playerMoving(0, 0, 1); } else if (e.getKeyCode()== KeyEvent.VK_S) { //back //moveAmount[2] += 0.1f; player.playerMoving(0, 0, -1); } else if (e.getKeyCode()== KeyEvent.VK_A) { //left //moveAmount[0] += -0.1f; player.playerMoving(-1, 0, 0); } else if (e.getKeyCode()== KeyEvent.VK_D) { //right //moveAmount[0] += 0.1f; player.playerMoving(1, 0, 0); } else if (e.getKeyCode()== KeyEvent.VK_E) { //moveAmount[0] += 0.1f; player.playerMoving(0, 1, 0); } else if (e.getKeyCode()== KeyEvent.VK_Q) { //moveAmount[0] += 0.1f; player.playerMoving(0, -1, 0); //camera move } else if (e.getKeyCode()== KeyEvent.VK_I) { //up player.cameraMoving(1, "x"); } else if (e.getKeyCode()== KeyEvent.VK_K) { //down player.cameraMoving(-1, "x"); } else if (e.getKeyCode()== KeyEvent.VK_L) { //right player.cameraMoving(-1, "y"); } else if (e.getKeyCode()== KeyEvent.VK_J) { //left player.cameraMoving(1, "y"); } else if (e.getKeyCode()== KeyEvent.VK_O) { //right round player.cameraMoving(-1, "z"); } else if (e.getKeyCode()== KeyEvent.VK_U) { //left round player.cameraMoving(1, "z"); } }  
      finallt found it.... i confused with the transformation matrix row and col. thanks anyway!
    • By Lewa
      So, i'm currently trying to implement an SSAO shader from THIS tutorial and i'm running into a few issues here.
      Now, this SSAO method requires view space positions and normals. I'm storing the normals in my deferred renderer in world-space so i had to do a conversion and reconstruct the position from the depth buffer.
      And something there goes horribly wrong (which has probably to do with worldspace to viewspace transformations).
      (here is the full shader source code if someone wants to take a look at it)
      Now, i suspect that the normals are the culprit.
      vec3 normal = ((uNormalViewMatrix*vec4(normalize(texture2D(sNormals, vTexcoord).rgb),1.0)).xyz); "sNormals" is a 2D texture which stores the normals in world space in a RGB FP16 buffer.
      Now i can't use the camera viewspace matrix to transform the normals into viewspace as the cameras position isn't set at (0,0,0), thus skewing the result.
      So what i did is to create a new viewmatrix specifically for this normal without the position at vec3(0,0,0);
      //"camera" is the camera which was used for rendering the normal buffer renderer.setUniform4m(ressources->shaderSSAO->getUniform("uNormalViewMatrix"), glmExt::createViewMatrix(glm::vec3(0,0,0),camera.getForward(),camera.getUp())//parameters are (position,forwardVector,upVector) ); Though i have the feeling this is the wrong approach. Is this right or is there a better/correct way of transforming a world space normal into viewspace?
    • By HawkDeath
      I'm trying mix two textures using own shader system, but I have a problem (I think) with uniforms.
      Code: https://github.com/HawkDeath/shader/tree/test
      To debug I use RenderDocs, but I did not receive good results. In the first attachment is my result, in the second attachment is what should be.
      PS. I base on this tutorial https://learnopengl.com/Getting-started/Textures.

    • By norman784
      I'm having issues loading textures, as I'm clueless on how to handle / load images maybe I missing something, but the past few days I just google a lot to try to find a solution. Well theres two issues I think, one I'm using Kotlin Native (EAP) and OpenGL wrapper / STB image, so I'm not quite sure wheres the issue, if someone with more experience could give me some hints on how to solve this issue?
      The code is here, if I'm not mistaken the workflow is pretty straight forward, stbi_load returns the pixels of the image (as char array or byte array) and you need to pass those pixels directly to glTexImage2D, so a I'm missing something here it seems.
    • By Hashbrown
      I've noticed in most post processing tutorials several shaders are used one after another: one for bloom, another for contrast, and so on. For example: 
      postprocessing.quad.bind() // Effect 1 effect1.shader.bind(); postprocessing.texture.bind(); postprocessing.quad.draw(); postprocessing.texture.unbind(); effect1.shader.unbind(); // Effect 2 effect2.shader.bind(); // ...and so on postprocessing.quad.unbind() Is this good practice, how many shaders can I bind and unbind before I hit performance issues? I'm afraid I don't know what the good practices are in open/webGL regarding binding and unbinding resources. 
      I'm guessing binding many shaders at post processing is okay since the scene has already been updated and I'm just working on a quad and texture at that moment. Or is it more optimal to put shader code in chunks and bind less frequently? I'd love to use several shaders at post though. 
      Another example of what I'm doing at the moment:
      1) Loop through GameObjects, bind its phong shader (send color, shadow, spec, normal samplers), unbind all.
      2) At post: bind post processor quad, and loop/bind through different shader effects, and so on ...
      Thanks all! 
    • By phil67rpg
      void collision(int v) { collision_bug_one(0.0f, 10.0f); glutPostRedisplay(); glutTimerFunc(1000, collision, 0); } void coll_sprite() { if (board[0][0] == 1) { collision(0); flag[0][0] = 1; } } void erase_sprite() { if (flag[0][0] == 1) { glColor3f(0.0f, 0.0f, 0.0f); glBegin(GL_POLYGON); glVertex3f(0.0f, 10.0f, 0.0f); glVertex3f(0.0f, 9.0f, 0.0f); glVertex3f(1.0f, 9.0f, 0.0f); glVertex3f(1.0f, 10.0f, 0.0f); glEnd(); } } I am using glutTimerFunc to wait a small amount of time to display a collision sprite before I black out the sprite. unfortunately my code only blacks out the said sprite without drawing the collision sprite, I have done a great deal of research on the glutTimerFunc and  animation.
  • Advertisement
  • Popular Now

  • Forum Statistics

    • Total Topics
    • Total Posts

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

Participate in the game development conversation and more when you create an account on GameDev.net!

Sign me up!