Q : Is there a way to use the default pipeline to blend the Alpha component properly?
Problem : I'm drawing semi-transparent surfaces into a texture, then I want to blit that texture into the main frame back buffer. Normally when you use straightforward alpha blending for transparency or antialiasing, it is to blend color components into the back buffer ( which has no alpha channel by deafult ).
However, when blending into a texture that has an alpha component, the usual GL_SRC_ALPHA, GL_ONE_MINUS_DST_ALHPA does not blend the "alpha" component properly :
Notice the problematic zone over the death star at the edge of the red and blue circles; the alpha component is supposed to be completely opaque. Note that the RGB components are blendend as expected.
So I played around with different settings for the BlendFunc and the closest to what I want is by using GL_DST_ALPHA as my dst alpha scaling factor.
There is still a problem with this approach because blending two surfaces with 50% opacity should give a combined surface of 75% opacity. But using OpenGL default formulas, I get (0.50.5) + (0.50.5) = 0.5. More disturbing, if I have 2 surfaces with 40% opacity, I get a LESS opaque surface : (.4*.4)+(.4*.4) = 0.32.
The real formula ( 1 - ( 1 - srcAlpha ) * ( 1 - dstAlpha ) ) would imply that I could use a GL_FUNC_MULT instead of GL_FUNC_ADD in the blend equation for the alpha component. Unfortunately this does not exist.
So I gave up and tried to do a solution using a shader. This becomes complicated really fast if the texture you are rendering to is also the texture you read from. Especially if you render multiple semi-transparent surfaces.
I'm looking for alternative solutions I have not already tried. Any idea is welcome.
EDIT : I also tried GL_ONE as both src and dst scaling factor. this makes the alpha component way too opaque as 1*.5 + 1*.5 will give 1 as the resulting alpha. Makes me wonder if GL_ONE and GL_DST_ALPHA would give better results.
p.s. : I used Anders Riggelsen's blending tool for the images.
The standard technique to do perfect blending is to use
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
Note that it requires both the source and destination to be premultiplied, and produces premultiplied result.
Premultiplication basically means performing color.rgb *= color.a on the color. Usually you apply it to the textures when you load them, or to the final color in your shader.
Corollary: if alpha == 1, premultiplication makes no difference.
If the source is not premultiplied, but the destination is, use this instead:
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
Yes, kind of.
Blending factors ONE_MINUS_DST_ALPHA, ONE may be used for blending alpha correctly. If you have glBlendFuncSeperate you can set different blend factors for color and alpha. Operation is add.
I say kind of because as I recall seperate blend func appeared late in the deprecated API.
Edit: after doing the math those blend factors are mathematically equivalent to HolyBlackCat answer.
I am trying to blend a white texture with varying alpha values with a colored background. I am expecting the result to retain colors from the background, and have alpha values replaced by ones from the blended texture.
So, for background I use:
Gdx.gl.glEnable(GL20.GL_BLEND);
Gdx.gl.glBlendEquationSeparate(GL20.GL_FUNC_ADD, GL20.GL_FUNC_ADD);
Gdx.gl.glBlendFuncSeparate(GL20.GL_ONE, GL20.GL_ZERO, GL20.GL_ONE, GL20.GL_ZERO);
I expect the background triangles mesh to override the destination, both color and alpha.
Question 1 - why with those blendFunc parameters, alpha value is being ignored?
If I set blendfunc to GL_ONE, GL_ONE, GL_ZERO, GL_ZERO then the filled mesh is rendered with proper alpha level - but both the source and dest alpha are supposed to be multiplied by zero - why does this work?
====
Now to blend the alpha map I use:
Gdx.gl.glBlendEquationSeparate(GL20.GL_FUNC_ADD, GL20.GL_FUNC_ADD);
Gdx.gl.glBlendFuncSeparate(GL20.GL_ZERO, GL20.GL_ONE, GL20.GL_ONE, GL20.GL_ZERO);
Question 2 - This supposed to keep the destination color and replace alpha. However, when I render the texture with those blendfunc params, I get no change to the output at all...
I've been reading the opengl blending chapter over and over again to understand what I fail to understand, please, share your insight on how those parameters actually work
I use this:
Gdx.gl.glEnable(GL20.GL_BLEND);
Gdx.gl.glBlendFunc(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
It works but only in the order of rendering. In order to account for depth you will need alpha testing which libGDX does not include.
