How to avoid black lines between triangles on modern GPUs? - opengl

Question

I am building a parametric 3d modeler with obj export.
I am really puzzled.
I have changed my GPU last night and now, there are cracks between the vertices, I can see what is behind. My old card was an Nvidia GTX275 and the one, NVIDIA GTX960. I didn't change anything in the code, shader or otherwise. I use only flat colors (not textures).
On the image below, the black lines that cut the pentagon into triangle shouldn't be there.
It seems to be purely an OpenGL problem as when I export the model and look in Blender, the faces are contiguous and there is no duplicate vertices.
the shader code is quite simple :
_VERTEX2 = """
#version 330
#extension GL_ARB_explicit_uniform_location : enable
layout(location = 0 ) in vec3 position;
layout(location = 1 ) in vec4 color;
layout(location = 2 ) in vec3 normal;
varying vec4 baseColor;
// uniform mat4 proj;
layout(location = 0) uniform mat4 view;
layout(location = 4) uniform mat4 proj;
varying vec3 fragVertexEc;
void main(void) {
gl_Position = proj * view * vec4(position, 1.0);
fragVertexEc = (view * vec4(position, 1.0)).xyz;
baseColor = color;
}
"""
_FRAGMENT2 = """
#version 330
#extension GL_OES_standard_derivatives : enable
varying vec3 fragVertexEc;
varying vec4 baseColor;
const vec3 lightPosEc = vec3(0,0,10);
const vec3 lightColor = vec3(1.0,1.0,1.0);
void main()
{
vec3 X = dFdx(fragVertexEc);
vec3 Y = dFdy(fragVertexEc);
vec3 normal=normalize(cross(X,Y));
vec3 lightDirection = normalize(lightPosEc - fragVertexEc);
float light = max(0.0, dot(lightDirection, normal));
gl_FragColor = vec4(normal, 1.0);
gl_FragColor = vec4(baseColor.xyz * light, baseColor.w);
}
"""
the rendering code is also quite simple :
def draw(self, view_transform, proj, transform):
self.shader.use()
gl_wrap.glBindVertexArray(self.vao)
try:
self.vbo.bind()
view_transform = view_transform * transform
GL.glUniformMatrix4fv(0, 1, False, (ctypes.c_float*16)(*view_transform.toList()))
GL.glUniformMatrix4fv(4, 1, False, (ctypes.c_float*16)(*proj.toList()))
GL.glEnableVertexAttribArray(self.shader.attrib['position'])
GL.glEnableVertexAttribArray(self.shader.attrib['color'])
GL.glEnableVertexAttribArray(self.shader.attrib['normal'])
STRIDE = 40
GL.glVertexAttribPointer(
self.shader.attrib['position'], len(Vector._fields), GL.GL_FLOAT,
False, STRIDE, self.vbo)
GL.glVertexAttribPointer(
self.shader.attrib['color'], len(Color._fields), GL.GL_FLOAT,
False, STRIDE, self.vbo+12)
GL.glVertexAttribPointer(
self.shader.attrib['normal'], len(Vector._fields), GL.GL_FLOAT,
False, STRIDE, self.vbo+28)
GL.glDrawElements(
GL.GL_TRIANGLES,
len(self.glindices),
self.index_type,
self.glindices)
finally:
self.vbo.unbind()
gl_wrap.glBindVertexArray(0)
self.shader.unuse()
A simple quad has the following data sent to OpenGL :
vertices ( flat array of pos+rgba+normal) :
[5.0, -3.061616997868383e-16, 5.0, 0.898, 0.0, 0.0, 1.0, 0.0, 1.0, 6.123233995736766e-17,
-5.0, 3.061616997868383e-16, -5.0, 0.898, 0.0, 0.0, 1.0, 0.0, 1.0, 6.123233995736766e-17,
-5.0, -3.061616997868383e-16, 5.0, 0.898, 0.0, 0.0, 1.0, 0.0, 1.0, 6.123233995736766e-17,
5.0, 3.061616997868383e-16, -5.0, 0.898, 0.0, 0.0, 1.0, 0.0, 1.0, 6.123233995736766e-17]
indices :: [0, 1, 2, 0, 3, 1]

Answer 1

ok, got it !
the strange display is due to glEnabled(GL_POLYGON_SMOOTH).
I had completely forgotten about it as my old card wasn't doing anything with this setting. It wasn't visible at least.
But the new one has the correct behaviour : it tries to anti-alias each triangle independently, hence the black lines.
Many thanks to #Jerem who helped me cover the other possibilities.