Blend and Smooth

Notes:

I combined examples of Blending and Smoothing. It's not fancy but shows both effects.
Pay close attention to the fidelity of one line verses the other.

Source:

// OpenGL Superbible 5 - Listing 3.3

#include <GLTools.h>	// OpenGL toolkit
#include <GLShaderManager.h>

#ifdef __APPLE__
#include <glut/glut.h>
#else
#define FREEGLUT_STATIC
#include <GL/glut.h>
#endif

GLBatch	squareBatch;
GLBatch greenBatch;
GLBatch redBatch;
GLBatch blueBatch;
GLBatch blackBatch;
GLBatch lineBatch;

GLShaderManager	shaderManager;


GLfloat blockSize = 0.4f;
GLfloat vVerts[] = { -blockSize, -blockSize, 0.0f, 
	                  blockSize, -blockSize, 0.0f,
					  blockSize,  blockSize, 0.0f,
					 -blockSize,  blockSize, 0.0f};

// Our lines
M3DVector3f vLine1[6] = {-0.0f,  0.0f, 0.0f, 
                         -0.8f,  0.6f, 0.0f};

M3DVector3f vLine2[6] = { 0.0f,  0.0f, 0.0f, 
                          0.8f,  0.6f, 0.0f};


///////////////////////////////////////////////////////////////////////////////
// This function does any needed initialization on the rendering context. 
// This is the first opportunity to do any OpenGL related tasks.
void SetupRC()
{
	// White background
	glClearColor(1.0f, 1.0f, 1.0f, 1.0f );
    
	shaderManager.InitializeStockShaders();

	// Load up a triangle fan
	squareBatch.Begin(GL_TRIANGLE_FAN, 4);
	squareBatch.CopyVertexData3f(vVerts);
	squareBatch.End();

    GLfloat vBlock[] = { 0.25f, 0.25f, 0.0f,
                         0.75f, 0.25f, 0.0f,
                         0.75f, 0.75f, 0.0f,
                         0.25f, 0.75f, 0.0f};
    
    greenBatch.Begin(GL_TRIANGLE_FAN, 4);
    greenBatch.CopyVertexData3f(vBlock);
    greenBatch.End();
    

    GLfloat vBlock2[] = { -0.75f, 0.25f, 0.0f,
                          -0.25f, 0.25f, 0.0f,
                          -0.25f, 0.75f, 0.0f,
                          -0.75f, 0.75f, 0.0f};
        
    redBatch.Begin(GL_TRIANGLE_FAN, 4);
    redBatch.CopyVertexData3f(vBlock2);
    redBatch.End();
    

    GLfloat vBlock3[] = { -0.75f, -0.75f, 0.0f,
                        -0.25f, -0.75f, 0.0f,
                        -0.25f, -0.25f, 0.0f,
                        -0.75f, -0.25f, 0.0f};
        
    blueBatch.Begin(GL_TRIANGLE_FAN, 4);
    blueBatch.CopyVertexData3f(vBlock3);
    blueBatch.End();


    GLfloat vBlock4[] = { 0.25f, -0.75f, 0.0f,
                        0.75f, -0.75f, 0.0f,
                        0.75f, -0.25f, 0.0f,
                        0.25f, -0.25f, 0.0f};
        
    blackBatch.Begin(GL_TRIANGLE_FAN, 4);
    blackBatch.CopyVertexData3f(vBlock4);
    blackBatch.End();


}

// Respond to arrow keys by moving the camera frame of reference
void SpecialKeys(int key, int x, int y)
{
	GLfloat stepSize = 0.025f;

	GLfloat blockX = vVerts[0];   // Upper left X
	GLfloat blockY = vVerts[7];  // Upper left Y

	if(key == GLUT_KEY_UP)
		blockY += stepSize;

	if(key == GLUT_KEY_DOWN)
		blockY -= stepSize;
	
	if(key == GLUT_KEY_LEFT)
		blockX -= stepSize;

	if(key == GLUT_KEY_RIGHT)
		blockX += stepSize;

	// Collision detection
	if(blockX < -1.0f) blockX = -1.0f;
	if(blockX > (1.0f - blockSize * 2)) blockX = 1.0f - blockSize * 2;;
	if(blockY < -1.0f + blockSize * 2)  blockY = -1.0f + blockSize * 2;
	if(blockY > 1.0f) blockY = 1.0f;

	// Recalculate vertex positions
	vVerts[0] = blockX;
	vVerts[1] = blockY - blockSize*2;
	
	vVerts[3] = blockX + blockSize*2;
	vVerts[4] = blockY - blockSize*2;
	
	vVerts[6] = blockX + blockSize*2;
	vVerts[7] = blockY;

	vVerts[9] = blockX;
	vVerts[10] = blockY;

	squareBatch.CopyVertexData3f(vVerts);

	glutPostRedisplay();
}


///////////////////////////////////////////////////////////////////////////////
// Called to draw scene
void RenderScene(void)
{
    // Clear the window with current clearing color
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

    GLfloat vRed[] = { 1.0f, 0.0f, 0.0f, 0.5f };
    GLfloat vGreen[] = { 0.0f, 1.0f, 0.0f, 1.0f };
    GLfloat vBlue[] = { 0.0f, 0.0f, 1.0f, 1.0f };
    GLfloat vBlack[] = { 0.0f, 0.0f, 0.0f, 1.0f };

    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vGreen);
    greenBatch.Draw();

    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vRed);
    redBatch.Draw();

    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlue);
    blueBatch.Draw();

    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlack);
    blackBatch.Draw();

    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vRed);
    squareBatch.Draw();
    glDisable(GL_BLEND);

    // Draw two lines
      
    // Line 1
    glLineWidth(4.0f);
    lineBatch.Begin(GL_LINES, 6);
    lineBatch.CopyVertexData3f(vLine1);
    lineBatch.End();
       
    // Enable smoothing
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_BLEND);
    glEnable(GL_LINE_SMOOTH);
    glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);
    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlack);
    lineBatch.Draw();

    // Line 2
    lineBatch.Begin(GL_LINES, 6);
    lineBatch.CopyVertexData3f(vLine2);
    lineBatch.End();
    // Turn off blending and all smoothing
    glDisable(GL_BLEND);
    glDisable(GL_POINT_SMOOTH);

    shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlack);
    lineBatch.Draw();

    // Flush drawing commands
    glutSwapBuffers();
}

///////////////////////////////////////////////////////////////////////////////
// Window has changed size, or has just been created. In either case, we need
// to use the window dimensions to set the viewport and the projection matrix.
void ChangeSize(int w, int h)
{
	glViewport(0, 0, w, h);
}

///////////////////////////////////////////////////////////////////////////////
// Main entry point for GLUT based programs
int main(int argc, char* argv[])
{
	gltSetWorkingDirectory(argv[0]);
	
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
	glutInitWindowSize(480, 480);
	glutCreateWindow("Move Block with Arrow Keys to see blending");
	
	GLenum err = glewInit();
	if (GLEW_OK != err)
	{
		// Problem: glewInit failed, something is seriously wrong.
		fprintf(stderr, "Error: %s\n", glewGetErrorString(err));
		return 1;
	}
	
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);
	glutSpecialFunc(SpecialKeys);

	SetupRC();

	glutMainLoop();
	return 0;
}

Answer:


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