glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
and used this in the fragment shader.
I've used Alpha blend to get the transparency working however it only seems to work from one side.
Not sure what the problem is, am new to programming and shading.
http://imgur.com/WDK4qjc Link to see the picture
I think you see 2 distant faces blending where color is darker.
Maybe Culling is not activated.
Face culling is the ability to discard face drawing upon certain condition.
To achieve what you want, You have to discard faces not facing the camera which is call backface culling. You do this:
glEnableGL(GL_CULL_FACE); //(enable face culling)
glCullFace(GL_BACK); //(discard back faces)
Everything is fine. It's called Transparency Sorting.
Some more info:
http://www.opengl.org/archives/resources/faq/technical/transparency.htm
Rendering transparent objects in OpenGL
opengl z-sorting transparency
Related
I am using gluCylinder() to create a cylinder in openGL and then plotting points inside the cylinder with Depth Test On .
When i see the front view of the cylinder, the points inside the cylinder are obstructed by front face.
To make front face of the cylinder translucent i am using Blending.
I am using below functions.
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
But whatever coloring or alpha value i assign to the cylinder the front face is not looking transparent due to its back face.
Tell whether it is possible to do with blending only or else i need to introduce lighting for both the faces of Cylinder.Here it clearly visible the change in the color of front face and back face of cylinder. And the points inside the cylinder are not visible due to being obstructed by front face of cylinder.
You should be able to accomplish this by drawing the cylinder twice, while culling the front faces the first time, and culling the back faces the second time. This way, you can draw the front and back parts differently, e.g. by making the front part transparent.
The code sequence could look like this:
// Draw back part of cylinder, opaque.
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
gluCylinder(...);
// Draw points.
// Draw front part of cylinder, transparent.
glCullFace(GL_BACK);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
gluCylinder(...);
glDisable(GL_BLEND);
If I understand you right then no, you can't do it with blending alone.
If the cylinder's normals all point outward then you also won't be able to see the cylinder's internal parts no matter what you do.
I do something similar to show characters behind walls and it goes like this - render your scene normally and save it all to a framebuffer. Then render what you want shown behind with the buffer contents on top, using a custom shader to make a bubble of transparency around the thing you want shown behind.
Not sure if I am explaining it well or not but it unfortunately requires multiple steps to get the results you want.
Your problem is still a bit unclear to me despite the image but I will attempt to answer based on my perception of your issue.
You are drawing a cylinder and have geometry (lines or other models) inside the cylinder. You want the cylinder to look translucent so the inner objects are visible. Here is one way to do it. Assuming your render functions are drawCylinder() and drawPoints().
init()
{
...
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
...
}
drawScene()
{
glDepthMask(GL_FALSE);
glEnable(GL_BLEND);
drawCylinder();
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);
drawPoints();
}
doing so will make sure that the points are drawn regardless of the cylinder. Try using lower values of alpha for your cylinder color.
Please note this is one way to do it. I suggest using shaders to have more control over blending as well as exploring fragment/pixel discard options.
I am trying to create the Light Scattering effect using OpenGL.
I am following this tutorial.
At some point it says:
Switch to Orthogonal projection and blend the FBO with the framebuffer, activating the shader in order to generate the "God's ray" effect .
I don't understand what's the meaning of "blend the FBO with the Framebuffer". I looked for this "Blending" and I noticed that's a OpenGL pipeline step.
I was thinking that I should use the function glEnablei(GL_BLEND, fbo), but I don't know where I should call it.
To draw the mesh (the scene has one mesh and one light source) I use glDrawArrays(GL_TRIANGLES, 0, n_of_verteces).
Can someone help me?
Ugh, the instructions you cited are written unneccessarily confusing. What it asks you to do is take the texture you've rendered your god rays to (using the FBO) and draw it on top what's in the main (non-FBO) frame buffer using a single, full viewport textured quad. And the instruction to "blend it" is asking you to enable blending (everything drawn thereafter will blend with what's been drawn in the steps before, including other blended stuff) and choose an appropriate blending function; (GL_ONE, GL_ONE) would be the obvious one for lighting effect stuff.
Is there a blending equation that can be made using the OpenGL glBlendFunc that would allow for a transparent color (RGBA) to be rendered behind an additive overlay.
Rendering Ontop:
This effect can be achieved using glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Rendering Underneath:
Is there an equation for this blending effect?
There are no glBlendFunc options for directly drawing overlay. Info can be found here:
http://benmcdowell.com/implementing-photoshop-blend-modes-in-opengl/
However, your effect doesn't seem to be overlay, seems to be either screen
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_COLOR);
or additive
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
I'd suggest you try all combinations, there aren't that many possible. If you need crazier effects however, you'll need to code shaders.
