OpenGL Shading Language (셰이딩) GLSL_(2)

OpenGL Shading Language

Passing of Vertex Attributes

• Per-Vertex attributes can be passed to a vertex shader using in variables
• Distinctions can be made regarding special in variables, which are automatically generated, and user-defined in variables, which must be passed by the application
• The vertex data is transferred to the graphics card via Vertex Buffer Objects (VBOs)
• To use the data of VBOs for the user-defined in variables, the usual procedure will be as follows:
• With glGetAttribLocation a location identifier for an in variable is queried. For example, when the variable in the shader code is called “inputColor”

int colorLoc = glGenAttribLocation(progID, "inputColor");

• The function glVertexAttribPointer(colorLoc, size, type, normalized, stride, offset) then defines the mapping between an in variable and VBO attribute
• Finally, the attribute needs to be activated by

glEnableVertexAttribArray(colorLoc);

• In a subsequent call to glDrawArray or glDrawElements with activated VBO the in variable of the vertex shader is then filled with the attribute data for each drawn vertex

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Data Input and Output in the Fragment Shader

• User-defined out variables of the vertex shader are interpolated in the rasterizer and the interpolated values are supplied in variables to the fragment shader
• The out variables of the fragment shader will be sent to the framebuffer
• For example, if the out variable “outputColor” should be written into the first color buffer of the framebuffer, this can be achieved by

glBindFragDataLocation(progID, 0, "outputColor");

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Uniform Variables

• In addition to in variables, uniform variables are another way to pass data to a shader
• In contrast to the in variables that contain the vertex attributes, the uniform variables include data that remains constant for all vertices during a call to glDrawArray or glDrawElements
• The location identifier of a uniform variable can be queried by

paraLoc = glGenUniformLocation(progID, "parameter");

• Afterwards, the uniform variable can be set with

glUniform1i(paraLoc, 123);

• or, if the variable is a float-vector

glUniform3fv(threeVectorLoc, 1, threeVetor);

• and in case of a matrix

glUniformMatrix4fv(modelviewLoc, 1, transpose, modelview);

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Example: Passing the Transformation Matrices As Uniforms

• In this example, the modelview matrix and the projection matrix are passed as uniform variables. The functionality of the GL_MODELVIEW and GL_PROJECTION matrices (which are known from the fixed-function pipeline) are simulated

• Vertex Shader

#version 140
in vec3 inputPosition;
in vec4 inputColor;
uniform mat4 projection;
uniform mat4 modelview;
out vec3 forFragColor;

void main() {
    forFragColor = inputColor.rgb;
    gl_Position = projection * modelview * vec4(inputPosition, 1.0);
}

• Fragment Shader

#version 140
in vec3 forFragColor;
out vec4 outputColor;

void main() {
    outputColor = vec4(forFragColor, 1.0);
}