I am trying to draw brush strokes made of quads with a rough texture into framebuffers that are then composited together. The problem is that the framebuffer texture initial color is 0,0,0,0 and it blends in creating a dark glow around the edges. here is an example image
im using
gl.blendEquationSeparate( gl.FUNC_ADD, gl.FUNC_ADD );
gl.blendFuncSeparate( gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA, gl.SRC_ALPHA, gl.ONE );
I think Iv tried every possible combination of blending settings and none work the way I want.
here is a demo of the problem
This looks like a pre-multiplication issue to me. Either your brush stroke textures aren't pre-multiplied and they should be, or they are pre-multiplied and they shouldn't be - I forget which.
For compositing you most likely want to use pre-multiplied alpha to get the effect you want. Either pre-multiply the texture when you upload the data
gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);
or pre-multiply in the shader
gl_FragColor = vec4(color.rgb * color.a, color.a);
And blend with
gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
A quick google of "pre-multiplied alpha blending" brought up this great explanation of the issue
I'm working on creating a transparent GUI in OpenGL, and am trying to get text rendered over some semi-transparent quads, but the results are odd.
If I render the text by itself, with nothing behind it, it looks fine:
However, if I render a semi-transparent quad behind it (rendering the quad before rendering the text), I get this:
I have blending set to (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA). The font texture is an all-white texture with the character shapes in the alpha channel.
Do I need to be doing something special when performing alpha-transparency over an existing layer of transparency? Or is there something else I need to check?
The alpha value of your font texture seems to be off. It should be 0 for texels that you want to be invisible and 1 (or 255 in bytes) for visible texels. You should check the texture and make sure alpha values are correct.
Instead of alpha blending, you can use alpha testing. This will completely get rid of fragments, that have a alpha value below a certain threshold and is often much faster than blending.
glDisbale(GL_BLEND);
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.96f); // Or some fitting threshold for your texture
This might work even if your texture's alpha is off in some places, but doesn't look like it is the case here, as the 's' and 't' seem to have a low alpha in places where it should be 1.
Thanks for the responses. There was nothing wrong with my font texture, but your suggestions led me to try a few other things. Turns out the problem wasn't the transparency at all. There was a problem with rendering the background quad, which caused it to also render the text quads, but using the background texture. Bah...
I am rendering to a texture through a framebuffer object, and when I draw transparent primitives, the primitives are blended properly with other primitives drawn in that single draw step, but they are not blended properly with the previous contents of the framebuffer.
Is there a way to properly blend the contents of the texture with the new data coming in?
EDIT: More information requsted, I will attempt to explain more clearly;
The blendmode I am using is GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA. (I believe that is typically the standard blendmode)
I am creating an application that tracks mouse movement. It draws lines connecting the previous mouse position to the current mouse position, and as I do not want to draw the lines over again each frame, I figured I would draw to a texture, never clear the texture and then just draw a rectangle with that texture on it to display it.
This all works fine, except that when I draw shapes with alpha less than 1 onto the texture, it does not blend properly with the texture's previous contents. Let's say I have some black lines with alpha = .6 drawn onto the texture. A couple draw cycles later, I then draw a black circle with alpha = .4 over those lines. The lines "underneath" the circle are completely overwritten. Although the circle is not flat black (It blends properly with the white background) there are no "darker lines" underneath the circle as you would expect.
If I draw the lines and the circle in the same frame however, they blend properly. My guess is that the texture just does not blend with it's previous contents. It's like it's only blending with the glclearcolor. (Which, in this case is <1.0f, 1.0f, 1.0f, 1.0f>)
I think there are two possible problems here.
Remember that all of the overlay lines are blended twice here. Once when they are blended into the FBO texture, and again when the FBO texture is blended over the scene.
So the first possibility is that you don't have blending enabled when drawing one line over another in the FBO overlay. When you draw into an RGBA surface with blending off, the current alpha is simply written directly into the FBO overlay's alpha channel. Then later when you blend the whole FBO texture over the scene, that alpha makes your lines translucent. So if you have blending against "the world" but not between overlay elements, it is possible that no blending is happening.
Another related problem: when you blend one line over another in "standard" blend mode (src alpha, 1 - src alpha) in the FBO, the alpha channel of the "blended" part is going to contain a blend of the alphas of the two overlay elements. This is probably not what you want.
For example, if you draw two 50% alpha lines over each other in the overlay, to get the equivalent effect when you blit the FBO, you need the FBO's alpha to be...75%. (That is, 1 - (1-.5) * (1-0.5), which is what would happen if you just drew two 50% alpha lines over your scene. But when you draw the two 50% lines, you'll get 50% alpha in the FBO (a blend of 50% with...50%.
