In a game I'm writing, I have a level, which is properly rendered to the on-screen render buffer provided to me by the OS. I can also render this to a framebuffer, then render this framebuffer onto the output render buffer.
To add a background, I want to render a different scene, an effect, or whatever to a second framebuffer, then have this "show through" wherever the framebuffer containing the level has no pixel set, i.e. the alpha value is 0. I think this is called alpha blending.
How would I go about doing this with OpenGL? I think glBlendFunc could be used to achieve this, but I am not sure how I can couple this with the framebuffer drawing routines to properly achieve the result I want.
glBlendFunc allows the application to blend (merge) the output of all your current draw operations (say, X) with the current "display" framebuffer (say, Y) that already exists.
ie,
New display output = X (blend) Y
You can control the blend function by gl as below snippet shows for example:
glEnable(GL_BLEND);
glBlendEquation(GL_FUNC_ADD);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Full usage shown here
https://github.com/prabindh/sgxperf/blob/master/sgxperf_test4.cpp
Note that the concepts of "showing through" and "blending" are a little different, you might just want to stick with "per pixel alpha blending" terminology.
FBOs are just a containers and are not storage. What you need to do is attach a texture target for each FBO and render your output to that texture, once you have done this. You can use your output textures on a fullscreen quad and do whatever you want with your blending.
Related
There may be an easier way to do this, I can do it with a shader I'd imagine, but I feel like setting the blend function is simpler.
Shader would maybe be gl_FragColor = vec4(texture.rgb, 0);
For a reason, I need to have a freshly cleared FBO filled with a transparent (0) texture. That way the pixels in the FBO are visually transparent, but contain the RGB values of the texture.
It's tough to explain, but it's essentially a glClear() but with a texture rather than a solid color. Is there a combination blend src/dst function that would accomplish this?
Basically it shouldn't matter what the FBO looks like beforehand. I then want to render a texture to it, but since I'm rendering it with alpha 0, it's not overriding the existing FBO pixels.
If you really want to do this via blending, you can do this via the separate blend factors introduced originally in GL_EXT_blend_func_separate (promoted to OpenGL core functionalitiy in Version 2.0):
glEnable(GL_BLEND);
glBlendFuncSeparate(GL_ONE, GL_ZERO, GL_ZERO, GL_ZERO);
glBlendEquation(GL_ADD); // is the default anyway
I'm trying to render a model in OpenGL. I'm on Day 4 of C++ and OpenGL (Yes, I have learned this quickly) and I'm at a bit of a stop with textures.
I'm having a bit of trouble making my texture alpha work. In this image, I have this character from Spiral Knights. As you can see on the top of his head, there's those white portions.
I've got Blending enabled and my blend function set to glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
What I'm assuming here, and this is why I ask this question, is that the texture transparency is working, but the triangles behind the texture are still showing.
How do I make those triangles invisible but still show my texture?
Thanks.
There are two important things to be done when using blending:
You must sort primitives back to front and render in that order (order independent transparency in depth buffer based renderers is still an ongoing research topic).
When using textures to control the alpha channel you must either write a shader that somehow gets the texture's alpha values passed down to the resulting fragment color, or – if you're using the fixed function pipeline – you have to use GL_MODULATE texture env mode, or GL_DECAL with the primitive color alpha value set to 0, or use GL_REPLACE.
There is some method to access the background pixel in a fragment shader in order to change the alpha blending function?
I try to implement the fragment shader from page 5 of Weighted Blended Order-Independent Transparency but I don't know how to get Ci.
In standard OpenGL, you can't read the current value in the color buffer in your fragment shader. As far as I'm aware, the only place this functionality is available is as an extension in OpenGL ES (EXT_shader_framebuffer_fetch).
I didn't study the paper you linked, but there are two main options to blend your current rendering with previously rendered content:
Fixed function blending
If the blending functionality you need is covered by the blending functions/equations supported by OpenGL, this is the easiest and likely most efficient option. You set up the blending with glBlendFunc() and glBlendEquation() (or there more flexible variations glBlendFuncSeparate() and glBlendEquationSeparate()), enable blending with glEnable(GL_BLEND), and you're ready to draw.
There are also extensions that enable more variations, like KHR_blend_equation_advanced. Of course, like with all extensions, you can't count on them being supported on all platforms.
