Consider a triangle in OpenGL or Vulkan with 2 of its vertices being green and one blue. Is there a way to disable the interpolation that GLSL does, and have the color of the entire triangle be green since there are more green vertices than blue ones?
I've heard something about the flat qualifier, but I'm not too sure if that is what I need.
Yes it is. See Interpolation qualifiers:
flat
The value will not be interpolated. The value given to the fragment shader is the value from the Provoking Vertex for that primitive.
That means if you use the flat interpolation qualifier for your color attributes, all fragments of the primitive get the same color.
Vertex shader:
flat out vec4 color;
Fragment shader
flat in vec4 color;
Related
How to disable color interpolation in GLFW and OpenGL 3.3 so that left half of the screen is pure white and the other half is completely black instead of having smooth transition through all shades of gray?
The interpolation of a vertex shader output can be changed with a Interpolation qualifier. Using the flat qualifier an output will not be interpolated.
e.g.:
Vertex shader
flat out vec3 color;
Fragment shader
flat in vec3 color;
I am learning opengl and I thought I pretty much understand fragment shader. My intuition is that fragment shader gets applied once to every pixel but recently when working with texture, I became confused on how they exactly work.
First of all, fragment shader typically takes in a series of texture coordinate so if I have a quad, the fragment shader would takes in the texture coordinates for the 4 corners of the quads. Now what I don't understand is the sampling process which is the process of taking the texture coordinates and getting the appropriate color value at that texture coordinates. Specifically, since I only supply 4 texture coordinates, how does opengl knows to samples the coordinates in between for color value.
This task is made even more confusing when you consider the fact that vertex shader goes straight to fragment shader and vertex shader gets applied per vertex. This means that at any given time, the fragment shader only knows about the texture coordinate corresponding to a single vertex rather than the whole 4 coordinates that make up the quads. Thus how exactly it knows to samples the values that fit the shapes on the screen when it only have one texture coordinates available at a time?
All varying variables are interpolated automatically.
Thus if you put texture coordinates for each vertex into a varying, you don't need to do anything special with them after that.
It could be as simple as this:
// Vertex
#version 330 compatibility
attribute vec2 a_texcoord;
varying vec2 v_texcoord;
void main()
{
v_texcoord = a_texcoord;
}
// Fragment
uniform vec2 u_texture;
varying vec2 v_texcoord;
void main()
{
gl_FragColor = texture2D(u_texture, v_texcoord);
}
Disclaimer: I used the old GLSL syntax. In newer GLSL versions, attribute would be replaced with in. varying would replaced with out in the vertex shader and with in in the fragment shader. gl_FragColor would be replaced with a custom out vec4 variable. texture2D() would be replaced with texture()
Notice how this fragment shader doesn't do any manual interpolation. It receives just a single vec2 v_texcoord, which was interpolated under the hood from v_texcoords of vertices comprising a primitive1 current fragment belongs to.
1. A primitive means a point, a line, a triangle or a quad.
first : in core context you still can use gl_FragColor.
second : you have texel ,fragment and real_monitor_pixel.These are different
things.
say this line is about convert texel to fragment(or to pixel idk exactly what it does):
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
when texel is less then fragment(pixel)
The gouraud-shader computes lighting at the corners of each triangle and linearly interpolates the resulting colours for each pixel covered by the triangle.
How would you program this certain interpolation (in GLSL-code) ?
Do you even have to code this yourself or does OpenGL interpolate (internally) single-handedly ?
From https://www.opengl.org/wiki/Type_Qualifier_(GLSL) :
Interpolation qualifiers
Certain inputs and outputs can use interpolation qualifiers. These are for any values which could be interpolated as a result of rasterization. These include:
Vertex shader outputs
Tessellation control shader inputs (to match with outputs from the
VS)
Tessellation evaluation shader outputs
Geometry shader inputs (to match with outputs from the TES/VS) and
outputs
Fragment shader inputs
Interpolation qualifiers control how interpolation of values happens across a triangle or other primitive. There are three basic interpolation qualifiers.
flat
The value will not be interpolated. The value given to the fragment shader is the value from the Provoking Vertex for that primitive.
noperspective
The value will be linearly interpolated in window-space. This is usually not what you want, but it can have its uses.
smooth
The value will be interpolated in a perspective-correct fashion. This is the default if no qualifier is present.
Since smooth is the default one, OpenGL interpolates it for you. If you would not want the interpolation, you would define your color in your shaders with keyword flat, something like this:
flat vec3 color;
I'm just starting to learn graphics using opengl and barely grasp the ideas of shaders and so forth. Following a set of tutorials, I've drawn a triangle on screen and assigned a color attribute to each vertex.
Using a vertex shader I forwarded the color values to a fragment shader which then simply assigned the vertex color to the fragment.
Vertex shader:
[.....]
layout(location = 1) in vec3 vertexColor;
out vec3 fragmentColor;
void main(){
[.....]
fragmentColor = vertexColor;
}
Fragment shader:
[.....]
out vec3 color;
in vec3 fragmentColor;
void main()
{
color = fragmentColor;
}
So I assigned a different colour to each vertex of the triangle. The result was a smoothly interpolated coloured triangle.
My question is: since I send a specific colour to the fragment shader, where did the smooth interpolation happen? Is it a state enabled by default in opengl? What other values can this state have and how do I switch among them? I would expect to have total control over the pixel colours using a fragment shader, but there seem to be calculations behind the scenes that alter the result. There are clearly things I don't understand, can anyone help on this matter?
Within the OpenGL pipeline, between the vertex shading stages (vertex, tesselation, and geometry shading) and fragment shading, is the rasterizer. Its job is to determine which screen locations are covered by a particular piece of geometry(point, line, or triangle). Knowing those locations, along with the input vertex data, the rasterizer linearly interpolates the data values for each varying variable in the fragment shader and sends those values as inputs into your fragment shader. When applied to color values, this is called Gouraud shading.
source : OpenGL Programming Guide, Eighth Edition.
If you want to see what happens without interpolation, call glShadeModel(GL_FLAT) before you draw. The default value is GL_SMOOTH.
How to transfer data from vertex shader to fragment shader without changes?
I need to say to the vertex pixels that they have this color. This color I can obtain only in the vertex shader.
You have to use a varying, because each fragment is "influenced" by more than one vertex (unless you are rendering GL_POINTS), so you have to interpolate them across the line/polygon. Recent versions of GLSL allow to specify flat shading interpolation, which doesn't interpolate the value throughout the primitive, ignoring the values from the other vertices.
I suspect thought that what you want to do is to render only the pixels corresponding to the vertices in a different color, is that correct? In that case it's not so easy, you would probably want to render the filled polygons first, and then re-render as GL_POINTS. At that point, varying variables are not interpolated because each fragment is influenced by a single vertex.
Here's a good tutorial on GLSL: NeHe GLSL tutorial
If you want to share data between vertex and fragment shaders use one of the built in types, for example gl_Color
If you want to pass through the color computed by the vertex shader to through the fragment shader you would create a fragment shader with the following line: gl_FragColor = gl_Color
gl_Color will be automatically set for you from the colors written by the vertex shader. You write a color from the vertex shader by setting one of the built-in variables, like gl_FrontColor, or one of it's peers: gl_BackColor etc.