我希望箭头围绕屏幕中心旋转。但实际效果是箭头在屏幕右侧的一个点上旋转，对吗？让我觉得很奇怪！我对功能的理解错了吗？
自https://lazyfoo.net/tutorials/SDL/15_rotation_and_flipping/index.php上的官方样本以来的代码更改

/*This source code copyrighted by Lazy Foo' Productions 2004-2023
and may not be redistributed without written permission.*/

// Using SDL, SDL_image, standard IO, math, and strings
#include <SDL.h>
#include <SDL_image.h>
#include <stdio.h>
#include <string>
#include <cmath>

// Screen dimension constants
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;

// Texture wrapper class
class LTexture
{
public:
    // Initializes variables
    LTexture();

    // Deallocates memory
    ~LTexture();

    // Loads image at specified path
    bool loadFromFile(std::string path);

    // Deallocates texture
    void free();

    // Set color modulation
    void setColor(Uint8 red, Uint8 green, Uint8 blue);

    // Set blending
    void setBlendMode(SDL_BlendMode blending);

    // Set alpha modulation
    void setAlpha(Uint8 alpha);

    // Renders texture at given point
    void render(int x, int y, SDL_Rect *clip = NULL, double angle = 0.0, SDL_Point *center = NULL, SDL_RendererFlip flip = SDL_FLIP_NONE);

    // Gets image dimensions
    int getWidth();
    int getHeight();

private:
    // The actual hardware texture
    SDL_Texture *mTexture;

    // Image dimensions
    int mWidth;
    int mHeight;
};

// Starts up SDL and creates window
bool init();

// Loads media
bool loadMedia();

// Frees media and shuts down SDL
void close();

// The window we'll be rendering to
SDL_Window *gWindow = NULL;

// The window renderer
SDL_Renderer *gRenderer = NULL;

// Scene texture
LTexture gArrowTexture;

LTexture::LTexture()
{
    // Initialize
    mTexture = NULL;
    mWidth = 0;
    mHeight = 0;
}

LTexture::~LTexture()
{
    // Deallocate
    free();
}

bool LTexture::loadFromFile(std::string path)
{
    // Get rid of preexisting texture
    free();

    // The final texture
    SDL_Texture *newTexture = NULL;

    // Load image at specified path
    SDL_Surface *loadedSurface = IMG_Load(path.c_str());
    if (loadedSurface == NULL)
    {
        printf("Unable to load image %s! SDL_image Error: %s\n", path.c_str(), IMG_GetError());
    }
    else
    {
        // Color key image
        SDL_SetColorKey(loadedSurface, SDL_TRUE, SDL_MapRGB(loadedSurface->format, 0, 0xFF, 0xFF));

        // Create texture from surface pixels
        newTexture = SDL_CreateTextureFromSurface(gRenderer, loadedSurface);
        if (newTexture == NULL)
        {
            printf("Unable to create texture from %s! SDL Error: %s\n", path.c_str(), SDL_GetError());
        }
        else
        {
            // Get image dimensions
            mWidth = loadedSurface->w;
            mHeight = loadedSurface->h;
        }

        // Get rid of old loaded surface
        SDL_FreeSurface(loadedSurface);
    }

    // Return success
    mTexture = newTexture;
    return mTexture != NULL;
}

void LTexture::free()
{
    // Free texture if it exists
    if (mTexture != NULL)
    {
        SDL_DestroyTexture(mTexture);
        mTexture = NULL;
        mWidth = 0;
        mHeight = 0;
    }
}

void LTexture::setColor(Uint8 red, Uint8 green, Uint8 blue)
{
    // Modulate texture rgb
    SDL_SetTextureColorMod(mTexture, red, green, blue);
}

void LTexture::setBlendMode(SDL_BlendMode blending)
{
    // Set blending function
    SDL_SetTextureBlendMode(mTexture, blending);
}

void LTexture::setAlpha(Uint8 alpha)
{
    // Modulate texture alpha
    SDL_SetTextureAlphaMod(mTexture, alpha);
}

void LTexture::render(int x, int y, SDL_Rect *clip, double angle, SDL_Point *center, SDL_RendererFlip flip)
{
    // Set rendering space and render to screen
    SDL_Rect renderQuad = {x, y, mWidth, mHeight};

    // Set clip rendering dimensions
    if (clip != NULL)
    {
        renderQuad.w = clip->w;
        renderQuad.h = clip->h;
    }

