opengl - Render to FBO + glReadPixels all black -

i trying render simple checkerboard in fbo , glreadpixels().

when without fbo, works fine. assume render function ok , glreadpixels(). fbo, lines draw after calls fbo have been done.

here code (python, aiming cross platform):

def renderfbo():     #whyyounoworking(gl_framebuffer) # degug function... error checking     glbindframebuffer( gl_draw_framebuffer, framebuffer)      glbindrenderbuffer( gl_renderbuffer, renderbuffera)     glrenderbufferstorage( gl_renderbuffer, gl_rgba, window.width, window.height)      glbindrenderbuffer( gl_renderbuffer, renderbufferb)     glrenderbufferstorage( gl_renderbuffer, gl_depth_component, window.width, window.height)          glbindframebuffer( gl_draw_framebuffer, framebuffer)     glframebufferrenderbuffer( gl_draw_framebuffer, gl_color_attachment0, gl_renderbuffer, renderbuffera)     glframebufferrenderbuffer( gl_draw_framebuffer, gl_depth_attachment, gl_renderbuffer, renderbufferb)      #whyyounoworking(gl_framebuffer)      gldrawbuffer(gl_color_attachment0)     glclear(gl_color_buffer_bit|gl_depth_buffer_bit)     glviewport( 0, 0, window.width, window.height)       drawchecker(nbr = 16, dark = 25.0/255, light = 75.0/255)     in range(len(labelsysinfo)):         pyglet.text.label(labelsysinfo[i], font_name='times new roman', font_size=26, x=(window.width*0.68), y= (window.height*0.04*i)+(window.height*2/3), anchor_x='left', anchor_y='center', color = (250, 250, 250, 150)).draw()      glreadpixels(0, 0, window.width, window.height, gl_rgba, gl_unsigned_byte, a)     glbindframebuffer( gl_framebuffer, 0) 

my other function:

def on_draw(dt):        gldrawbuffer(gl_back)     glclear(gl_color_buffer_bit)     glclearcolor( 0.0, 0.0, 0.0, 1.0)     glloadidentity()     glenable(gl_texture_2d)       gldisable(gl_texture_2d)     blueline() # draw simple line. works fine     dropframetest()   # draw simple line. works fine 

in main, call renderfbo() done once, , on_draw called periodically.

dt = pyglet.clock.tick() renderfbo()   pyglet.clock.schedule_interval(on_draw, 0.007) pyglet.app.run() 

at guess, you've bound framebuffer gl_draw_framebuffer only. use

glbindframebuffer(gl_framebuffer, ... 

and

glframebufferrenderbuffer(gl_framebuffer, ... 

to both read , write same fbo.

i'm sure have checking framebuffer completeness (glcheckframebufferstatus) , gl errors (glgeterror, or new extension) useful.

[edit]
(the shotgun problem solving tactics from comments)

if see image on first frame, none on next there must staying behind previous frame.

• the common problem forgetting clear depth buffer - haven't.
• next stencil buffers , blending (neither they're enabled begin with).
• maybe new fbo handle being generated each frame , you're running out?
• another common problem accumulating matrix transforms, have glloadidentity should no issue there.