用纯C语言实现3D空间中的点坐标转化为屏幕二维点坐标，包含主视图、侧视图、俯视图、正等轴投影

要实现3D空间中的点坐标转换为屏幕二维点坐标，需要进行透视变换和投影变换。以下是一些基本的思路和示例代码，可以用于实现主视图、侧视图、俯视图、正等轴投影。

1. 主视图投影

主视图投影是指以一个点作为视点，从一个方向观察物体，投影到一个平面上。通常情况下，主视图的观察方向是从正面，也就是Z轴负方向。投影平面一般是平行于X-Y平面。

具体实现可以通过以下步骤完成：

• 定义观察点坐标和投影平面距离
• 对3D坐标进行透视变换
• 对透视变换后的坐标进行投影变换
• 将投影后的坐标映射到屏幕上

示例代码：

int x_2d = (int) (x_3d / (z_3d - view_point_z) * distance_to_projection_plane);
int y_2d = (int) (y_3d / (z_3d - view_point_z) * distance_to_projection_plane);

2. 侧视图投影

侧视图投影是指以一个点作为视点，从一个方向观察物体，投影到一个平面上。通常情况下，侧视图的观察方向是从侧面，也就是X轴正方向。投影平面一般是平行于Y-Z平面。

具体实现可以通过以下步骤完成：

• 定义观察点坐标和投影平面距离
• 对3D坐标进行透视变换
• 对透视变换后的坐标进行投影变换
• 将投影后的坐标映射到屏幕上

示例代码：

int x_2d = (int) (y_3d / (x_3d - view_point_x) * distance_to_projection_plane);
int y_2d = (int) (z_3d / (x_3d - view_point_x) * distance_to_projection_plane);

3. 俯视图投影

俯视图投影是指以一个点作为视点，从一个方向观察物体，投影到一个平面上。通常情况下，俯视图的观察方向是从上方，也就是Y轴正方向。投影平面一般是平行于X-Z平面。

具体实现可以通过以下步骤完成：

• 定义观察点坐标和投影平面距离
• 对3D坐标进行透视变换
• 对透视变换后的坐标进行投影变换
• 将投影后的坐标映射到屏幕上

以下是一个简单的示例代码，用于将3D空间中的点坐标转化为屏幕二维点坐标。这里包括了主视图、侧视图、俯视图、正等轴投影的实现。

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 480

typedef struct {
    double x, y, z;
} Point3D;

typedef struct {
    int x, y;
} Point2D;

void project_ortho(Point3D point_3d, Point2D *point_2d, double distance_to_projection_plane) {
    point_2d->x = (int) point_3d.x;
    point_2d->y = (int) point_3d.y;
}

void project_isometric(Point3D point_3d, Point2D *point_2d, double distance_to_projection_plane) {
    point_2d->x = (int) ((point_3d.x - point_3d.z) * cos(30 * M_PI / 180));
    point_2d->y = (int) ((point_3d.y - (point_3d.x + point_3d.z) * sin(30 * M_PI / 180)) * cos(30 * M_PI / 180));
}

void project_main(Point3D point_3d, Point2D *point_2d, Point3D view_point, double distance_to_projection_plane) {
    double z_3d = point_3d.z - view_point.z;
    point_2d->x = (int) (point_3d.x - view_point.x) * distance_to_projection_plane / z_3d + SCREEN_WIDTH / 2;
    point_2d->y = (int) (point_3d.y - view_point.y) * distance_to_projection_plane / z_3d + SCREEN_HEIGHT / 2;
}

void project_side(Point3D point_3d, Point2D *point_2d, Point3D view_point, double distance_to_projection_plane) {
    double x_3d = point_3d.x - view_point.x;
    point_2d->x = (int) (point_3d.y - view_point.y) * distance_to_projection_plane / x_3d + SCREEN_WIDTH / 2;
    point_2d->y = (int) (point_3d.z - view_point.z) * distance_to_projection_plane / x_3d + SCREEN_HEIGHT / 2;
}

void project_top(Point3D point_3d, Point2D *point_2d, Point3D view_point, double distance_to_projection_plane) {
    double y_3d = point_3d.y - view_point.y;
    point_2d->x = (int) (point_3d.x - view_point.x) * distance_to_projection_plane / y_3d + SCREEN_WIDTH / 2;
    point_2d->y = (int) (point_3d.z - view_point.z) * distance_to_projection_plane / y_3d + SCREEN_HEIGHT / 2;
}

int main() {
    // Define the 3D points of a cube
    Point3D cube[8] = {
        {-50, -50, -50},
        {50, -50, -50},
        {50, 50, -50},
        {-50, 50, -50},
        {50, -50, 50},
        {50, 50, 50},
        {-50, 50, 50},
        {-50, -50, 50},
    };

    // Define the view point for the main, side, and top projections
    Point3D main_view_point = {0, 0, 200};
    Point3D side_view_point = {-200, 0, 0};
    Point3D top_view_point = {0, 200, 0};

    // Define the distance from the projection plane for the orthogonal and isometric projections
    double distance_to_ortho_projection_plane = 200;
    double distance_to_isometric_projection_plane = 200 / cos(30 * M_PI / 180);

    // Project the 3D points to 2D points for each projection
    Point2D main_projection[8];
    Point2D side_projection[8];
    Point2D top_projection[8];
    Point2D ortho_projection[8];
    Point2D iso_projection[8];

    int i;
    for (i = 0; i < 8; i++) {
        project_main(cube[i], &main_projection[i], main_view_point, distance_to_ortho_projection_plane);
        project_side(cube[i], &side_projection[i], side_view_point, distance_to_ortho_projection_plane);
        project_top(cube[i], &top_projection[i], top_view_point, distance_to_ortho_projection_plane);
        project_ortho(cube[i], &ortho_projection[i], distance_to_ortho_projection_plane);
        project_isometric(cube[i], &iso_projection[i], distance_to_isometric_projection_plane);
    }

    // Draw the 2D projections
    // ...
    // Your code to draw the projections goes here
    // ...

    return 0;
}