• Advertisement
Sign in to follow this  

OpenGL Alpha buffer With 2D Soft Shadows (SOLVED!)

This topic is 4632 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

I feel like such an idiot but I can't seem to figure this one out! On a bit of a whim I decided to try and implement the 2D soft shadows described Here in my current program. The setup of my program seemed a prefect fit for the alorithim but... I can't get it to work. At all. Not even the first step! AUGH! I haven't the foggiest idea where I'm going wrong. What I've been trying to do thus far is: a) Write the (alpha only) "light" to the framebuffers alpha component. b) Render my normal geometry modulating it by the newly written to alpha channel. This doesn't work, and all I seem to get is the normal geometry, thouh it blends additively if I've got any overlaping polys. I've tried a billion different combinations but none of them work. I've searched through the fourms and all throughout the web, but no one else seems to be having this problem. So, a few ideas of where my problem might be. For one: I don't need to do anything special to create an alpha channel in my frame buffer, do I? Here's my current code:

ZeroMemory( &pfd, sizeof(PIXELFORMATDESCRIPTOR) );

pfd.nVersion = 1;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = createParams.colorDepth; //32 in this case
pfd.cDepthBits = createParams.zDepth; //16 here (don't need a 24 bit one)
pfd.cStencilBits = 1;

I would ASSUME that specifying a 32 bit RGBA format should do the trick, but I've been known to be wrong before... Next, Rendering the light (only one for now) I run through something like this:
int cLight::Render( void )
	int numSubdivisions = 32;
	float angle;
        glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
        //Clear the alpha buffer to 0	
	glColor4f(0.0f, 0.0f, 0.0f, 0.0f);

		glVertex2f( 0, 0 );
		glVertex2f( 1024, 0 );
		glVertex2f( 1024, 768 );
		glVertex2f( 0, 768 );

        //This stuff is pretty much direct from the tutorial
        glColor4f(1.0f, 1.0f, 1.0f, intensity);
        glVertex3f(position.x, position.y, 0.0f);
        // Set edge colour for rest of shape
        glColor4f(0.0f, 0.0f, 0.0f, 0.0f);
        for (angle=0; angle<=PI*2; angle+=((PI*2)/numSubdivisions) )
            glVertex3f( range*(float)cos(angle) + position.x, range*(float)sin(angle) + position.y, 0.0f);  
        glVertex3f(position.x+range, position.y, 0.0f);



	return 1;

All's well so far, yes?
        glBlendFunc(GL_DST_ALPHA, GL_ONE);

	glColor4f(0.3f, 1.0f, 0.3f, 0.0f);

		glVertex2f( 512, 512 );
		glVertex2f( 1024, 512 );
		glVertex2f( 1024, 1024 );
		glVertex2f( 512, 1024 );

This is just a stand in for my real geometry, but I figure that it's best to test with something simple. Yes, the coordinates overlap the lights radius, so it should be affected. Yet.... it shows up perfectly normal, not a whit of difference made by the alpha buffer. So why is that? Am I missing something massively obvious? Does anyone have any insight as to why this isn't working for me? Thanks in advance for any advice! [Edited by - Toji on June 15, 2005 9:21:43 AM]

Share this post

Link to post
Share on other sites
Okay, I'm usually not one for bumping my own posts, but I'm seriously lost on this one guys. I've been working at it and STILL haven't made any progress, and I really don't have any other insights to add.

Anyone out there with any suggestions? Please?

Share this post

Link to post
Share on other sites
Don't know if you've tried this, and if this is it, but try changing your blendFunc to glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA).

if you have the dest at GL_ONE, then it will keep whatever color is already there and add to it. Of course if it's already black, then you're fine. Looks like you're starting out with green, so as long as you're not drawing to a white background, it should at least change the green channel. You don't mention your ClearColor anywhere.

Also, I just realized, do you have depth test enabled? If you do, then your colors won't draw over your light because that will have filled the depth buffer.

[Edited by - renderer on June 14, 2005 1:37:32 PM]

Share this post

Link to post
Share on other sites
Just tested your code, and it works fine as long as you don't have depth test on.

Share this post

Link to post
Share on other sites
Heh, thanks for the help, but I've already got it worked out. All the gory details Here if you're interested. Thanks for the help, though. I was really feeling lost there.

The long and the short of it is that different cards seem to require different format descriptions. ATI cards, for example, seem to assume that you always want an alpha buffer, whereas Nvidia cards require you to explicitly state that you want one. I ended up figuring this out when my program worked on my work computer, but not my home.

A few other notes: I am using depth testing for early z-out and correct depth sorting, but I'm controlling the states very carefully. My current order of operations:

Enable Depth Testing/Writing, Disable Blending
-Initial depth/ambient pass.
Disable Depth Writing
-For each light:
Disable Depth Testing/Color writing.
-Clear the Alpha buffer
-Render the lighting information to the alpha buffer
Enable Depth Testing
-Render Shadow Hulls
Enable Blending/Color Writing, Disable Alpha Writing
-Render All Geometry visible from this light.
Render the rest of the scene normally.

A few warnings I'd like to put out to people wanting to use this method. It's a great effect, but the fillrate requirements are HUGE. You're pretty much writing to each pixle on the screen an absolute mimimum of once per-light just for the alpha clearing pass plus once more for the ambient pass. On average you'll proably be redrawing the entire screen maybe 7-8 times for 3-4 visible lights.

Also, the soft shadowing portion of the code is anything but trivial. I'll post my actual code for it once I've completed it (getting close!) but it requires a lot of careful state-setting that's not covered in the article.

Not trying to discourge anyone here, because I love the effect itself, just a few things to be aware of!

Share this post

Link to post
Share on other sites
Glad you got it working, looks rather nice. :)

Original post by Toji
A few warnings I'd like to put out to people wanting to use this method. It's a great effect, but the fillrate requirements are HUGE. You're pretty much writing to each pixle on the screen an absolute mimimum of once per-light just for the alpha clearing pass plus once more for the ambient pass. On average you'll proably be redrawing the entire screen maybe 7-8 times for 3-4 visible lights.

Yeah, the fillrate needed is pretty heavy. Having a fairly bright ambient (or maybe whole-world directional) would reduce the number of lights some. I mentioned this in the comments thread for the article, but you can get *big* speedups by using scissor testing around the light bounds.

Share this post

Link to post
Share on other sites
Wow, the man himself! Thanks for the compliment!

I have a question if you don't mind: in your code what blending options do you use when rendering the shadow fins? I've tried a myriad of combinations, but have yet to get something that looks completely "right". I think that part of this is a bug in my penumbra position generation, but I'm certain that part of it is simply the blending itself.
Strangely enough I've found that glBlendFunc(GL_ZERO, GL_SRC_ALPHA) seems to give me the best results thus far.

Share this post

Link to post
Share on other sites
Shadow fins should be using the same blending func as the shadow hulls. So since you're basically masking off areas you need to multiply the shadow geometry alpha by that already in the framebuffer. So glBlendFunc(GL_ZERO, GL_SRC_ALPHA) should IIRC be the correct function to use. The only difference being that hulls always have an alpha of 0 (fully shadowed) whereas the fins will have a varying alpha for the fade out.

Share this post

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

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
      Thank you
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

      int rgbValue = int(textureSample.w);//4 bytes of data packed as color
      // algorithm might not be correct and endianness might need switching.
      vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
      extractedData /= 255.0f;
    • By Devashish Khandelwal
      While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
      Anyone has any idea .. what should I do?
  • Advertisement