Passing color to fragment shader from javascript

Question

I am currently learning webgl and have a question. I am trying to make a triangle and passing the color info into fragment shader from js file. The following is my js code.

var VSHADER_SOURCE = 
    'attribute vec4 a_Position;\n'+
    'attribute vec4 a_Color;\n'+
    'varying vec4 v_Color;\n'+
    'void main(){\n'+
        'gl_Position = a_Position;\n'+
        'v_Color = a_Color;\n'+
    '}\n';


var FSHADER_SOURCE = 
    'precision highp float;\n'+
    'varying vec4 v_Color;\n'+
    'void main() {\n'+
        'gl_FragColor = v_Color;\n'+
    '}\n';

function main(){
    var canvas = document.getElementById('webgl');
    var gl = getWebGLContext(canvas);
    if(!gl){
        console.log('Error!');
        return;
    }
    //Init shaders.
    if(!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)){
        console.log('Error!');
        return;
    }
    var vertices = new Float32Array([-0.8, -0.8, 0.8, -0.8, 0.0, 0.8]);
    var color = new Float32Array([0.0, 0.0, 1.0, 1.0]);
    var buffer_object = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, buffer_object);
    gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(a_Position);

    var color_object = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, color_object);
    gl.bufferData(gl.ARRAY_BUFFER, color, gl.STATIC_DRAW);
    var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
    gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(a_Color);

    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);
    gl.drawArrays(gl.TRIANGLES, 0, 3);


    return 0;
}

This have to create a blue triangle but the only thing I see is a canvas filled with black color. Can anyone tell me what's missing?? I created two buffer objects and used one for vertex and the other for color.

Answer 1

There's a lot of issues with your example but... specific problems.

You're only supplying colors for the first vertex.

You have 3 vertices but only 1 color. You should be getting an error for that. Did you check the JavaScript Console for errors?

You have 3 options to fix that

1. Provide a color for each vertex

   new Float32Array([
     0.0, 0.0, 1.0, 1.0,
     0.0, 0.0, 1.0, 1.0,
     0.0, 0.0, 1.0, 1.0,
   ]);
   

2. Turn off the a_Color attribute and supply a constant value

   gl.disableVertexAtttibArray(a_Color);
   gl.vertexAttrib4f(a_Color, 0, 0, 1, 1);
   

3. Use a uniform instead of an attribute + varying

   remove all references of a_Color and v_color and instead have your fragment shader be

   precision highp float;
   uniform vec4 u_Color;
   void main() {
     gl_FragColor = u_Color;
   }
   

   Now you'd set the color with

   At Init time

   // Lookup the location
   var u_colorLocation = gl.getUniformLocation(program, "u_Color");
   

   At render time

   // Set the uniform
   gl.uniform4f(u_colorLocation, 0, 0, 1, 1);
   

If you choose #2 you'll likely run into another issue that you'll get a warning that attirbute 0 is not enabled because, at least on my computer, a_Color is assigned to attribute 0. Turning it off means it has to be emulated which is slow. The solution is to make sure a_Position is in attribute 0 by calling gl.bindAttribLocation before linking the program.

Other issues:

Your initShader function is apparently creating a program and attaching it to the WebGLRenderContext (gl.program). Most WebGL projects have many shader programs so it would probably be best to just return the program. In other words, instead of

initShader(...);
gl.getAttribLocation(gl.program, ...)

You probably want

var program = initShader(...);
gl.getAttribLocation(program, ...)

You'll need to fix initShader so it returns the program that was created rather than hacking it on to the WebGLRenderingContext.

Also you're using precision highp float. That won't work on many phones. Unless you're sure you need highp it's better to use mediump.