SDL_SetRenderTarget (SDL2)

Question

在我看来，有两个例子应该同样有效。，但它只在一种情况下工作，在第二种情况下不起作用。，我需要在我的代码中使用第二种情况。我需要将纹理导入到另一个函数以分配纹理，并对纹理进行一些更改并将其复制到渲染器。但不起作用。有人能帮上忙吗。我是个初学者，所以我想这一定是一些基本的错误。谢谢所有看过这个的人。

这里有两种情况下的简化代码：

First case works:

/*---------------------------------in main.c----------------------------*/

#include "global.h"

int main(/*...*/)
{
    create_txt_texture();
    to_screen(); /*Here there is red color on a screen*/

    return 0;
}

    /*-------------------------------in program.c-------------------------*/

#include "global.h"

int create_txt_texture()
{
    texture = SDL_CreateTexture(renderer, /*...*/);

    txt_to_bitmap();

    return 0;
}

void txt_to_bitmap()
{
    SDL_SetRenderTarget(renderer, texture);

    SDL_SetRenderDrawColor(renderer,255,0,0,255); 
    SDL_RenderClear(renderer);  /*filling target with red color*/

    SDL_SetRenderTarget(renderer, NULL);
}

void to_screen()
{
    SDL_RenderCopy(renderer, texture, NULL, NULL);
    SDL_RenderPresent(renderer); /*Here there is red color on a screen*/
}

    /*-------------------------------in global.h-------------------------*/

#ifndef __GLOBAL_H__
#define __GLOBAL_H__

SDL_Renderer *renderer;
SDL_Texture *texture;

void to_screen();
void txt_to_bitmap();
int create_txt_texture();

#endif

第二种情况无效：

/*-------------------------------in main.c-------------------------------*/

#include "global.h"

int main(/*...*/)
{
    create_txt_texture(texture);
    to_screen(); /*Here there is not red color on a screen*/

    return 0;
}

    /*-------------------------------in program.c-------------------------*/

#include "global.h"

int create_txt_texture(SDL_Texture *final_texture)
{
    final_texture = SDL_CreateTexture(renderer, /*...*/);

    txt_to_bitmap(final_texture);

    return 0;
}

void txt_to_bitmap(SDL_Texture *final_texture)
{
    SDL_SetRenderTarget(renderer, final_texture);

    SDL_SetRenderDrawColor(renderer,255,0,0,255); 
    SDL_RenderClear(renderer);  /*filling target with red color*/

    SDL_SetRenderTarget(renderer, NULL);
}

void to_screen()
{
    SDL_RenderCopy(renderer, texture, NULL, NULL);
    SDL_RenderPresent(renderer); /*Here there is not red color on a screen*/
}

    /*-------------------------------in global.h---------------------------*/

#ifndef __GLOBAL_H__
#define __GLOBAL_H__

SDL_Renderer *renderer;
SDL_Texture *texture;

void to_screen();
void txt_to_bitmap(SDL_Texture *final_texture);
int create_txt_texture(SDL_Texture *final_texture);

#endif

Answer 1

尝尝这个

    /*-------------------------------in main.c-------------------------------*/

#include "global.h"

int main(/*...*/)
{
    create_txt_texture(texture);
    to_screen(); /*Here there is not red color on a screen*/

    return 0;
}

    /*-------------------------------in program.c-------------------------*/

#include "global.h"

int create_txt_texture(SDL_Texture*& final_texture)
{
    final_texture = SDL_CreateTexture(renderer, /*...*/);

    txt_to_bitmap(final_texture);

    return 0;
}

void txt_to_bitmap(SDL_Texture *final_texture)
{
    SDL_SetRenderTarget(renderer, final_texture);

    SDL_SetRenderDrawColor(renderer,255,0,0,255); 
    SDL_RenderClear(renderer);  /*filling target with red color*/

    SDL_SetRenderTarget(renderer, NULL);
}

void to_screen()
{
    SDL_RenderCopy(renderer, texture, NULL, NULL);
    SDL_RenderPresent(renderer); /*Here there is not red color on a screen*/
}

    /*-------------------------------in global.h---------------------------*/

#ifndef __GLOBAL_H__
#define __GLOBAL_H__

SDL_Renderer *renderer;
SDL_Texture *texture;

void to_screen();
void txt_to_bitmap(SDL_Texture*& final_texture);
int create_txt_texture(SDL_Texture *final_texture);

#endif

解释：，您是通过值传递纹理指针，而不是通过引用传递纹理指针，这导致了这个问题。当您按值传递任何东西时，会创建另一个副本，因此在您的情况下，在按值传递指针之后，有两个指针(纹理和final_texture)指向相同的位置，但在步骤之后。

final_texture = SDL_CreateTexture(renderer, /*...*/);

final_texture指向新分配的纹理，而纹理指针保持不变。因此，要使纹理指针也指向新创建的纹理位置，必须将其作为引用传递。