Calculate point projection in GLSL shader - opengl

Question

I need to calculate projection of point on specific line segment in shader (OpenGL ES 2).
Here is how I test the algorithm:
I draw simple triangle with points A(0, 0.5), B(1, -0.5), C(-1, -0.5).
I calculate projection of every point on line segment AC.
I draw points with a projection in the middle of a line segment AC in blue. And the remaining points in green.
I expect to get a green triangle with a blue line perpendicular to the side AC. But blue line is not perpendicular to AC.
I check projection formula in code with drawing on canvas and got expected result.
What's my mistake?
Result of shader:
Vertex shader:
uniform mat4 matrix;
attribute vec4 position;
varying vec4 vPosition;
void main()
{
vPosition = matrix * position;
gl_Position = matrix * position;
}
Fragment shader:
precision mediump float;
varying vec4 vPosition;
void main()
{
vec2 P = vPosition.xy;
vec2 A = vec2(0.0, 0.5);
vec2 B = vec2(-1.0, -0.5);
vec2 AP = P - A;
vec2 AB = B - A;
vec2 projection = A + dot(AP, AB) / dot(AB, AB) * AB;
if(projection.x > -0.51 && projection.x < -0.49 && projection.y > -0.01 && projection.y < 0.01) {
gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);
} else {
gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
}
}

Answer 1

You didn't consider the rectangular aspect of the of the window. When the normalized device coordinates in the range [-1, 1] are mapped to the viewport rectangle (see glViewport) then the triangle gets stretched. This causes that angles of 90 degree are not maintained.
Add a uniform variable to the fragment shader which contains the width and height of the viewport:
uniform vec2 u_resolution;
Calculate the aspect ratio:
float aspect = u_resolution.x / u_resolution.y;
Of course you can initialize the variable float aspect, by a constant value, too.
e.g. float aspect = 16.0/9.0;
Correct the coordinates of the points A, B and P according to the aspect ratio:
vec2 P = vPosition.xy;
vec2 A = vec2(0.0, 0.5);
vec2 B = vec2(-1.0, -0.5);
A.x *= aspect;
B.x *= aspect;
P.x *= aspect;
And consider the aspect ration when evaluating the result projection:
vec2 projection = A + dot(AP, AB) / dot(AB, AB) * AB;
projection.x /= aspect;
The final fragment shader may look like this:
precision mediump float;
varying vec4 vPosition;
uniform vec2 u_resolution;
void main()
{
float aspect = u_resolution.x / u_resolution.y;
vec2 as = vec2(aspect, 1.0);
vec2 P = as * vPosition.xy;
vec2 A = as * vec2(0.0, 0.5);
vec2 B = as * vec2(-1.0, -0.5);
vec2 AP = P - A;
vec2 AB = B - A;
vec2 projection = A + dot(AP, AB) / dot(AB, AB) * AB / as;
if(projection.x > -0.51 && projection.x < -0.49 && projection.y > -0.01 && projection.y < 0.01) {
gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);
} else {
gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
}
}