Without touching the broader question of bitmap formats:
> On Mar 15, 2017, at 12:01 PM, Nevin Brackett-Rozinsky via swift-users
> <[email protected]> wrote:
>
> Also, for my specific application, I need to take an array of floating point
> numbers and say, “Hey, the underlying bits that make up the IEEE-754
> representation of these numbers are exactly the bits that I want to use for
> the colors of the pixels.” I do not know how to do that in Swift.
>
> In C I would malloc a buffer, write to it as (float*), then pass it to a
> function which takes (char*) and saves a PNG. Thus there is only one
> allocation, the buffer is filled with float values, and the exact bit-pattern
> is interpreted as RGBA pixels.
>
> How can I do the equivalent in Swift?
Create and fill an Array<Float> (actually, a ContiguousArray<Float> would be a
little better for this use) with your data. Then use `withUnsafeBytes` to
access the raw bytes as an `UnsafeRawBufferPointer`, a type which behaves sort
of like a fixed-size array of `UInt8`.
You could access the bytes one at at time by looping (or indexing, or doing
many other things):
myFloats.withUnsafeBytes { buffer in
for byte in buffer {
putchar(byte)
}
}
Or you can pull out the start pointer and count and use them:
myFloats.withUnsafeBytes { buffer in
guard write(fd, buffer.baseAddress, buffer.count) != -1 else {
throw MyError.IOError(errno) }
}
Or you can copy it into a `Data` or `Array` and return it to the outer context:
let bytes = myFloats.withUnsafeBytes { buffer in
return Data(buffer: buffer) // or Array(buffer)
}
One thing you should *not* do is hold on to `buffer` or its `baseAddress`
beyond the closing bracket. Once `withUnsafeBytes` returns, the `Array` or
`ContiguousArray` is free to move or delete its data, so you can no longer
depend on the pointer to be correct. Copy the data to an `Array` or `Data`, or
allocate and copy it to your own `UnsafeRawBufferPointer`, if you want to hold
on to it longer.
--
Brent Royal-Gordon
Architechies
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