This brings up the final issue: by pre-mixing the lines with each other before you blend them over the world, you are changing the draw order. Whereas you might have had:
blend(blend(blend(background color, model), first line), second line);
now you will have
blend(blend(first line, second line), blend(background color, model)).
In other words, pre-mixing the overlay lines into an FBO changes the order of blending and thus changes the final look in a way you may not want.
First, the simple way to get around this: don't use an FBO. I realize this is a "go redesign your app" kind of answer, but using an FBO is not the cheapest thing, and modern GL cards are very good at drawing lines. So one option would be: instead of blending lines into an FBO, write the line geometry into a vertex buffer object (VBO). Simply extend the VBO a little bit each time. If you are drawing less than, say, 40,000 lines at a time, this will almost certainly be as fast as what you were doing before.
(One tip if you go this route: use glBufferSubData to write the lines in, not glMapBuffer - mapping can be expensive and doesn't work on sub-ranges on many drivers...better to just let the driver copy the few new vertices.)
If that isn't an option (for example, if you draw a mix of shape types or use a mix of GL state, such that "remembering" what you did is a lot more complex than just accumulating vertices) then you may want to change how you draw into the VBO.
Basically what you'll need to do is enable separate blending; initialize the overlay to black + 0% alpha (0,0,0,0) and blend by "standard blending" the RGB but additive blending the alpha channels. This still isn't quite correct for the alpha channel but it's generally a lot closer - without this, over-drawn areas will be too transparent.
Then, when drawing the FBO, use "pre-multiplied" alpha, that is, (one, one-minus-src-alph).
Here's why that last step is needed: when you draw into the FBO, you have already multiplied every draw call by its alpha channel (if blending is on). Since you are drawing over black, a green (0,1,0,0.5) line is now dark green (0,0.5,0,0.5). If alpha is on and you blend normally again, the alpha is reapplied and you'l have 0,0.25,0,0.5.). By simply using the FBO color as is, you avoid the second alpha multiplication.
This is sometimes called "pre-multiplied" alpha because the alpha has already been multiplied into the RGB color. In this case you want it to get correct results, but in other cases, programmers use it for speed. (By pre-multiplying, it removes a mult per pixel when the blend op is performed.)
Hope that helps! Getting blending right when the layers are not mixed in order gets really tricky, and separate blend isn't available on old hardware, so simply drawing the lines every time may be the path of least misery.
Clear the FBO with transparent black (0, 0, 0, 0), draw into it back-to-front with
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
and draw the FBO with
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
to get the exact result.
As Ben Supnik wrote, the FBO contains colour already multiplied with the alpha channel, so instead of doing that again with GL_SRC_ALPHA, it is drawn with GL_ONE. The destination colour is attenuated normally with GL_ONE_MINUS_SRC_ALPHA.
The reason for blending the alpha channel in the buffer this way is different:
The formula to combine transparency is
resultTr = sTr * dTr
(I use s and d because of the parallel to OpenGL's source and destination, but as you can see the order doesn't matter.)
Written with opacities (alpha values) this becomes
1 - rA = (1 - sA) * (1 - dA)
<=> rA = 1 - (1 - sA) * (1 - dA)
= 1 - 1 + sA + dA - sA * dA
= sA + (1 - sA) * dA
which is the same as the blend function (source and destination factors) (GL_ONE, GL_ONE_MINUS_SRC_ALPHA) with the default blend equation GL_FUNC_ADD.
As an aside:
The above answers the specific problem from the question, but if you can easily choose the draw order it may in theory be better to draw premultiplied colour into the buffer front-to-back with
glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ONE);
and otherwise use the same method.
My reasoning behind this is that the graphics card may be able to skip shader execution for regions that are already solid. I haven't tested this though, so it may make no difference in practice.
As Ben Supnik said, the best way to do this is rendering the entire scene with separate blend functions for color and alpha. If you are using the classic non premultiplied blend function try glBlendFuncSeparateOES(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE) to render your scene to FBO. and glBlendFuncSeparateOES(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) to render the FBO to screen.
It is not 100% accurate, but in most of the cases that will create no unexpected transparency.
Keep in mind that old Hardware and some mobile devices (mostly OpenGL ES 1.x devices, like the original iPhone and 3G) does not support separated blend functions. :(