如何使用 SDL、OpenGL 和 C++ 加载具有透明度的 .png 图像？

Question

我正在尝试加载并显示具有透明度的 .png 图像。整个图像不是透明的，它只是我在 Photoshop 中裁剪的一个包的图片，周围是透明的。我需要将其加载到在 C++ 上使用 SDL 和 OpenGL 的程序中，但是当我尝试它时，图像中袋子周围的空间不是正确显示，而是“模糊”（我只能这样说）。我想说的是，透明度不起作用。

到目前为止，这是我的代码：

init() 方法：

SDL_Surface *surface;
GLenum texture_format;
GLint  nOfColors;

if ( (surface = IMG_Load("bag.png")) ) { 

    // Check that the image's width is a power of 2
    if ( (surface->w & (surface->w - 1)) != 0 ) {
        printf("warning: bag.png's width is not a power of 2\n");
    }

    // Also check if the height is a power of 2
    if ( (surface->h & (surface->h - 1)) != 0 ) {
        printf("warning: bag.png's height is not a power of 2\n");
    }

        // get the number of channels in the SDL surface
        nOfColors = surface->format->BytesPerPixel;
        if (nOfColors == 4)     // contains an alpha channel
        {
                if (surface->format->Rmask == 0x000000ff)
                        texture_format = GL_RGBA;
                //else
                        //texture_format = GL_BGRA;
        } else if (nOfColors == 3)     // no alpha channel
        {
                if (surface->format->Rmask == 0x000000ff)
                        texture_format = GL_RGB;
                //else
                        //texture_format = GL_BGR;
        } else {
                printf("warning: the image is not truecolor..  this will probably break\n");
                // this error should not go unhandled
        }

    // Have OpenGL generate a texture object handle for us
    glGenTextures( 1, &texture );

    // Bind the texture object
    glBindTexture( GL_TEXTURE_2D, texture );

    // Set the texture's stretching properties
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );

    // Edit the texture object's image data using the information SDL_Surface gives us
    glTexImage2D( GL_TEXTURE_2D, 0, nOfColors, surface->w, surface->h, 0,
                      texture_format, GL_UNSIGNED_BYTE, surface->pixels );
} 
else {
    printf("SDL could not load bag.png: %s\n", SDL_GetError());
    SDL_Quit();
    //return 1;
}    

// Free the SDL_Surface only if it was successfully created
if ( surface ) { 
    SDL_FreeSurface( surface );
}

drawScene() 方法：

float xMin, xMax, yMin, yMax;

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glTranslatef(0.0f, 0.0f, -6.0f);

glEnable(GL_TEXTURE_2D);

glBindTexture(GL_TEXTURE_2D, texture); 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); // Use nice (linear) scaling 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); // Use nice (linear) scaling

glBegin (GL_QUADS);
glNormal3f(0.0f, 0.0f, 1.0f);
if (texturesOn) glTexCoord2f(0.0f, 1.0f);
glVertex3f(4.3f, -3.5f, 0.0f); //bottom left

if (texturesOn) glTexCoord2f(1.0f, 1.0f);
glVertex3f(5.3f, -3.5f, 0.0f); //bottom right

if (texturesOn) glTexCoord2f(1.0f, 0.0f);
glVertex3f(5.3f, -4.5f, 0.0f); //top right

if (texturesOn) glTexCoord2f(0.0f, 0.0f);
glVertex3f(4.3f, -4.5f, 0.0f); //top left
glEnd();

glDisable(GL_TEXTURE_2D);

glutSwapBuffers();

Answer 1

您需要启用混合才能使透明度起作用：

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

此外，您应该检查加载的 PNG 文件是否包含 Alpha 通道。