ios - pass a 4x4 matrix to glUniformMatrix4fv

问题

I'm trying to pass a 4x4 matrix to glUniformMatrix4fv but can't figure out the last bit. I create a 4x4 by directly inputing 16 values. glUniformMatrix4fv excepts an UnsafePointer<GLfloat>! as its last argument

var proj = GLKMatrix4(m: (
    -1.1269710063934326,
    0.0,
    -1.380141455272968e-16,
    0.0,
    0.0,
    0.800000011920929,
    0.0,
    0.0,
    0.0,
    -0.0,
    0.0,
    -4.950000286102295,
    -1.2246468525851679e-16,
    0.0,
    1.0,
    5.050000190734863)
)
var loc = GLint(_locations.uniforms.projection)
var f = GLboolean(GL_FALSE)

First try:

glUniformMatrix4fv(loc, 1, f, proj.m)

raises

Cannot convert value of type '(Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float)' to expected argument type 'UnsafePointer<GLfloat>!'

second try:

glUniformMatrix4fv(loc, 1, f, &proj.m)

raises

Cannot convert value of type '(Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float)' to expected argument type 'GLfloat' (aka 'Float')

third try

glUniformMatrix4fv(loc, 1, f, &proj)

raises

Cannot convert value of type 'GLKMatrix4' (aka '_GLKMatrix4') to expected argument type 'GLfloat' (aka 'Float')

and finally

glUniformMatrix4fv(loc, 1, f, proj)

raises

Cannot convert value of type 'GLKMatrix4' (aka '_GLKMatrix4') to expected argument type 'UnsafePointer<GLfloat>!'

Any idea?

回答1:

The "problem" is that C arrays like the

float m[16];

in struct _GLKMatrix4 are mapped to Swift as a tuple:

public var m: (Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float, Float)

but the glUniformMatrix4fv() function expects UnsafePointer<GLfloat> (float * in C) as the last argument.

In C, an array "decays" to a pointer to the first element when passed to a function, but not in Swift. But Swift preserves the memory layout of imported C structures, therefore you can pass a pointer to the tuple, converted to a pointer to GLfloat:

// Swift 2:
withUnsafePointer(&proj.m) {
    glUniformMatrix4fv(loc, 1, f, UnsafePointer($0))
}

// Swift 3/4:
let components = MemoryLayout.size(ofValue: proj.m)/MemoryLayout.size(ofValue: proj.m.0)
withUnsafePointer(to: &proj.m) {
    $0.withMemoryRebound(to: GLfloat.self, capacity: components) {
        glUniformMatrix4fv(loc, 1, f, $0)
    }
}

来源：https://stackoverflow.com/questions/38261094/ios-pass-a-4x4-matrix-to-gluniformmatrix4fv

标签

ios

swift

opengl-es

shader