Image coordinates in opengl behaves strange

Question

I'm trying to render an video from OpenCV using OpenGL with the following vertices and indices:

static const GLint ImageIndices[] {
    0, 1, 2,
    0, 2, 3
};

static const GLfloat ImageVertices[] = {
    // positions   // texCoords
    -1.0f,  1.0f,  0.0f, 1.0f,
    -1.0f, -1.0f,  0.0f, 0.0f,
     1.0f, -1.0f,  1.0f, 0.0f,
     1.0f,  1.0f,  1.0f, 1.0f
};

and following vertex and fragment shader:

#version 330 core
layout(location = 0) in vec2 vert_pos;
layout(location = 1) in vec2 tex_pos;

uniform mat3 trans1;
uniform mat3 trans2;

out vec2 texPos;

void main()
{
    vec3 pos = vec3(-vert_pos.y, vert_pos.x, 0.0f);
    vec3 rst;
    if(pos.y < 0.0f)
    {
        rst = pos;
        texPos = tex_pos;
    }
    else if(pos.y > 0.0f)
    {
        rst = pos;
        texPos = tex_pos;
    }
    gl_Position = vec4(rst.x, rst.y, 0.0f, 1.0f);
    //texPos = tex_pos;
}

#version 330 core

in vec2 texPos;

out vec4 fragColor;

uniform sampler2D tex;

uniform float width;
uniform float height;

void main()
{
    fragColor = texture(tex, texPos);
}

and everything works well:

However, since I want to rotate the image using different matrix on the top and the bottom part, I changed the vertex shader to debug the coordinates of the image where texPos is vec2(1.0f, 1.0f) when pos.y > 0.0f:

#version 330 core
layout(location = 0) in vec2 vert_pos;
layout(location = 1) in vec2 tex_pos;

uniform mat3 trans1;
uniform mat3 trans2;

out vec2 texPos;

void main()
{
    vec3 pos = vec3(-vert_pos.y, vert_pos.x, 0.0f);
    vec3 rst;
    if(pos.y < 0.0f)
    {
        rst = pos;
        texPos = tex_pos;
    }
    else if(pos.y > 0.0f)
    {
        rst = pos;
        texPos = vec2(1.0f, 1.0f);
    }
    gl_Position = vec4(rst.x, rst.y, 0.0f, 1.0f);
    //texPos = tex_pos;
}

and the output of the video is strange:

Why the video turned out to be like this and how can I fix it?

Answer 1

The vertex shader is executed per vertex, not per fragment. It is executed 6 times for the 6 vertices of the 2 triangles. You have changed the texture coordinates of the 3 vertices where pos.y > 0.0f

Since pos = vec3(-vert_pos.y, vert_pos.x, 0.0)) you have changed the texture coordinates of the vertices, where x > 0.0:

 x  y   u  v          x  y   u  v
-1  1   0  1    ->   -1  1   0  1
-1 -1   0  0    ->   -1 -1   0  0
 1 -1   1  0    ->    1 -1   1  1
 1  1   1  1    ->    1  1   1  1

Actually only the texture coordinate of the vertex attribute with index 2 has changed. Hence, just the 1st triangle is effected: