question about modify the shader in model.js

CesiumJS
1

1. A concise explanation of the problem you’re experiencing.

I was trying to modify the model shader in the example: http://localhost:8080/Apps/Sandcastle/index.html?src=3D%20Models.html&label=Showcases. I change the code in Model.js like this:

var tmpVS = ‘precision highp float; \n’ +

‘attribute vec3 a_position; \n’ +

‘attribute vec3 a_normal; \n’ +

‘varying vec3 v_normal; \n’ +

‘uniform mat3 u_normalMatrix; \n’ +

‘uniform mat4 u_modelViewMatrix; \n’ +

‘uniform mat4 u_projectionMatrix; \n’ +

‘attribute vec2 a_texcoord0; \n’ +

‘varying vec2 v_texcoord0; \n’ +

‘varying vec4 v_color; \n’ +

‘void main() { \n’ +

‘vec4 pos = u_modelViewMatrix * vec4(a_position,1.0); \n’ +

‘v_normal = u_normalMatrix * a_normal; \n’ +

‘v_texcoord0 = a_texcoord0; \n’ +

‘v_color = pos; \n’ +

‘pos.z = sin(5.0*pos.x)*0.25; \n’ +

‘gl_Position = u_projectionMatrix * pos; \n’ +

‘}’;

var tmpFS = ‘precision highp float; \n’ +

‘varying vec4 v_color; \n’ +

‘void main() { \n’ +

‘gl_FragColor = v_color; \n’ +

‘}’;

}

model._rendererResources.programs[id] = ShaderProgram.fromCache({

context : context,

vertexShaderSource : tmpVS,

fragmentShaderSource : tmpFS,

attributeLocations : attributeLocations

});

But the model “Cesium_Air” was invisible if I ran the code. I don’t know what’s the mistake in the code?

4. The Cesium version you’re using, your operating system and browser.

Cesium 1.41

2

pos.z is setting the model’s position in eye space, which looks into the -z direction. Either the result of that equation is a positive number, in which case the airplane won’t be visible, or the result is a small negative number, in which case the camera is “inside” the airplane but appears invisible because back faces are culled. If you set pos.z to something like -30.0 you should be able to see it.

3

Hi, Sean, thank you for your reply. I tried the value like -30.0, and the airplane was visible. If so, how could I flatten a 3D model in Cesium by setting its z coordinate to zero like this:

void main(void)
{
	vec4 v = vec4(gl_Vertex);
	v.z = 0.0;

	gl_Position = gl_ModelViewProjectionMatrix * v;
}

在 2018年2月3日星期六 UTC+8上午8:37:22，Sean Lilley写道：

4

Hi, Sean, I think I have got it by setting the a_position value. But I still don’t understand why was the airplane flattened by setting its y coordinate to zero?

在 2018年2月3日星期六 UTC+8上午8:37:22，Sean Lilley写道：

5

Cool that you got the effect you wanted.

That worked because the model’s up axis is Y, so scaling a_position.y is basically scaling the height of the original vertices.