ios - glUniform. Fragment shader fails to compile with uniform or varying compares -

i've created subclass of glkviewcontroller render waveforms using glkbaseeffect works fine.

i want use own vertex , fragment shaders, used boilerplate code default opengl game project. both shaders compile, link, , usable. can alter color in vertex shader without issue.

the problem i'm having passing own uniforms fragment shader. can pass vertex shader , use there, when declare same variable in

code:

// vertex shader

attribute vec4 position; uniform float amplitudemax;  void main() {     gl_position = position + amplitudemax; // works , offsets drawing } 

// fragment shader

uniform float amplitudemax; // fails compile //uniform float amplitudemax; // compiles fine  void main() {     gl_fragcolor = vec4(0.0, 0.0, 1.0, 1.0); } 

if curious, here how set uniforms , shaders

func loadshaders() -> bool {      // copied default opengl game project      // link program ...      uniforms[uniform_amplitude_max] = glgetuniformlocation(program, "amplitudemax")      // release shaders ...  }  // draw loop func render() {      configureshaders()      // ... draw }  func configureshaders() {      gluseprogram(program)      let max = audiomanager.sharedinstance.buffermanager.currentpower)     gluniform1f(uniforms[uniform_amplitude_max], max)  } 

i had idea of passing value vertex shader fragment shader using varying float. again can declare , use variable in vertex shader delaring in fragment shader cause fail compiling.

edit: --------------------------------------------------

through trial , error found if (in fragment shader) qualify uniform declaration precision works both uniforms , varying (can pass vertex shader).

uniform lowp float amplitudemax;  void main() {     gl_fragcolor = vec4(amplitudemax, 0.0, 1.0, 1.0); }