Sign in to follow this  
Toji

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

Recommended Posts

Toji    535
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:
PIXELFORMATDESCRIPTOR pfd;

ZeroMemory( &pfd, sizeof(PIXELFORMATDESCRIPTOR) );

pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
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;
    
	glDisable(GL_BLEND);
        glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
        
        //Clear the alpha buffer to 0	
	glPushMatrix();
	glLoadIdentity();
	
	glColor4f(0.0f, 0.0f, 0.0f, 0.0f);

	glBegin(GL_QUADS);
		glVertex2f( 0, 0 );
		glVertex2f( 1024, 0 );
		glVertex2f( 1024, 768 );
		glVertex2f( 0, 768 );
	glEnd();

	glPopMatrix();
	
	glBegin(GL_TRIANGLE_FAN);
	
        //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);

        glEnd();

	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);

	return 1;
}



All's well so far, yes?
        glBlendFunc(GL_DST_ALPHA, GL_ONE);
	glEnable(GL_BLEND);
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);

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

	glBegin(GL_QUADS);
		glVertex2f( 512, 512 );
		glVertex2f( 1024, 512 );
		glVertex2f( 1024, 1024 );
		glVertex2f( 512, 1024 );
	glEnd();



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
Toji    535
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
renderer    202
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
Toji    535
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
OrangyTang    1298
Glad you got it working, looks rather nice. :)

Quote:
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
Toji    535
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
OrangyTang    1298
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

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 pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • 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. 
       
      Thanks
    • 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,
      http://www.informit.com/articles/article.aspx?p=2120983
      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.
      Thanks!
  • Popular Now