SDL2 透明背景

Question

我正在尝试从更大的.PNG 图像创建一个.PNG 图像。 基本上剪辑原始的矩形区域并将结果另存为 another.PNG 有点像纹理解包器，如果你愿意的话。

我的问题是原始图像中透明的部分在剪切图像中是彩色的。

最初我使用硬件加速，背景是带有斑点的白色，切换到软件渲染器只是将背景更改为黑色。

我想保持原始图像的透明度。

#include <SDL2/SDL.h>
#include <SDL2_image/SDL_image.h>


void save_texture(const char* file_name, SDL_Renderer* renderer, SDL_Texture* texture); // 
save png to disk
SDL_Texture* clipTexture(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source); // get 
a new texture that is clipped from the original


int main(int argc, const char * argv[])
{

    SDL_Init(SDL_INIT_VIDEO);

    IMG_Init(IMG_INIT_PNG);
    SDL_Surface* surface = IMG_Load("LevelItems.png");
    SDL_Renderer* renderer = SDL_CreateSoftwareRenderer(surface);

    SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, surface);
    SDL_Rect frame_rect = { 189, 243,115, 50 };
    SDL_Texture* tex_clip = clipTexture( frame_rect, renderer, texture );

    save_texture("test1.png", renderer, tex_clip);

    SDL_FreeSurface(surface);

    return 0;
}

// return a new texture that is a part of the original texture.

SDL_Texture* clipTexture(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source)
{
    SDL_Texture* result = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, 
SDL_TEXTUREACCESS_TARGET, rect.w, rect.h);
SDL_SetRenderTarget(renderer, result);
SDL_RenderCopy(renderer, source, &rect, NULL);

return result;
}


// save png to disk
void save_texture(const char* file_name, SDL_Renderer* renderer, SDL_Texture* texture)
{
    int width, height;
    SDL_QueryTexture(texture, NULL, NULL, &width, &height);
    SDL_Surface* surface = SDL_CreateRGBSurface(0, width, height, 32, 0, 0, 0, 0);

    SDL_RenderReadPixels(renderer, NULL, surface->format->format, surface->pixels, surface- 
  >pitch);
    IMG_SavePNG(surface, file_name);
    SDL_FreeSurface(surface);
}

Answer 1

有两个问题：

1. 禁用混合。 使用SDL_SetTextureBlendMode(source, SDL_BLENDMODE_NONE); 之前RenderCopy ，否则你会得到错误的颜色（与默认黑色混合 - 结果会比它应该的更暗）。 混合与纹理相关，因此如果您想稍后使用此纹理，您可能应该立即重置混合模式。 有关在不同混合模式中使用的公式，请参阅SDL_SetTextureBlendMode文档。

2. 您生成的表面没有 Alpha 通道，因此无处复制结果。 检查SDL_CreateRGBSurface的文档：虽然它允许将0用于颜色蒙版以推断默认值，但它明确不允许将其用于 alpha 通道，因此0代替amask会导致“我不想要 alpha 通道，只是 RGB” . 您生成的格式是 RGB888 打包为 32 位。 你想要 alpha，所以你应该使用正确的颜色掩码 - 从文档中获取一个，在你的情况下，你甚至不需要检查字节序，但它永远不会受到伤害。

把它们加起来：

#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>


void save_texture(const char* file_name, SDL_Renderer* renderer, SDL_Texture* texture);
SDL_Texture* clipTexture(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source);

int main(int argc, const char * argv[])
{

    SDL_Init(SDL_INIT_VIDEO);

    IMG_Init(IMG_INIT_PNG);
    SDL_Surface* surface = IMG_Load("test.png");
    SDL_Renderer* renderer = SDL_CreateSoftwareRenderer(surface);

    SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, surface);
    SDL_Rect frame_rect = { 189, 243,115, 50 };
    SDL_Texture* tex_clip = clipTexture( frame_rect, renderer, texture );

    save_texture("test1.png", renderer, tex_clip);

    SDL_FreeSurface(surface);

    return 0;
}

// return a new texture that is a part of the original texture.

SDL_Texture* clipTexture(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source)
{
    SDL_Texture* result = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, 
SDL_TEXTUREACCESS_TARGET, rect.w, rect.h);
SDL_SetRenderTarget(renderer, result);
SDL_SetTextureBlendMode(source, SDL_BLENDMODE_NONE);
SDL_RenderCopy(renderer, source, &rect, NULL);

return result;
}


// save png to disk
void save_texture(const char* file_name, SDL_Renderer* renderer, SDL_Texture* texture)
{
    int width, height;
    SDL_QueryTexture(texture, NULL, NULL, &width, &height);

    Uint32 rmask, gmask, bmask, amask;
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
    rmask = 0xff000000;
    gmask = 0x00ff0000;
    bmask = 0x0000ff00;
    amask = 0x000000ff;
#else
    rmask = 0x000000ff;
    gmask = 0x0000ff00;
    bmask = 0x00ff0000;
    amask = 0xff000000;
#endif

    SDL_Surface* surface = SDL_CreateRGBSurface(0, width, height, 32, rmask, gmask, bmask, amask);

    SDL_RenderReadPixels(renderer, NULL, surface->format->format, surface->pixels, surface->pitch);
    IMG_SavePNG(surface, file_name);
    SDL_FreeSurface(surface);
}