Why do this yourself, when QC has core image blend modes to do all of this for
you? Make a CVOpenGLTextureRef or more ideally, publish an image output from
your source comps and then use a 3rd comp to do arbitrary mixing. Caveat: all
the QCRenderers should be on the same thread if you use an image output /
QCImage due to API contractual obligations.
Otherwise, with a texture attachment from various FBO's, assuming the
CGLContext objects are shared, you should be able to call a GL Flush on both
threads (to ensure the FBO's are in sync, and then pass the GLuint texture ID
to a thread that does the compositing. Syncing may be problematic, and
honestly, im not 100% up to speed on the nuances. For sanity, get it working in
a single thread 1st.
Additionally "Blend" means a lot of different things. Do you want a simple
source over? Do you want Porter / Duff blend modes? Do you want a mixer amount
to animate between a and b channels?
You can just draw two quads, one on top of the other, and use basic OpenGL
blend mode blending to do alpha blending.
glEnable(GL_BLEND)
glColor4f(1.0, 1.0, 1.0, 1.0);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
// bind texture channel a
// draw quad one
glColor4f(1.0, 1.0, 1.0, mix_amount);
//bind texture channel b
// draw quad 2
You can get more fancy and use GLSL shaders and bind both textures at once via
multitexturing, and render to a quad however you want.
On Apr 15, 2012, at 5:17 AM, Nisar Ahmed wrote:
> Dear list,
>
> How do I blend 2 textures rendered by 2 different compositions, these
> compositions are rendered in separate FBOs in separate threads.
>
> I have initialized FBO in the following code
>
> #import <OpenGL/CGLMacro.h>
> int createFBO(GLuint* fbo, GLuint* color, GLuint* depth, uint32_t width,
> uint32_t height, CGLContextObj cgl_ctx)
> {
> // texture / color attachment
> glGenTextures(1, color);
> glEnable(GL_TEXTURE_2D);
> glBindTexture(GL_TEXTURE_2D, *color);
>
> glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
> glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
> glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
> glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_RGBA,
> GL_UNSIGNED_BYTE, NULL);
> glBindTexture(GL_TEXTURE_2D, 0);
>
> /*
> // depth buffer
> glGenRenderbuffersEXT(1, depth);
> glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, *depth);
> glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, width,
> height);
> glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, 0);
> */
> // FBO and connect attachments
> glGenFramebuffersEXT(1, fbo);
> glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, *fbo);
> glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0,
> GL_TEXTURE_2D, *color, 0);
> //glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT,
> GL_RENDERBUFFER_EXT, *depth);
> // Draw black so we have output if the renderer isn't loaded
> glClearColor(0.0, 0.0, 0.0, 0.0);
> glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
>
> glViewport(0, 0, width, height);
>
> glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
>
> GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
> if(status != GL_FRAMEBUFFER_COMPLETE)
> {
> //NSLog(@"Simple Client: OpenGL error %04X", status);
> glDeleteTextures(1, color);
> glDeleteFramebuffersEXT(1, fbo);
> glDeleteRenderbuffersEXT(1, depth);
> *color = 0;
> *fbo = 0;
> *depth = 0;
> return 0;
> }
> return 1;
> }
> The compositions are rendered in this FBO and the textures returned are
> rendered in a separate rendering loop
> In my rendering loop I am enabling
>
> glEnable(GL_DEPTH_TEST);
> glEnable(GL_BLEND);
> glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
>
> before binding and rendering each texture into a quad.
>
> Tanks
> NIsar
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