c++ - 使用旋转矩阵旋转三角形 - 从窗口中消失

Question

我尝试使用旋转矩阵围绕其中心顺时针旋转三角形。但是，三角形会移出屏幕并移向左下角。 这是我的代码

class Triangle {
    public:
    vector<glm::vec3> verts; // positions of the triangles vertices.
    glm::vec3 pos; // position of the triangle's origin.

    void rotate(float rotationMat[4]) {
        // Transform vert 1
        glm::vec3 vert1 = verts[0]; // Get the vertices (centred about the triangle origin)
        vert1 = vert1 - pos; // Get the vertices centered about the origin (0, 0)
        // Rotate the vertices
        vert1.x = (vert1.x * rotationMat[0]) + (vert1.y * rotationMat[1]);
        vert1.y = (vert1.x * rotationMat[2]) + (vert1.x * rotationMat[3]);
        // Get the vertices centered about the triangle origin
        vert1 = vert1 + pos;

        // Set the new vertices value
        verts[0] = vert1;



        // Transform vert2
        glm::vec3 vert2 = verts[1]; // Get the vertices (centered about the triangle origin)
        vert2 = vert2 - pos; // Get the vertices centered about the origin (0, 0)
        // Rotate the vertices
        vert2.x = (vert2.x * rotationMat[0]) + (vert2.y * rotationMat[1]);
        vert2.y = (vert2.x * rotationMat[2]) + (vert2.x * rotationMat[3]);
        // Get the vertices centered about the triangle origin
        vert2 = vert2 + pos;

        // Set the new vertices value
        verts[1] = vert2;



        // Transform vert3
        glm::vec3 vert3 = verts[2]; // Get the vertices (centered about the triangle origin)
        vert3 = vert3 - pos; // Get the vertices centered about the origin (0, 0)
        // Rotate the vertices
        vert3.x = (vert3.x * rotationMat[0]) + (vert3.y * rotationMat[1]);
        vert3.y = (vert3.x * rotationMat[2]) + (vert3.x * rotationMat[3]);
        // Get the vertices centered about the triangle origin
        vert3 = vert3 + pos;

        // Set the new vertices value
        verts[2] = vert3;

    }
};

我这样调用方法:

float rotationMat[4] = { (float) cos(1), (float) -sin(1), (float) sin(1), (float) cos(1)};
triangle.rotate(rotationMat);

作为测试，我打印出旋转前后的顶点，结果如下:

BEFORE: 
(100,-100)
(0,100)
(-100,-100)

AFTER: 
(788.762,899.003)
(566.437,591.801)
(680.701,749.688)

我哪里错了？为什么三角形会飞出屏幕？

Answer 1

你不能那样做:

vert1.x = (vert1.x * rotationMat[0]) + (vert1.y * rotationMat[1]);
vert1.y = (vert1.x * rotationMat[2]) + (vert1.x * rotationMat[3]);

在第一行之后，vert1.x 将发生变化，您在第二行所做的计算将无效。做:

auto newx = (vert1.x * rotationMat[0]) + (vert1.y * rotationMat[1]);
auto newy = (vert1.x * rotationMat[2]) + (vert1.x * rotationMat[3]);
vert1.x = newx;
vert1.y = newy;

(可能还有其他问题)