OpenGL Study 2

最新推荐文章于 2024-04-20 10:04:52 发布

Anglewing_nwpu

最新推荐文章于 2024-04-20 10:04:52 发布

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分类专栏： CG 文章标签： buffer 图形

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本文介绍如何使用OpenGL在窗口中绘制基本图形，包括三角形和正方形，并演示如何为这些图形添加颜色，通过Flatcoloring和平滑着色实现不同的视觉效果。

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Lesson02:

在前一课建立的Opengl窗口中绘制一个三角形和一个正方形。

//here we do all the drawings
bool DrawGLScene(GLvoid)
{
    //clear the screen and the depth buffer
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    //reset the current modelview matrix
    glLoadIdentity();

    //move left 1.5 units,deep 6.0 units from current position
    //move the model from (0,0,0) to (-1.5,0.0,-6.0),it can also be implemented by move the view point
    glTranslatef(-1.5f,0.0f,-6.0f);

    //draw a triangle
    glBegin(GL_TRIANGLES);
        glVertex3f(0.0f,1.0f,0.0f);
        glVertex3f(-1.0f,-1.0f,0.0f);
        glVertex3f(1.0f,-1.0f,0.0f);
    glEnd();

    //move from (-1.5,0.0,-6.0) to (1.5,0.0,-6.0)
    glTranslatef(3.0f,0.0f,0.0f);

    //draw a quad,the vertex is represented in the model coordinate
    glBegin(GL_QUADS);
        glVertex3f(-1.0f,1.0f,0.0f);
        glVertex3f(-1.0f,-1.0f,0.0f);
        glVertex3f(1.0f,-1.0f,0.0f);
        glVertex3f(1.0f,1.0f,0.0f);
    glEnd();

    return true;
}

绘制前先清除颜色缓存和深度缓存，将当前矩阵置为单位矩阵。

在该代码中没有使用视图变换，而是采用模型变换实现的。对于每个要绘制的图形，现使用glTranslatef()将局部坐标系的原点移动到适当的位置，然后在绘制时，就可以在局部坐标系中以原点为中心绘制相应图形了。

注意，glTranslatef（）的参数是移动增量，是将当前位置移动指定的增量到目标位置，移动后当前矩阵值为目标位置的值。

绘制图形：在glBegin（）和glEnd（）指定图形的顶点坐标。

绘制三角形：采用GL_TRIANGLES参数，使用三个顶点绘制一个三角形，顶点要按逆时针的顺序列出；

绘制四边形：采用GL_QUADS参数，使用四个顶点，顶点按逆时针顺序列出

Lesson03

给图形着色，采用Flat coloring 和 Smooth Coloring 两种着色模式

//here we do all the drawings
bool DrawGLScene(GLvoid)
{
    //clear the screen and the depth buffer
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    //reset the current modelview matrix
    glLoadIdentity();

    //move left 1.5 units,deep 6.0 units from current position
    //move the model from (0,0,0) to (-1.5,0.0,-6.0),it can also be implemented by move the view point
    glTranslatef(-1.5f,0.0f,-6.0f);

    //draw a triangle,with smooth coloring
    glBegin(GL_TRIANGLES);
        glColor3f(1.0f,0.0f,0.0f);
        glVertex3f(0.0f,1.0f,0.0f);
        glColor3f(0.0f,1.0f,0.0f);
        glVertex3f(-1.0f,-1.0f,0.0f);
        glColor3f(0.0f,0.0f,1.0f);
        glVertex3f(1.0f,-1.0f,0.0f);
    glEnd();

    //move from (-1.5,0.0,-6.0) to (1.5,0.0,-6.0)
    glTranslatef(3.0f,0.0f,0.0f);

    //draw a quad,the vertex is represented in the model coordinate
    //with flat coloring
    glColor3f(0.5f,0.5f,0.0f);
    glBegin(GL_QUADS);
        glVertex3f(-1.0f,1.0f,0.0f);
        glVertex3f(-1.0f,-1.0f,0.0f);
        glVertex3f(1.0f,-1.0f,0.0f);
        glVertex3f(1.0f,1.0f,0.0f);
    glEnd();

    return true;
}

对每个顶点都设置颜色后，Opengl会自动在不同顶点的不同颜色间进行插值，从而使颜色线性变换；

如果对某个顶点不设置颜色，则应用之前最后设置的颜色，例如在绘制四边形时，只在绘制第一个顶点时设置了颜色，则其他顶点采用相同颜色。

在Opengl中，视图矩阵，投影矩阵，颜色矩阵和纹理矩阵都只记录最后一次修改后的值。