Why does golang gomobile basic example sets 3-float size for a vec4 attribute?

Question

Golang gomobile basic example [1] uses VertexAttribPointer to set 3 x FLOATS per vertex.

However the vertex shader attribute type is vec4. Shouldn't it be vec3?

Why?

Within render loop:

glctx.VertexAttribPointer(position, coordsPerVertex, gl.FLOAT, false, 0, 0)

Triangle data:

var triangleData = f32.Bytes(binary.LittleEndian,
    0.0, 0.4, 0.0, // top left
    0.0, 0.0, 0.0, // bottom left
    0.4, 0.0, 0.0, // bottom right
)

Constant declaration:

const (
    coordsPerVertex = 3
    vertexCount     = 3
)

In vertex shader:

attribute vec4 position;

[1] gomobile basic example: https://github.com/golang/mobile/blob/master/example/basic/main.go

derhass derhass · Accepted Answer · 2017-06-28T20:21:33

Vertex attributes are conceptually always 4 component vectors. There is no requirement that the number of components you use in the shader and the one you set up for the attribute pointer have to match. If your array has more components than your shader consumes, the additional components are just ignored. If your array supplies less components, the attribute is filled to a vector of the form (0,0,0,1) (which makes sense for homogeneous position vectors as well as RGBA colors).

In the usual case, you want w=1 for every input position anyway, there is no need to store that in an array. But you usually need the full 4D form when applying the transformation matrices (or even when directly forwarding the value as gl_Position). So your shader could conceptually do

in vec3 pos;
gl_Position=vec4(pos,1);

but that would be equivalent of just writing

in vec4 pos;
gl_Position=pos;

Why does golang gomobile basic example sets 3-float size for a vec4 attribute?

1 Answers