alpha blending looking wrong/different on iPhone/Pad - iphone

Question

I'm drawing some alpha blended sprites. I have the same assets rendering on Win32 using D3D and OpenGL, and the correct result looks like this when the vertex color is 0xFFFFFF and the background colour is 0xC0D0E0
On Win32/OpenGL I enable blending as follows:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
and on Win32/D3D as follows:
CD3D11_BLEND_DESC blendDesc(D3D11_DEFAULT);
D3D11_RENDER_TARGET_BLEND_DESC rtBlendDesc;
rtBlendDesc.BlendEnable = true;
rtBlendDesc.BlendOp = D3D11_BLEND_OP_ADD;
rtBlendDesc.BlendOpAlpha = D3D11_BLEND_OP_ADD;
rtBlendDesc.DestBlend = D3D11_BLEND_INV_SRC_ALPHA;
rtBlendDesc.DestBlendAlpha = D3D11_BLEND_ZERO;
rtBlendDesc.RenderTargetWriteMask = D3D11_COLOR_WRITE_ENABLE_ALL;
rtBlendDesc.SrcBlend = D3D11_BLEND_SRC_ALPHA;
rtBlendDesc.SrcBlendAlpha = D3D11_BLEND_ZERO;
blendDesc.RenderTarget[0] = rtBlendDesc;
gDevice->CreateBlendState(&blendDesc, &mBlendState);
then, at render time:
gContext->OMSetBlendState(mBlendState, NULL, 0xffffffff);
On Win32/OpenGL I use the fixed function pipeline, and on DX11 I use very simple vert/pixel shaders:
float4 SpritePixelShader(PS_INPUT input) : SV_Target
{
return (txDiffuse.Sample(samLinear, input.Tex)) * input.Col;
}
PS_INPUT SpriteVertexShader(VS_INPUT input)
{
PS_INPUT output = (PS_INPUT)0;
output.Pos = mul(float4(input.Pos.x, input.Pos.y, 0.5, 1.0), projection);
output.Tex = input.Tex;
output.Col = input.Col;
return output;
}
All well and good. OpenGL and D3D give me identical results.
On OpenGL ES 2.0 on the iPhone simulator (not sure if this is a simulator thing or not, I can't test on a device yet):
I enable blending similarly:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
My fragment/vertex shaders are similar:
// fragment shader
void main()
{
gl_FragColor = color * texture2D(s_texture, texCoord);
}
// vertex shader
void main()
{
color = vertexColor;
texCoord = vertexTexCoord;
gl_Position = matrix * vec4(vertexPosition, 0, 1);
}
But I get this:
Which seems to be using modulate rather than additive. I've tried calling:
glBlendEquation(GL_FUNC_ADD);
but that doesn't seem to do anything, nor does
glBlendEquationOES(GL_FUNC_ADD_OES);
If I use:
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
then it looks right but the alpha component of the vertex colors is ignored (and I need it to be used).
Does anyone have any ideas?
Thanks!
Charlie.