P.S. I lied. You don't HAVE to code shaders to do crazy effects like overlay, but you have to draw so many times it becomes unusable in real time. I should have said it's better to use shaders.
I'm making a 2D game using OpenGL. I recently tried implementing Framebuffer-objects, and I am having some problems regarding blending.
I'm creating an FBO (using GL_RGBA as format).
When I render to the FBO, I first clear it to fully transparent black, and disable GL_BLEND. I then draw my textures and then I enable GL_BLEND again.
When I'm drawing the FBO-texture, I use GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA as source and destination pixels respectively, as the blending-function. I am rendering it as a textured quad.
This does not work properly, as white pixels appear transparent. I have tried experimenting with different blend-function values, but all that I have tried have had issues. I do not fully understand how blending works, so it's hard for me to wrap my head around this. Maybe I'm missing something obvious here?
Here's an image of how it looks right now. There is supposed to be a glow around the button when it is being highlighted, but instead the pixels around it appear transparent: http://i.snag.gy/RnV4s.jpg
You can also see two boxes of text in the image. The top one is drawn normally, without an FBO. The textures are also rendered normally without an FBO, so I know that the problem lies within my framebuffer-code somewhere.
I have pasted my "RenderTarget" class to pastebin (I'm used to calling it a rendertarget instead of FBO): http://pastebin.com/dBXgjrUX
This is how I use it:
RT->Begin();
// draw stuff
RT->End();
RT->Draw();
Can someone help me? Let me know if you need any more info about my issue.
Edit:
Here are the properties of OpenGL that I set on startup:
// Initialize shaders
shaderManager.InitializeStockShaders();
// Set some OpenGL properties
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glShadeModel(GL_SMOOTH);
glAlphaFunc(GL_GREATER, 0.0f);
// Enables/disables
glEnable(GL_ALPHA_TEST);
glEnable(GL_BLEND);
glDisable(GL_DEPTH_TEST);
glDisable(GL_DITHER);
glDisable(GL_LIGHTING);
I'ts a bit difficult to tell what your problem is exactly, because you didn't provide source code. Alas, I see several potential troublemakers:
First you told that you want to draw a glow around the button. I presume, that all the buttons are drawn into the FBO, merging them into a UI overlay. Glow sounds to me, like you want to blend something, so you probably also want to have blending enabled, drawing to the FBO.
Next be aware of depth buffer issues. Blending and Depth Buffering have peculiar interactions. In your case I suggest disabling depth testing and depth writes to the FBO (or not using a depth buffer attachment to the FBO at all). Draw the glowing button last, so that it won't block the other buttons from being drawn. Also you must make sure, that your glow comes out with a nonzero alpha value, otherwise it will blend transparent. This is something you control in your shaders, or texture environment (depending on what you use).
Update 1:
Your FBO class doesn't propperly ensure, that textures attached to a bound framebuffer must not be bound themself. It's easy to fix though, by moving attachment code into bind, where the textures are also unbound apropriately. See my edited pastebin http://pastebin.com/1uVT7VkR (I probably missed a few things).
I am trying to create a simple ray tracer. I have a perspective view which shows the rays visibly for debugging purposes.
In my example screenshot below I have a single white sphere to be raytraced and a green sphere representing the eye.
Rays are drawn as lines with
glLineWidth(10.0f)
If a ray misses the sphere it is given color glColor4ub(100,100,100,100);
in my initialization code I have the following:
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0f);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_SRC_ALPHA);
You can see in the screen shot that for some reason, the rays passing between the perspective view point and the sphere are being color blended with the axis line behind the sphere, rather than with the sphere itself.
Here is a screenshot:
Can anyone explain what I am doing wrong here?
Thanks!!
Is it a possibility you cast those rays before you draw the sphere?
Then if Z-buffer is enabled, the sphere's fragments simply won't be rendered, as those parts of rays are closer. When you are drawing something semi-transparent (using blending), you should watch the order you draw things carefully.
In fact I think you cannot use Z-buffer in any sensible way together with ray-tracing process. You'll have to track Z-order manually. While we are at it OpenGL might not be the best API to visualize ray-tracing process. (It will do so possibly much slower than pure software ray-tracer)
You dont need the glAlphaFunc, disable it.
Light rays should be blended by adding to the buffer: glBlendFunc(GL_ONE, GL_ONE) (for premultiplied alpha, which you chose.
Turn off depth buffer writing (not testing) when rendering the rays: glDepthMask(GL_FALSE)
Render the rays last.
AlphaTest is only for discarding fragments - not for blending them. Check the spec
By using it, you are telling OpenGL that you want it to throw away the pixels instead of drawing them, so you won't can any transparent blending. The most common blending function is
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); You can also check out the OpenGL Transparency FAQ.