I know gl_FragDepth will take the value of gl_FragCoord.z from opengl wiki.

https://www.khronos.org/opengl/wiki/Fragment_Shader/Defined_Outputs

But I have a problem. If I enable MSAA and write gl_FragDepth = gl_FragCoord.z in fragment shader, the display will not work fine. You can see a black line on the white triangle.

If I don't write gl_FragDepth in fragment shader, it will work fine.

If I disalbe MSAA, it also work fine no matter if I write gl_FragDepth.

The correct display image is no black line:

The render scene is easy, I just draw 2 white triangles and they are intersected on an edge.

I add a simple light in vertex shader. The codes show as below:

const char *vertexShaderSource[] = {

"#version 120\n",

"varying vec4 lightColor;\n",

"void main()\n",

"{\n",

" vec3 n = normalize(gl_NormalMatrix * gl_Normal);\n",

" vec3 l = normalize(vec3(0.0, 1.0, 1.0));\n",

" float NdotL = clamp(dot(n, l), 0.001, 1.0);\n",

" lightColor = vec4(1.0)*(NdotL + 0.2);\n",

" gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;\n",

"}\n"

};

const char *fragmentShaderSource[] = {

"#version 120\n",

"varying vec4 lightColor;\n",

"void main(void)\n",

"{\n",

" gl_FragColor = vec4(lightColor.rgb, 1.0);\n",

" gl_FragDepth = gl_FragCoord.z;\n"

"}\n"

};