OpenGL中的透明帧缓冲背景

Question

我想用glClear和glClearColor来给一个帧缓冲区填充颜色，包括透明度。但是，当这个帧缓冲区绑定到一个纹理上并渲染到屏幕时，它总是显示为不透明的。

我希望渲染到帧缓冲区的所有内容都能保持它们的透明度。我只是想改变背景。

请看以下代码：

def create_texture(surface):
surface.texture = glGenTextures(1)
glMatrixMode(GL_MODELVIEW)
glLoadIdentity() #Loads model matrix
glBindTexture(GL_TEXTURE_2D, surface.texture) #Binds the current 2D texture to the texture to be drawn
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR) #Required to be set for maping the pixel data
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR) #Similar as above
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, surface.surface_size[0], surface.surface_size[1], 0, GL_RGBA,GL_UNSIGNED_BYTE, surface.data) #Put surface pixel data into texture
if surface.data == None:
    setup_framebuffer(surface)
    c = [float(sc)/255.0 for sc in surface.colour] #Divide colours by 255 because OpenGL uses 0-1
    if surface.background_alpha:
        c[3] = float(surface.background_alpha)/255.0
    glClearColor(*c)
    glClear(GL_COLOR_BUFFER_BIT)
    end_framebuffer()
Surface.texture_ready.append(surface)
def setup_framebuffer(surface):
#Create texture if not done already
if surface.texture == None:
    create_texture(surface)
#Render child to parent
if surface.frame_buffer == None:
    surface.frame_buffer =  glGenFramebuffersEXT(1)
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, surface.frame_buffer)
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, surface.texture, 0)
glPushAttrib(GL_VIEWPORT_BIT)
glViewport(0,0,surface._scale[0],surface._scale[1])
glMatrixMode(GL_PROJECTION)
glLoadIdentity() #Load the projection matrix
gluOrtho2D(0,surface._scale[0],0,surface._scale[1])
def end_framebuffer():
    glPopAttrib()
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity() #Load the projection matrix
    gluOrtho2D(0,1280,720,0) #Set an orthorgraphic view

surface.background_alpha应该是帧缓冲区背景的透明度。以下是我的初始化代码：

def __init__(self,title,game_size,on_exit = sys.exit):
        self.keys = [False] * 323
        self.events = []
        pygame.font.init()
        pygame.mixer.init()
        self.title = title
        self.game_size = game_size
        self.first_screen = (1280,720) #Take 120 pixels from the height because the menu bar, window bar and dock takes space
        glutInit(sys.argv)
        glutInitWindowPosition(0,0)
        glutInitWindowSize(*game_size)
        glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA)
        glutGameModeString("1280x720:32@60") #720 HD
        glutCreateWindow(title)
        glutSetIconTitle(title)
        self.callbacks()
        self.game_gap = (0,0)
        self.on_exit = on_exit
        self.mod_key = 1024 if sys.platform == "darwin" else 64
        Surface.__init__(self,game_size)
        self.screen_change = True
        self.frames = [time.time()]
        self.fps = 60
        self.last_time = 0
        self.fade_surface = Surface([1280,720])
    def callbacks(self):
        glutReshapeFunc(self.reshaped)
        glutKeyboardFunc(self.keydown)
        glutKeyboardUpFunc(self.keyup)
        glutSpecialFunc(self.specialdown)
        glutSpecialUpFunc(self.specialup)
        glutDisplayFunc(self.game_loop)
        glutIdleFunc(self.game_loop)
        glutMouseFunc(self.mouse_func)
        glutPassiveMotionFunc(self.mouse_move)
        glViewport(0,0,self.first_screen[0],self.first_screen[1]) #Creates the viewport which is mapped to the window
        glEnable(GL_BLEND) #Enable alpha blending
        glEnable(GL_TEXTURE_2D) #Enable 2D Textures
        glEnable(GL_POLYGON_SMOOTH) #Enable antialiased polygons
        glHint(GL_POLYGON_SMOOTH_HINT, GL_NICEST)
        glHint(GL_LINE_SMOOTH_HINT, GL_NICEST)
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
        glMatrixMode(GL_PROJECTION)
        glLoadIdentity() #Load the projection matrix
        gluOrtho2D(0,1280,720,0) #Set an orthorgraphic view

这段代码有点乱，因为我做了很多调整来让它工作，但还没有好好整理一下。

如果有人能帮我，我将非常感谢。

Answer 1

FBO的清除颜色的透明度（alpha）必须设置为0：

glColorMask(TRUE, TRUE, TRUE, TRUE);
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);

Answer 2

如果你想让帧缓冲区变得透明，这样就可以在桌面上渲染一个物体，你需要知道通过Glut是做不到的。

这种效果是特定于你所使用的平台的。你是在用什么？Linux？Windows？还是Mac OS X？你需要深入了解一下（放弃使用Glut），并多了解一些关于如何创建窗口的知识。

顺便说一下，如果你的目标是Windows，你应该看看这个：

(win32) 如何创建一个带透明背景的OpenGL渲染上下文？

Answer 3

我觉得这里对帧缓冲区有个基本的误解。帧缓冲区本身并不是一个数据的存储区，它并不保存任何数据。你需要把其他的缓冲区（比如纹理）连接到它上面，这样你就可以像在屏幕上绘图一样，在离屏缓冲区上进行渲染。所以当你说想要“用glClear和glClearColor填充一个帧缓冲区的颜色，包括透明度”，这其实不太对，因为帧缓冲区本身并不保存任何颜色数据。

当你用下面的代码把纹理连接到帧缓冲区时：

glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, surface.texture, 0)

你实际上是在把“surface.texture”设为帧缓冲区的渲染目标。换句话说，你就是在说“当我在这个帧缓冲区上绘图时，要绘制到这个纹理上。”