android - Draw to framebuffer object, then blit to primary display (GLES 3.0) -

i attempting create framebuffer object, , blit primary display. purpose of to cache screen shot can blit display whenever want without re-rendering object. using open gles 3.0 android ndk.

i created frame buffer in normal way.

gluint framebuffer = null; glgenframebuffers( 1, &framebuffer ); glbindframebuffer( gl_framebuffer, framebuffer );  gluint colorrenderbuffer = null;         glgenrenderbuffers( 1, &colorrenderbuffer ); glbindrenderbuffer( gl_renderbuffer, colorrenderbuffer ); glrenderbufferstorage( gl_renderbuffer, gl_rgba8, engine->width, engine->height ); glframebufferrenderbuffer( gl_framebuffer, gl_color_attachment0, gl_renderbuffer, colorrenderbuffer );  gluint depthrenderbuffer = null; glgenrenderbuffers( 1, &depthrenderbuffer ); glbindrenderbuffer( gl_renderbuffer, depthrenderbuffer ); glrenderbufferstorage( gl_renderbuffer, gl_depth_component16, engine->width, engine->height ); glframebufferrenderbuffer( gl_framebuffer, gl_depth_attachment, gl_renderbuffer, depthrenderbuffer );  glenum status = glcheckframebufferstatus( gl_framebuffer ); if (status != gl_framebuffer_complete) {     logi( "failed make complete framebuffer object %d", status ); } else {     logi( "frame buffer complete." ); } 

this works without error. able edit frame buffer, successfully.

glbindframebuffer( gl_draw_framebuffer, framebuffer ); glbindframebuffer( gl_read_framebuffer, framebuffer );  // fill screen color. glclearcolor( red, green, blue, 255 ); glclear( gl_color_buffer_bit );  read_pixels( ); //wrapper glreadpixels. shows pixels in frame buffer object have desired color. 

however, attempting blit main draw buffer fails.

glbindframebuffer( gl_draw_framebuffer, 0 ); glbindframebuffer( gl_read_framebuffer, framebuffer );  glreadbuffer( gl_color_attachment0 );  glblitframebuffer( 0, 0, engine->width, engine->height, 0, 0, engine->width, engine->height, gl_color_buffer_bit, gl_linear );  logi("gl error after blit: %d", glgeterror()); //no error reported  glbindframebuffer( gl_draw_framebuffer, 0 ); glbindframebuffer( gl_read_framebuffer, 0 );  read_pixels( engine ); //gives zero'd buffer (wrong result)  eglswapbuffers( engine->display, engine->surface ); //shows black screen, unless use glclearcolor directly on primary display buffer. 

i able around problem using texture (instead of render buffer) frame buffer object, drawing frame buffer, , drawing frame buffer texture default buffer. isn't answer, work around, i'd still appreciate if has answer original question.

for reference, include key code below:

gluint loadframebuffertexture( escontext *context, int width, int height ) {      glgenframebuffers( 1, &(context->framebuffer) );     glbindframebuffer( gl_framebuffer, context->framebuffer );      // create texture     gluint texture;     glgentextures( 1, &texture );     glbindtexture( gl_texture_2d, texture );     gltexparameteri( gl_texture_2d, gl_texture_min_filter, gl_linear );     glteximage2d( gl_texture_2d, 0, gl_rgba, width, height, 0, gl_rgba, gl_unsigned_byte, null );     glframebuffertexture2d( gl_framebuffer, gl_color_attachment0, gl_texture_2d, texture, 0 );      gluint depthrenderbuffer;     glgenrenderbuffers( 1, &depthrenderbuffer );     glbindrenderbuffer( gl_renderbuffer, depthrenderbuffer );     glrenderbufferstorage( gl_renderbuffer, gl_depth_component16, width, height );     glframebufferrenderbuffer( gl_framebuffer, gl_depth_attachment, gl_renderbuffer, depthrenderbuffer );       glenum status = glcheckframebufferstatus( gl_framebuffer );     if (status != gl_framebuffer_complete) {         logi("failed make complete framebuffer object ");     }   else {         logi("frame buffer complete.");     }      glbindframebuffer( gl_framebuffer, 0 );      return texture; }  void draw_frame(int red, int green, int blue) {     glbindframebuffer( gl_framebuffer, escontext->framebuffer );     glclearcolor( red, green, blue, 255 );     glclear( gl_color_buffer_bit );      glbindframebuffer(gl_framebuffer, 0 );     drawframe( escontext );      eglswapbuffers( engine->display, engine->surface ); }     void drawframe( escontext *escontext ) {     userdata *userdata = (userdata *)escontext->userdata;     glfloat vvertices[] = {-1.0f, 1.0f, 0.0f, 0.0f,  1.0f, -1.0f, -1.0f, 0.0f, 0.0f,  0.0f, 1.0f, -1.0f, 0.0f, 1.0f,  0.0f, 1.0f,  1.0f, 0.0f, 1.0f, 1.0f };     glushort indices[] = { 0, 1, 2, 0, 2, 3 };     glviewport( 0, 0, escontext->width, escontext->height );     gluseprogram( userdata->programobject );     glvertexattribpointer( 0, 3, gl_float, gl_false, 5 * sizeof( glfloat ), vvertices );     glvertexattribpointer( 1, 2, gl_float, gl_false, 5 * sizeof( glfloat ), &vvertices[3] );     glenablevertexattribarray( 0 );     glenablevertexattribarray( 1 );     glactivetexture( gl_texture0 );     glbindtexture( gl_texture_2d, userdata->frametexid );     gluniform1i( userdata->basemaploc, 0 );      gldrawelements( gl_triangles, 6, gl_unsigned_short, indices ); }