透明度Opengl + SDL不起作用C ++

Question

I've been trying to get transparency working with SDL+ OpenGL.

Here are my functions that initialize OpenGL and SDL, draw an image, and create a texture

void initGL()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho( 0.0, Screen_Width,Screen_Height, 0.0, 1.0, -1.0 );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, Screen_Width, Screen_Height);
}

void initSDL()
{
SDL_Init(SDL_INIT_EVERYTHING);
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BUFFER_SIZE, 32);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
Bob=SDL_SetVideoMode(0, 0, 32, SDL_OPENGL);
Screen_Width= Bob->w;
Screen_Height= Bob->h;
SDL_WM_SetCaption("Project", NULL);;
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
}

void Texture::draw(int x1, int y1,int x2, int y2,std::string filename)
{
glClear( GL_COLOR_BUFFER_BIT );
CreateTexture(filename.c_str());
glEnable( GL_TEXTURE_2D );
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
glBindTexture(GL_TEXTURE_2D, Tex);
glBegin(GL_QUADS);
    glTexCoord2f(0.f, 0.f);
    glVertex2f( 0.f, 0.f );
    glTexCoord2f(1.f, 0.f);
    glVertex2f(  1920.f, 0.f );
    glTexCoord2f(1.f, 1.f);
    glVertex2f(  1920.f,  1080.f );
    glTexCoord2f(0.f, 1.f);
    glVertex2f( 0.f,  1080.f );
glEnd();
glDisable(GL_TEXTURE_2D);
SDL_GL_SwapBuffers();
}

void Texture::CreateTexture(std::string filename)
{
SDL_Surface* image =NULL;
image=IMG_Load( filename.c_str() );
glClearColor( 0, 0, 0, 0 );
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glGenTextures(1, &Tex);
glBindTexture(GL_TEXTURE_2D, Tex);
int mode = GL_RGB;
if(image->format->BytesPerPixel == 4)
    mode = GL_RGBA;
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, 4, image->w, image->h, 0, mode, GL_UNSIGNED_BYTE, image-  >pixels);
SDL_FreeSurface(image);
}

Answer 1

You're drawing exactly one quad, with one texture. What exactly do you expect to be visible in the translucent parts? The background color?

Or your desktop, or the windows beneath? That takes quite some additional work.

First you need a compositor running (on Windows this requires Windows Vista with Aero or later).

And the window must be configured in a way, that its alpha channel is actually considered when compositing; AFAIK SDL doesn't take those preparations.

Answer 2

You can check the texture format is should be RGBA and you can test a white texture(RGB(255,255,255)) that has alpha channel of 128, so the whole texture is just white with alpha half white. Drawing this on a black background should result a half white texture being drawn.

Also the vertex positions might not be right, you could try something like this (with identity matrix for projection and world transform) to see if everything is in order with opengl setup

glTexCoord2f(0.f, 0.f);
glVertex2f( 0.25f, 0.25f );
glTexCoord2f(1.f, 0.f);
glVertex2f(  0.25f,0.5f );
glTexCoord2f(1.f, 1.f);
glVertex2f(  0.5f,0.5f );
glTexCoord2f(0.f, 1.f);
glVertex2f(  0.5f,0.25f );

Hope this helps. Razvan.