GL_QUAD_STRIPS平滑色

Question

我试着用OpenGL绘制这张彩色地图

地图

当使用QUADS时，结果看起来很好：

int colorArr[] = { 255, 224, 192, 160, 128, 96, 64, 32 };
gluOrtho2D(-2, 2, -2, 2);
glPolygonMode(GL_FRONT, GL_FILL);

glDisable(GL_COLOR_LOGIC_OP);
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_QUADS);
for (int i = 0; i < 8; i++) {
    glColor3ub(colorArr[7 - i], 0, 0);
    glVertex2f(-4, i - 4);
    glVertex2f(4, i - 4);
    glVertex2f(4, i - 4 + 1);
    glVertex2f(-4, i - 4 + 1);
}
glEnd();

glEnable(GL_COLOR_LOGIC_OP);
glLogicOp(GL_OR);

glBegin(GL_QUADS);
for (int i = 0; i < 8; i++) {
    glColor3ub(0, colorArr[i], 0);
    glVertex2f(i - 4, 4);
    glVertex2f(i - 4, -4);
    glVertex2f(i - 4 + 1, -4);
    glVertex2f(i - 4 + 1, 4);
}
glEnd();

glFlush();

但是，如果我使用QUAD_STRIPS绘制多边形，颜色就会在线条之间变得平滑：

int colorArr[] = { 255, 224, 192, 160, 128, 96, 64, 32 };
gluOrtho2D(-2, 2, -2, 2);
glPolygonMode(GL_FRONT, GL_FILL);

glDisable(GL_COLOR_LOGIC_OP);
glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_QUAD_STRIP);
for (int i = 0; i < 8; i++) {
    glColor3ub(colorArr[7 - i], 0, 0);
    glVertex2f(-4, i - 4);
    glVertex2f(4, i - 4);
}
glVertex2f(-4, 4);
glVertex2f(4, 4);
glEnd();

glEnable(GL_COLOR_LOGIC_OP);
glLogicOp(GL_OR);

glBegin(GL_QUAD_STRIP);
for (int i = 0; i < 8; i++) {
    glColor3ub(0, colorArr[i], 0);
    glVertex2f(i - 4, 4);
    glVertex2f(i - 4, -4);
}
glVertex2f(4, 4);
glVertex2f(4, -4);
glEnd();
glFlush();

你知道为什么吗？

Answer 1

您正在绘制的四层条形图与此相当：

glBegin(GL_QUADS);
for (int i = 0; i < 7; i++) {
    glColor3ub(colorArr[7 - i], 0, 0);
    glVertex2f(-4, i - 4);
    glVertex2f(4, i - 4);
    glColor3ub(colorArr[7 - i+1], 0, 0);
    glVertex2f(4, i - 4 + 1);
    glVertex2f(-4, i - 4 + 1);
}

您将每两个顶点更改一次颜色，并将该颜色作为下一个/先前点的属性进行内插。