Multiple Passes
If you really do need programmable control over the blending, you can always do that with more rendering passes.
Say you render two passes that need to be blended together, and want the result in framebuffer C. The conventional sequence would be:
set current framebuffer to C
render pass 1
set up and enable blending
render pass 2
Now if this is not enough, you can render pass 1 and pass 2 into separate framebuffers, and then combine them:
set current framebuffer to A
render pass 1
set current framebuffer to B
render pass 2
set current framebuffer to C
bind color buffers from framebuffer A and B as textures
draw screen size quad, and sample/combine A and B in fragment shader
A and B in this sequence are FBOs with texture attachments. So you end up with the result of each rendering pass in a texture. You can then bind both of the textures for a final pass, sample them both in your fragment shader, and combine the colors in a fully programmable fashion to produce the final output.
At office we're working with an old GLX/Motif software that uses OpenGL's AccumulationBuffer to implement anti-aliasing for saving images.
Our problem is that Apple removed the AccumulationBuffer from all of its drivers (starting from OS X 10.7.5), and some Linux drivers like Intel HDxxxx don't support it neither.
Then I would like to update the anti-aliasing code of the software for making it compatible with most actual OSs and GPUs, but keeping the generated images as beautiful as they were before (because we need them for scientific publications).
SuperSampling seems to be the oldest and the best quality anti-aliasing method, but I can't find any example of SSAA that doesn't use AccumulationBuffer. Is there a different way to implement SuperSampling with OpenGL/GLX ???
You can use FBOs to implement the same kind of anti-aliasing that you most likely used with accumulation buffers. The process is almost the same, except that you use a texture/renderbuffer as your "accumulation buffer". You can either use two FBOs for the process, or change the attached render target of a single render FBO.
In pseudo-code, using two FBOs, the flow looks roughly like this:
create renderbuffer rbA
create fboA (will be used for accumulation)
bind fboA
attach rbA to fboA
clear
create texture texB
create fboB (will be used for rendering)
attach texB to fboB
(create and attach a renderbuffer for the depth buffer)
loop over jitter offsets
bind fboB
clear
render scene, with jitter offset applied
bind fboA
bind texB for texturing
set blend function GL_CONSTANT_ALPHA, GL_ONE
set blend color 0.0, 0.0, 0.0, 1.0 / #passes
enable blending
render screen size quad with simple texture sampling shader
disable blending
end loop
bind fboA as read_framebuffer
bind default framebuffer as draw framebuffer
blit framebuffer
Full super-sampling is also possible. As Andon in the comment above suggested, you create an FBO with a render target that is a multiple of your window size in each dimension, and in the end do a down-scaling blit to your window. The whole thing tends to be slow and use a lot of memory, even with just a factor of 2.
To clarify, when I say 'default framebuffer' I mean the one provided by the windowing system and what ends up on your monitor.
To improve my rendering speeds for a CAD app, I've managed to separate out the 3D elements from the Qt-handled 2D ones, and they now each render into their own FBO. When the time comes to get them onto the screen, I blit the 3D FBO onto the default FB, and then I want to blend my 2D FBO on top of it.
I've gotten to the blitting part fine, but I can't see how to blend my 2D FBO onto it? Both FBOs are identical in size and format, and they are both the same as the default FB.
I'm sure it's a simple operation, but I can't find anything on the net - presumably I'm missing the right term for what I am trying to do. Although I'm using Qt, I can use native OpenGL commands without issue.
A blit operation is ultimately a pixel copy operation. If you want to layer one image on top of another, you can't blit it. You must instead render a full-screen quad as a texture and use the proper blending parameters for your blending operation.
You can use GL_EXT_framebuffer_blit to blit contents of the framebuffer object to the application framebuffer (or to any other). Although, as the spec states, it is not possible to use blending:
The pixel copy bypasses the fragment pipeline. The only fragment
operations which affect the blit are the pixel ownership test and
the scissor test.
So any blending means to use fragment shader as suggested. One fullscreen pass with blending should be pretty cheap, I believe there is nothing to worry about.
use shader to read back from frame buffer. this is OpenGL ES extension, not support by all hardware.
https://www.khronos.org/registry/gles/extensions/EXT/EXT_shader_framebuffer_fetch.txt