当尝试使用SDL/C++进行与帧无关的移动时，在特定方向上移动缓慢或没有移动

我正在尝试制作一个简单的蛇游戏，并从尝试让一个对象在屏幕上独立于帧速率移动开始。我的问题是，当我将速度设置为负值以在屏幕上向左或向上移动对象时，我会得到移动。然而，当我试图向右或向下移动时，这涉及到正值，我在对象上几乎没有移动。如果我设置的值明显更高，那么是的，物体会左右移动，但我得到的移动速度在正值和负值之间的差异对我来说很奇怪。

这是我的密码。到目前为止，我只尝试过让一个对象移动，所以有一两个奇怪的函数没有定义或需要工作。我只是暂时关心我的运动问题。

Snake.h

#pragma once
#include <vector>
#include <SDL.h>
#include <iostream>
typedef SDL_Rect snakeSegment;
enum orientation { ORIENTATION_LEFT = 0, ORIENTATION_UP = 1, ORIENTATION_RIGHT = 2, 
ORIENTATION_DOWN = 3 };

class Snake
{
private:
snakeSegment head;

std::vector<snakeSegment> segments;
SDL_Surface* snakeSurface;
SDL_Texture* snakeTexture;
const char* filepath = "assets/sprites/snake_body.bmp";

public:

Snake();
void init(SDL_Renderer* renderer);
void move(float delta);
void appendSegment();
void render(SDL_Renderer* renderer);

int orientation;


};

Snake.cpp

#include "Snake.h"

Snake::Snake()
{

} 

void Snake::init(SDL_Renderer *renderer)
{
snakeSurface = SDL_LoadBMP(filepath);
if (snakeSurface == nullptr)
{
throw(SDL_GetError());
}
else {
snakeTexture = SDL_CreateTextureFromSurface(renderer, snakeSurface);
if (snakeTexture == nullptr)
{
SDL_FreeSurface(snakeSurface);
throw(SDL_GetError());
}
SDL_FreeSurface(snakeSurface);
}
SDL_Window* w = SDL_RenderGetWindow(renderer);

int width;
int height;
SDL_GetWindowSize(w, &width, &height);
head.w = 16;
head.h = 32;
head.y = height / 2 - (head.h / 2);
head.x = width / 2 - (head.w / 2);
orientation = ORIENTATION_UP;
}

void Snake::render(SDL_Renderer* renderer)
{
SDL_RenderCopy(renderer, snakeTexture, nullptr, &head);
}

void Snake::move(float delta)
{
float positionYDelta = 0;
float positionXDelta = 0;

switch (orientation) {
case ORIENTATION_UP:
positionYDelta = -3 * delta;
positionXDelta = 0;
break;
case ORIENTATION_DOWN:
positionYDelta = 3 * delta; 
positionXDelta = 0;
break;
case ORIENTATION_RIGHT:
positionYDelta = 0;
positionXDelta = 3 * delta;
break;
case ORIENTATION_LEFT:
positionYDelta = 0;
positionXDelta = -3 * delta;
break;
}
head.y += positionYDelta;
head.x += positionXDelta;

}
void Snake::appendSegment()
{
}

main.cpp:

#include <iostream>
#include <SDL.h>
#include "Snake.h"


int main(int argc, char* args[])
{
if (SDL_Init(SDL_INIT_EVERYTHING) != 0)
{
std::cout << "failed to initialize" << std::endl;
}
SDL_Window* w = SDL_CreateWindow("Snake Clone", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640, 480, SDL_WINDOW_SHOWN);
SDL_Renderer* ren = SDL_CreateRenderer(w, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
Snake snake;
try {
snake.init(ren);
}
catch (std::exception e)
{
std::cout << e.what();
}
SDL_Event e;

bool quit = false;
float deltaTime;
float time_seconds = SDL_GetTicks() / 1000.f;

while (!quit)
{
float now;
now = SDL_GetTicks() / 1000.f;
deltaTime = now - time_seconds;
time_seconds = now;
const Uint8* state = SDL_GetKeyboardState(NULL);


while (SDL_PollEvent(&e))
{
if (e.type == SDL_QUIT)
{
quit = true;
}
if (e.type == SDL_KEYDOWN)
{
if (state[SDL_SCANCODE_W])
{
if (snake.orientation == ORIENTATION_LEFT || snake.orientation == ORIENTATION_RIGHT)
{
snake.orientation = ORIENTATION_UP;
std::cout << "W pressed" << std::endl;
}
}
if (state[SDL_SCANCODE_S])
{
if (snake.orientation == ORIENTATION_LEFT || snake.orientation == ORIENTATION_RIGHT)
{
snake.orientation = ORIENTATION_DOWN;
std::cout << "S pressed" << std::endl;
}
}
if (state[SDL_SCANCODE_D])
{
if (snake.orientation == ORIENTATION_UP || snake.orientation == ORIENTATION_DOWN)
{
snake.orientation = ORIENTATION_RIGHT;
std::cout << "D pressed" << std::endl;
}
}
if (state[SDL_SCANCODE_A])
{
if (snake.orientation == ORIENTATION_UP || snake.orientation == ORIENTATION_DOWN)
{
snake.orientation = ORIENTATION_LEFT;
std::cout << "A pressed" << std::endl;
}
}
}
}
snake.move(deltaTime);

SDL_RenderClear(ren);
snake.render(ren);
SDL_RenderPresent(ren);

}
SDL_DestroyRenderer(ren);
SDL_DestroyWindow(w);
return 0;
}

我在这里所要做的就是让我的精灵以一致的速度向各个方向移动。