opengl - Transform feedback without a framebuffer? -

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i'm interested in using vertex shader process buffer without producing rendered output. here's relevant snippet:

gluseprogram(program);  gluint tfoutputbuffer; glgenbuffers(1, &tfoutputbuffer); glbindbuffer(gl_array_buffer, tfoutputbuffer); glbufferdata(gl_array_buffer, sizeof(double)*4*3, null, gl_static_read);  glenable(gl_rasterizer_discard_ext);  glbindbufferbase(gl_transform_feedback_buffer, 0, tfoutputbuffer); glbegintransformfeedbackext(gl_triangles);  glbindbuffer(gl_array_buffer, positionbuffer); glenablevertexattribarray(positionattribute); glvertexattribpointer(positionattribute, 4, gl_float, gl_false, sizeof(double)*4, 0);  glbindbuffer(gl_element_array_buffer, elementbuffer);  gldrawelements(gl_triangles, 1, gl_unsigned_int, 0);

this works fine until gldrawelements() call, results in gl_invalid_framebuffer_operation. , glcheckframebufferstatusext(gl_framebuffer_ext); returns gl_framebuffer_undefined.

i presume because gl context not have default framebuffer, , have not bound fbo. but, since don't care rendered output , i've enabled gl_rasterizer_discard_ext, thought framebuffer shouldn't necessary.

so, there way use transform feedback without framebuffer, or need generate , bind framebuffer though don't care contents?

this valid behavior, as-per specification.

opengl 4.4 core specification - 9.4.4 effects of framebuffer completeness on framebuffer operations

a gl_invalid_framebuffer_operation error generated attempts render or read framebuffer is not framebuffer complete. error generated regardless of whether fragments read or written framebuffer. example, generated when rendering command called , framebuffer incomplete, even if gl_rasterizer_discard enabled.

what need work around create fbo 1 pixel color attachment , bind that. must have complete fbo bound or gl_invalid_framebuffer_operation , 1 of rules completeness @ least 1 complete image attached.

opengl 4.3 allows skirt around issue defining fbo no attachments of sort (see: gl_arb_framebuffer_no_attachments). however, because using ext form of fbos , transform feedback, doubt have 4.3 implementation.


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