I'm trying to draw a rectangle with a texture in OpenGL. I'm simply trying to render an entire .jpg image, so I specify the texture coordinates as [0, 0] to [1, 1] in the vertex buffer. I expect all the interpolated texture coordinates in the fragment shader to be between [0, 0] and [1, 1], however, depending on where the texture is drawn, I sometimes get a texture coordinate that is less than 0 (I know this is the case because I tried outputting red from the fragment shader if the tex coord is less than 0).
How come I get an interpolated value outside of the specified range? I currently visualize vertices/fragments like the following image (https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing):
If I imagine a rectangle instead, then if the pixel sample is inside the rectangle, then the interpolated texture coord must be at least 0, since the very left of the rectangle represents 0, right? So how do I end up with a value less than 0?
Edit: after some basic testing, it looks like the fragment shader is called if a shape simply intersects that pixel, not if the pixel sample point is inside the shape. I tested this by placing the start of the rectangle slightly before and slightly after the middle of a pixel - when slightly behind the middle of the pixel, I don't get a negative value, but if I place it slightly after the middle, then I do get a negative value. This contradicts what the website I linked to said - perhaps it's driver-dependent?
Edit: the previous test I did was with multisampling on. If I turn multisampling off, then even if the shape is past the middle, I don't get a negative value...
Turns out I just needed to keep reading the article I linked:
This is where multisampling becomes interesting. We determined that 2 subsamples were covered by the triangle so the next step is to determine a color for this specific pixel. Our initial guess would be that we run the fragment shader for each covered subsample and later average the colors of each subsample per pixel. In this case we'd run the fragment shader twice on the interpolated vertex data at each subsample and store the resulting color in those sample points. This is (fortunately) not how it works, because this basically means we need to run a lot more fragment shaders than without multisampling, drastically reducing performance.
How MSAA really works is that the fragment shader is only run once per pixel (for each primitive) regardless of how many subsamples the triangle covers. The fragment shader is run with the vertex data interpolated to the center of the pixel and the resulting color is then stored inside each of the covered subsamples. Once the color buffer's subsamples are filled with all the colors of the primitives we've rendered, all these colors are then averaged per pixel resulting in a single color per pixel. Because only two of the 4 samples were covered in the previous image, the color of the pixel was averaged with the triangle's color and the color stored at the other 2 sample points (in this case: the clear color) resulting in a light blue-ish color.
So I was getting a negative value because the fragment shader was being run on a pixel that had at least one of its sub-sample points covered by the shape, but the shape was slightly after the mid-point of the pixel, and since "the fragment shader is run with the vertex data interpolated to the center of the pixel", I was getting a negative value.
Here is a good explanation what is a fragment:
https://gamedev.stackexchange.com/questions/8977/what-is-a-fragment/8981#8981
But here (and not only here) I have read that "I want to stress the fact that one pixel is not necessarily one fragment, multiple fragment can be combined to make one pixel....". But I don't understand clearly what are fragments and why they are not necessarily correspond to pixels one to one?
EDIT: When multiple fragments form one pixel it is only in the case when they overlap after projection, or it is because the pixel is bigger than the fragment, hence you need to put together next to each other multiple fragments with the same color to form a pixel?
A fragment has a location that can be queried via its built-in gl_FragCoord variable where the x and y component directly correspond to pixels on your screen. So you could say that a fragment indeed corresponds to a pixel.
However, a fragment outputs a color and stores that color in a color buffer at its coordinates. This does not mean this color is the actual pixel color that is shown to the viewer.
Because a fragment shader is run for each object, it could happen that other objects are drawn after your first object that also output a fragment at the same screen coordinate. When taking depth-testing, stencil testing and blending into account, the resulting color value in the color buffer might get overwritten and/or merged with new colors.
Think of it like this:
Object 1 gets drawn and draws the color purple at screen coordinate (200, 300);
Object 2 gets drawn and draws the color red at same coordinate, overwriting it.
Object 3 (is blue) has transparency of 50% at same coordinate, and merges colors.
Final fragment color output is then combination of red and blue (50%).
The final resulting pixel could then be a color from a single fragment shader run, a color that is overwritten by many other fragment shader runs, or a combination of colors via blending.
A fragment is not equal to a pixel when multi sample anti-aliasing (MSAA) or any of the other modes that change the ratio of rendered pixels to screen pixels is activated.
In the case of 4x MSAA, each screen pixel will be represented by 4 (2x2) fragments in the display buffer. The fragment shader for a particular polygon will only be run once for the screen pixel no matter how many of the fragments are covered by the polygon. Since a polygon may not cover all the fragments within a pixel it will only store color into the fragments it covers. This is repeated for every polygon that may cover one or more of the fragments. Then at the final display all 4 fragments are blended to produce the final screen pixel.
I am having a scene containing of thousands of little planes. The setup is that the plane can occlude each other in the depth.
The planes are red and green. Now I want to do the following in a shader:
Render all the planes. As soon as a plane is red, substract 0.5 from the currently bound framebuffer and if the texture is green, add 0.5 to the framebuffer.
Therefore I should be able to see for each pixel in the texture of the framebuffer: < 0 => more red planes at this pixel, = 0 => Same amount of red and green and for the last case >0 => more green planes, as well as I can tell the difference.
This is just a very rough simplification of what I need to do, but the core is to write change a pixel of a texture/framebuffer depending on the given values of planes in the scene influencing the current fragment. This should happen in the fragment shader.
So how do I change the values of the framebuffer using GLSL? using gl_FragColor just sets a new color, but does not manipulate the color set before.
PS I also gonna deactivate depth testing.
The fragment shader cannot read the (old) value from the framebuffer; it just generates a new value to put into the framebuffer. When multiple fragments output to the same pixel (overlapping planes in your example), how those value combine is controlled by the BLEND function of the pipeline.
What you appear to want can be done by setting a custom blending function. The GL_FUNC_ADD blending function allows adding the old value and new value (with weights); what you want is probably something like:
glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_ONE, GL_ONE, GL_ONE, GL_ONE);
this will simply add each output pixel to the old pixel in the framebuffer (in all four channels; its not clear from your question whether you're using a 1-channel, 3-channel, or 4-channel frame buffer). Then, you just have your fragment shader output 0.5 or -0.5 depending. In order for this to make sense, you need a framebuffer format that supports values outside the normal [0..1] range, such as GL_RGBA32F or GL_R32F
So i want my texture colors to multiply with the colors under it, how this can be done with glBlendFunc() ?
For example: if i have a texture color (0.31, 0.51, 0.124) and there is a color (0.83, 0.64, 0.12) under it, the resulting color would be 0.31*0.83, 0.51*0.64, 0.124*0.12. How to do this?
So i want my texture colors to multiply with the colors under it, how this can be done with glBlendFunc() ?
By "texture colors", I'm going to assume that you mean "the color output by the glTexEnv process". Or, using more rigorous OpenGL spec language, the fragment color.
In any case, this is a simple matter of blending. If you're using OpenGL 2.1 or above:
glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD); //No, that's not a typo. Read the wiki article.
glBlendFuncSeparate(GL_DST_COLOR, GL_ZERO, GL_DST_ALPHA, GL_ZERO);
This causes the destination color (what's in the framebuffer) to be multiplied with the source color (what comes out of glTexEnv), and added to zero (technically, zero times the destination color).
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. :(