1.7.2 Exercises

1

Using the last transformation on the container, try switching the order around by first rotating and then translating. See what happens and try to reason why this happens

注

这里题目应该是搞反了，示例中的代码是先旋转后移动，所以练习中调换顺序的话应该是先移动后旋转

2

Try drawing a second container with another call to glDrawElements but place it at a different position using transformations only. Make sure this second container is placed at the top-left of the window and instead of rotating, scale it over time (using the sin function is useful here; note that using sin will cause the object to invert as soon as a negative scale is applied)

|401x319

1

调换变换的顺序，先旋转后移位
实现：

	glm::mat4 trans(1.0f);
	trans = glm::rotate(trans, (float)glfwGetTime(), glm::vec3(0.0f, 0.0f, 1.0f));
	trans = glm::translate(trans, glm::vec3(0.5f, -0.5f, 0.0f));

效果：

可以看到笑脸是绕着原点在进行旋转而不是自身的中心点

2

再绘制一个container，然后应用不同的变换（移动到左上角，动态放大或缩小）
两个container除了trans 不一样，其余的均相同。

参考答案

        // second transformation
        // ---------------------
        transform = glm::mat4(1.0f); // reset it to identity matrix
        transform = glm::translate(transform, glm::vec3(-0.5f, 0.5f, 0.0f));
        float scaleAmount = static_cast<float>(sin(glfwGetTime()));
        transform = glm::scale(transform, glm::vec3(scaleAmount, scaleAmount, scaleAmount));
        glUniformMatrix4fv(transformLoc, 1, GL_FALSE, &transform[0][0]); // this time take the matrix value array's first element as its memory pointer value

        // now with the uniform matrix being replaced with new transformations, draw it again.
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

• 仍然使用trans，重新对其进行赋值
• 数组首元素的地址：&transform - 数组名； &transform[0][0] -对数组首元素取地址

我的实现

while(!glfwWindowShouldClose(window))
{
	//...
	glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

	glm::mat4 trans2(1.0f);
	trans2 = glm::translate(trans2, glm::vec3(-0.5f, 0.5f, 0.0f));
	auto s = sin((float)glfwGetTime());
	trans2 = glm::scale(trans2, glm::vec3(s, s, 0.0f));
	glUniformMatrix4fv(transformLoc, 1, GL_FALSE, glm::value_ptr(trans2));

	glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
}

行号	功能	说明
4	绘制第一个container
6-9	创建第二个container的变换矩阵

如果先进行translate，那么scale的效果会作用到translate上，移动的距离也跟着放大或缩小 以及反向。效果如下：

矩阵乘法的特性-后面的变换会影响前面的变换 和 万向节的物理特性（外环可以带动内环转，但是内环不能带动外环转）一致。