how to draw a semi-transparent triangle to back-buffer then render it?

I'm trying to draw a line with semi-transparent color to an offscreen framebuffer, use glCopyTexSubImage2D to copy it into a texture, then draw that texture to onscreen framebuffer. I tried many configuration but only got an opaque line.

For more information, this is how I setup my OpenGL:

Firstly, I subclass EAGLView, then add an offscreen framebuffer:

glGenFramebuffersOES(1, &offscreenFramebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreenFramebuffer);

glGenRenderbuffersOES(1, &offscreenRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, offscreenRenderbuffer);

glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, kTextureOriginalSize, kTextureOriginalSize);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, offscreenRenderbuffer); 

glGenRenderbuffersOES(1, &offscreenDepthBuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, offscreenDepthBuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, kTextureOriginalSize, kTextureOriginalSize);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, offscreenDepthBuffer);

// Offscreen framebuffer texture target
glGenTextures(1, &offscreenRenderTexture);
glBindTexture(GL_TEXTURE_2D, offscreenRenderTexture);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); 
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); 

unsigned char * data = (unsigned char *)malloc( kTextureOriginalSize * kTextureO开发者_StackOverflow中文版riginalSize * 4 ); 
memset( data,0xff, kTextureSizeWidth * kTextureSizeHeight * 4 );
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, kTextureOriginalSize, kTextureOriginalSize, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);    

glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, offscreenRenderTexture, 0);

glEnable( GL_TEXTURE_2D);
glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glEnableClientState(GL_VERTEX_ARRAY);
glEnable(GL_BLEND);
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );    

And copy data from offscreen framebuffer to texture

glBindTexture( GL_TEXTURE_2D, offscreenRenderTexture);
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, kTextureOriginalSize, kTextureOriginalSize);

I used glColor4f to change the alpha of color before drawing.

Thanks for your help!

I found the answer:

• First, I need to set alpha to very small: like 0.1 to see the effect.

• Second, glBlendFunc() has effect only on active framebuffer, so I need to call glBlendFunc() twice for both offscreen and onscreen framebuffer. Or actually, what I do is disable GL_BLEND when rendering the texture to onscreen framebuffer.