modify attribute vec2 variable - html

Question

In my vertex shader I would love to modify attribute vec2 a_position variable that is shared in fragment shader. By this modifycation I should get image into cylindrical projection.
This is what I'm doing in my shaders:
<!-- vertex shader -->
<script id="2d-vertex-shader" type="x-shader/x-vertex">
attribute vec2 a_position;
uniform vec2 u_resolution;
uniform mat3 u_matrix;
varying vec2 v_texCoord;
void main() {
// modifying START
float angle = atan(a_position.y, a_position.x);
float r = sqrt(a_position.x*a_position.x + a_position.y*a_position.y);
a_position.x = r*cos(angle);
a_position.y = r*sin(angle);
// modifying STOP
gl_Position = vec4(u_matrix * vec3(a_position, 1), 1);
v_texCoord = a_position;
}
</script>
<!-- fragment shader -->
<script id="2d-fragment-shader" type="x-shader/x-fragment">
precision mediump float;
// our texture
uniform sampler2D u_image;
// the texCoords passed in from the vertex shader.
varying vec2 v_texCoord;
void main() {
gl_FragColor = texture2D(u_image, v_texCoord);
}
</script>
But I'm getting this error:
compiling shader '[object WebGLShader]':ERROR: 0:12: 'assign' : l-value required "a_position" (can't modify an attribute)
ERROR: 0:13: 'assign' : l-value required "a_position" (can't modify an attribute)
Don't you have any idea how to fix that?

Answer 1

Just use another variable
attribute vec2 a_position;
uniform vec2 u_resolution;
uniform mat3 u_matrix;
varying vec2 v_texCoord;
void main() {
// modifying START
float angle = atan(a_position.y, a_position.x);
float r = sqrt(a_position.x*a_position.x + a_position.y*a_position.y);
vec3 p = vec3(
r*cos(angle),
r*sin(angle),
1);
// modifying STOP
gl_Position = vec4(u_matrix * p, 1);
v_texCoord = a_position;
}