    // Render to screen
    SDL_RenderCopyEx(gRenderer, mTexture, clip, &renderQuad, angle, center, flip);
}

int LTexture::getWidth()
{
    return mWidth;
}

int LTexture::getHeight()
{
    return mHeight;
}

bool init()
{
    // Initialization flag
    bool success = true;

    // Initialize SDL
    if (SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        printf("SDL could not initialize! SDL Error: %s\n", SDL_GetError());
        success = false;
    }
    else
    {
        // Set texture filtering to linear
        if (!SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "1"))
        {
            printf("Warning: Linear texture filtering not enabled!");
        }

        // Create window
        gWindow = SDL_CreateWindow("SDL Tutorial", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
        if (gWindow == NULL)
        {
            printf("Window could not be created! SDL Error: %s\n", SDL_GetError());
            success = false;
        }
        else
        {
            // Create vsynced renderer for window
            gRenderer = SDL_CreateRenderer(gWindow, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
            if (gRenderer == NULL)
            {
                printf("Renderer could not be created! SDL Error: %s\n", SDL_GetError());
                success = false;
            }
            else
            {
                // Initialize renderer color
                SDL_SetRenderDrawColor(gRenderer, 0xFF, 0xFF, 0xFF, 0xFF);

                // Initialize PNG loading
                int imgFlags = IMG_INIT_PNG;
                if (!(IMG_Init(imgFlags) & imgFlags))
                {
                    printf("SDL_image could not initialize! SDL_image Error: %s\n", IMG_GetError());
                    success = false;
                }
            }
        }
    }

    return success;
}

bool loadMedia()
{
    // Loading success flag
    bool success = true;

    // Load arrow
    if (!gArrowTexture.loadFromFile("./test/asset/arrow.png"))
    {
        printf("Failed to load arrow texture!\n");
        success = false;
    }

    return success;
}

void close()
{
    // Free loaded images
    gArrowTexture.free();

    // Destroy window
    SDL_DestroyRenderer(gRenderer);
    SDL_DestroyWindow(gWindow);
    gWindow = NULL;
    gRenderer = NULL;

    // Quit SDL subsystems
    IMG_Quit();
    SDL_Quit();
}

int main(int argc, char *args[])
{
    // Start up SDL and create window
    if (!init())
    {
        printf("Failed to initialize!\n");
    }
    else
    {
        // Load media
        if (!loadMedia())
        {
            printf("Failed to load media!\n");
        }
        else
        {
            // Main loop flag
            bool quit = false;

            // Event handler
            SDL_Event e;

            // Angle of rotation
            double degrees = 0;

            // Flip type
            SDL_RendererFlip flipType = SDL_FLIP_NONE;

            // While application is running
            while (!quit)
            {
                // Handle events on queue
                while (SDL_PollEvent(&e) != 0)
                {
                    // User requests quit
                    if (e.type == SDL_QUIT)
                    {
                        quit = true;
                    }
                    else if (e.type == SDL_KEYDOWN)
                    {
                        switch (e.key.keysym.sym)
                        {
                        case SDLK_a:
                            degrees -= 15;
                            break;

                        case SDLK_d:
                            degrees += 15;
                            break;

                        case SDLK_q:
                            flipType = SDL_FLIP_HORIZONTAL;
                            break;

                        case SDLK_w:
                            flipType = SDL_FLIP_NONE;
                            break;

                        case SDLK_e:
                            flipType = SDL_FLIP_VERTICAL;
                            break;
                        }
                    }
                }

                // Clear screen
                SDL_SetRenderDrawColor(gRenderer, 0xFF, 0xFF, 0xFF, 0xFF);
                SDL_RenderClear(gRenderer);

                SDL_Point center = {SCREEN_WIDTH / 2, SCREEN_HEIGHT / 2};
                SDL_Point arrowPoint = {center.x, center.y - (gArrowTexture.getHeight() / 2)};

                // Render arrow
                gArrowTexture.render(arrowPoint.x, arrowPoint.y, NULL, degrees, &center, flipType);

                SDL_Rect filledRect = {center.x - 3, center.y - 3, 5, 5};
                SDL_SetRenderDrawColor(gRenderer, 255, 0, 0, 255); 
                SDL_RenderFillRect(gRenderer, &filledRect);

                // Update screen
                SDL_RenderPresent(gRenderer);
            }
        }
    }

    // Free resources and close SDL
    close();

    return 0;
}

我想让箭头在我按a，d时在屏幕中心旋转