OpenGL jobs - shaders

One 、 In the vertex shader , Invert a triangle

#version 330 core
layout (location = 0) in vec3 aPos;
out vec4 ourcolor;
void main()
{
    gl_Position = vec4(aPos.x, -aPos.y, aPos.z, 1.0);
    ourcolor=vec4(1.0, 0,0.2, 1.0);
}

Two 、 Use uniform Define a horizontal offset , Use this offset in the vertex shader to move the triangle to the right of the screen

#version 330 core
layout (location = 0) in vec3 aPos;
out vec4 ourcolor;
uniform float Pianyi_x;
void main()
{
    gl_Position = vec4(aPos.x+Pianyi_x, aPos.y, aPos.z, 1.0);
    ourcolor=vec4(1.0, 0,0.2, 1.0);
}

int vertexColorLocation = glGetUniformLocation(myshader.ID, "Pianyi_x");
myshader.use();
glUniform1f(vertexColorLocation,0.7f);

3、 ... and Use out Keyword to output vertex positions to fragment shaders , And set the color of the clip to be equal to the vertex position （ Let's look at the result that even the vertex position values are interpolated in the triangle ）. After that , Try to answer the following questions ： Why is it black in the lower left corner of the triangle ?

I made a triangle

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