iOs OpenGL ES 2.0 adding textures with low opacity on device and on simulator -

i have problem multiple drawing of textures in program.

blending mode is

glblendfuncseparate(gl_src_alpha, gl_one_minus_src_alpha, gl_one, gl_one_minus_src_alpha); glblendequation(gl_func_add); 

the value of a-channel passed shader cpu-code.

precision highp float;  uniform sampler2d tshape;  uniform vec4      vcolor; uniform float     sopacity;  varying vec4 texcoords;\n"  void main() {     float = texture2d(tshape, texcoords.xy).x * sopacity;     gl_fragcolor = vec4(vcolor.rgb, a); } 

it's calculated with

o = pow(o, 1.3); 

for best visual effect.

i draw color (0; 0; 0) on black transparent canvas (0;0;0;0), low opacity:

0.03 -> 0.01048 0.06 -> 0.0258 0.09 -> 0.0437 0.12 -> 0.0635 ... 

i expect, maximal value of point's color (0;0;0;1) (black, no transparent) after multiple drawings on simulator:

but isn't on device:

do have ideas, why so?

update:

also manual blending works incorrect (and difference standard).

glblendfunc(gl_one, gl_zero); 

fragment shader code:

#extension gl_ext_shader_framebuffer_fetch : require precision highp float;  uniform sampler2d tshape;  uniform vec4      vcolor; uniform float     sopacity;  varying vec4 texcoords;  void main() {     float = texture2d(tshape, texcoords.xy).x * sopacity;     gl_fragcolor = vec4(vcolor.rgb * a, a) + (gl_lastfragdata[0] * (1.0 - a)); } 

result on simulator:

result on device:

i'm trying understand approach wrote down equations:

this how new drawing performed (if didn't make mistake):

color = {pencil_shape}*sourcealpha + {old_paint}*(1-sourcealpha) alpha = {pencil_shape} + {old_paint}*(1-sourcealpha) 

so alpha getting closer 1 on each frame, , color blended each time based on src alpha in *pencil_shape*.

questions:

• do intend use alpha in output image anything?
• is *pencil_shape* black (0, 0, 0, 0)? (besides cornes suppose has antialiasing effect)