Getting ready

Start with an OpenGL program that draws two triangles to form a quad. Provide the position at vertex attribute location 0, and the texture coordinate (0 to 1 in each direction) at vertex attribute location 1 (see the Sending data to a shader using vertex attributes and vertex buffer objects recipe).

We'll use the following vertex shader:

#version 430

layout (location = 0) in vec3 VertexPosition;
layout (location = 1) in vec3 VertexTexCoord;

out vec3 TexCoord;

void main()
{
  TexCoord = VertexTexCoord;
  gl_Position = vec4(VertexPosition,1.0);
}

The fragment shader contains the uniform block, and is responsible for drawing our fuzzy circle:

#version 430

in vec3 TexCoord;
layout (location = 0) out vec4 FragColor;

layout (binding = 0) uniform BlobSettings {
  vec4 InnerColor;
  vec4 OuterColor;
  float RadiusInner;
  float RadiusOuter;
};

void main() {
  float dx = TexCoord.x - 0.5;
  float dy = TexCoord.y - 0.5;
  float dist = sqrt(dx * dx + dy * dy);
  FragColor =mix( InnerColor, OuterColor,smoothstep( RadiusInner, RadiusOuter, dist ));
}

Note the uniform block named BlobSettings. The variables within this block define the parameters of our fuzzy circle. The variable OuterColor defines the color outside of the circle. InnerColor is the color inside of the circle. RadiusInner is the radius defining the part of the circle that is a solid color (inside the fuzzy edge), and the distance from the center of the circle to the inner edge of the fuzzy boundary. RadiusOuter is the outer edge of the fuzzy boundary of the circle (when the color is equal to OuterColor).

The code within the main function computes the distance of the texture coordinate to the center of the quad located at (0.5, 0.5). It then uses that distance to compute the color by using the smoothstep function. This function provides a value that smoothly varies between 0.0 and 1.0 when the value of the third argument is between the values of the first two arguments. Otherwise it returns 0.0 or 1.0 depending on whether dist is less than the first or greater than the second, respectively. The mix function is then used to linearly interpolate between InnerColor and OuterColor based on the value returned by the smoothstep function.

How to do it...

In the OpenGL program, after linking the shader program, use the following steps to assign data to the uniform block in the fragment shader:

How it works...

Phew! This seems like a lot of work! However, the real advantage comes when using multiple programs where the same buffer object can be used for each program. Let's take a look at each step individually.

First we get the index of the uniform block by calling glGetUniformBlockIndex, then we query for the size of the block by calling glGetActiveUniformBlockiv. After getting the size, we allocate a temporary buffer named blockBuffer to hold the data for our block.

The layout of data within a uniform block is implementation dependent, and implementations may use different padding and/or byte alignment. So in order to accurately layout our data, we need to query for the offset of each variable within the block. This is done in two steps. First, we query for the index of each variable within the block by calling glGetUniformIndices. This accepts an array of variable names (third argument) and returns the indices of the variables in the array indices (fourth argument). Then we use the indices to query for the offsets by calling glGetActiveUniformsiv. When the fourth argument is GL_UNIFORM_OFFSET, this returns the offset of each variable in the array pointed to by the fifth argument. This function can also be used to query for the size and type, however, in this case we choose not to do so, to keep the code simple (albeit less general).

The next step involves filling our temporary buffer blockBuffer with the data for the uniforms at the appropriate offsets. Here we use the standard library function memcpy to accomplish this.

Now that the temporary buffer is populated with the data with the appropriate layout, we can create our buffer object and copy the data into the buffer object. We call glGenBuffers to generate a buffer handle, and then bind that buffer to the GL_UNIFORM_BUFFER binding point by calling glBindBuffer. The space is allocated within the buffer object and the data is copied when glBufferData is called. We use GL_DYNAMIC_DRAW as the usage hint here because uniform data may be changed somewhat often during rendering. Of course, this is entirely dependent on the situation.

Finally, we associate the buffer object with the uniform block by calling glBindBufferBase. This function binds to an index within a buffer binding point. Certain binding points are also so-called "indexed buffer targets". This means that the target is actually an array of targets, and glBindBufferBase allows us to bind to one index within the array. In this case, we bind it to the index that we specified in the layout qualifier in the fragment shader: layout (binding = 0) (see the "Getting ready…" section). These two indices must match.

There's more...

If the data for a uniform block needs to be changed at some later time, one can call glBufferSubData to replace all or part of the data within the buffer. If you do so, don't forget to first bind the buffer to the generic binding point GL_UNIFORM_BUFFER.

uniform BlobSettings {
  vec4 InnerColor;
  vec4 OuterColor;
  float RadiusInner;
  float RadiusOuter;
} Blob;

FragColor =mix( Blob.InnerColor, Blob.OuterColor,smoothstep( Blob.RadiusInner, Blob.RadiusOuter, dist )
  );

const GLchar *names[] = { "BlobSettings.InnerColor", 
      "BlobSettings.OuterColor", "BlobSettings. RadiusInner",
      "BlobSettings.RadiusOuter" };
GLuint indices[4];
glGetUniformIndices(programHandle, 4, names, indices);

layout( std140 ) uniform BlobSettings {
  
};

layout( row_major, shared ) uniform BlobSettings {
  
};

See also