Notes

in changes per vertex (or per pixel); uniform is one value shared by the whole draw call. Ask: does each vertex own a copy, or does the whole object share one?

One draw, six vertex shader runs

Run	vs_Pos (in)	u_Time (uniform)
1	(0.5, 0.0, 1)	3.2
2	(0.25, 0.43, 1)	3.2
…	different every run	same every run

C++ analogy: in is a function parameter, uniform is a global set before the calls.

Comparison

	in attribute	uniform
Shape	Array, one element per vertex	Single value
CPU sends via	Buffer + glVertexAttribPointer	glUniform*
Send how often	Upload once if geometry is static	Every time the value changes
Lookup	glGetAttribLocation (vertex shader only)	glGetUniformLocation
Typical	Position, color, normal, UV	Time, screen size, matrices, mouse

Where each kind lives

• Attribute in: vertex shader only. Fed from buffers.
• Uniform: whole program. Declared in both vert and frag → merged into one at link; set once, both read it. Types must match.
• vert out → frag in: matched by name and type at link, interpolated by hardware. CPU never touches these.
• frag out: written to the framebuffer. One output = location 0, nothing to set.

Location lookup

// init, once per program
attribs["vs_Pos"]  = glGetAttribLocation(prog, "vs_Pos");
uniforms["u_Time"] = glGetUniformLocation(prog, "u_Time");

// draw
GLint loc = uniforms.at("u_Time");

• Lookup is scoped to the program in the first argument. Same name in another program may have another number.
• Locations are fixed after link. Query once, cache in a name → location map.
• -1 means the name is wrong, or the variable is unused and the compiler removed it.
• Read with .at(): map["name"] inserts 0 when the key is missing, and 0 is a valid location.

Where to set uniforms

Changes when	Set in
Never	initializeGL
Window resize	resizeGL
Every frame	paintGL / draw function

A uniform keeps its last value until set again.

Related

OpenGL Hub · GLSL shader conventions · Barycentric interpolation · Associative containers