104 lines
3.6 KiB
Swift
104 lines
3.6 KiB
Swift
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import Foundation
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/// Minimal dependency-free PNG encoder: 8-bit truecolor, stored (uncompressed)
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/// deflate blocks inside a valid zlib stream. Bigger files than real deflate,
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/// but universally decodable — browsers, ffmpeg, Python all accept it.
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/// Purpose: shot-report thumbnails + still grabs without an image library.
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public enum PNGWriter {
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// MARK: CRC32 (PNG chunk checksums)
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private static let crcTable: [UInt32] = {
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(0..<256).map { n -> UInt32 in
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var c = UInt32(n)
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for _ in 0..<8 {
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c = (c & 1) != 0 ? 0xEDB88320 ^ (c >> 1) : c >> 1
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}
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return c
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}
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}()
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static func crc32(_ data: Data) -> UInt32 {
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var c: UInt32 = 0xFFFFFFFF
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for byte in data {
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c = crcTable[Int((c ^ UInt32(byte)) & 0xFF)] ^ (c >> 8)
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}
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return c ^ 0xFFFFFFFF
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}
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// MARK: Adler32 (zlib checksum)
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static func adler32(_ data: Data) -> UInt32 {
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var a: UInt32 = 1, b: UInt32 = 0
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for byte in data {
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a = (a + UInt32(byte)) % 65521
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b = (b + a) % 65521
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}
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return (b << 16) | a
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}
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private static func putU32BE(_ v: UInt32, into data: inout Data) {
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data.append(UInt8((v >> 24) & 0xFF))
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data.append(UInt8((v >> 16) & 0xFF))
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data.append(UInt8((v >> 8) & 0xFF))
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data.append(UInt8(v & 0xFF))
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}
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private static func chunk(_ type: String, _ payload: Data) -> Data {
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var out = Data()
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putU32BE(UInt32(payload.count), into: &out)
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var typed = Data(type.utf8)
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typed.append(payload)
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out.append(typed)
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putU32BE(crc32(typed), into: &out)
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return out
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}
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/// zlib stream with stored deflate blocks (BTYPE=00).
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static func zlibStored(_ raw: Data) -> Data {
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var out = Data([0x78, 0x01]) // CMF/FLG: 32K window, no preset, check bits valid
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var offset = 0
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let maxBlock = 65535
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while offset < raw.count {
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let size = min(maxBlock, raw.count - offset)
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let isLast: UInt8 = (offset + size >= raw.count) ? 1 : 0
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out.append(isLast) // BFINAL + BTYPE=00
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out.append(UInt8(size & 0xFF))
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out.append(UInt8((size >> 8) & 0xFF))
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out.append(UInt8(~size & 0xFF))
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out.append(UInt8((~size >> 8) & 0xFF))
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out.append(raw.subdata(in: raw.startIndex + offset..<raw.startIndex + offset + size))
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offset += size
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}
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putU32BE(adler32(raw), into: &out)
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return out
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}
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/// Encode frame as 8-bit RGB PNG.
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public static func encode(_ frame: VideoFrame) -> Data {
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var png = Data([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])
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// IHDR
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var ihdr = Data()
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putU32BE(UInt32(frame.width), into: &ihdr)
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putU32BE(UInt32(frame.height), into: &ihdr)
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ihdr.append(contentsOf: [8, 2, 0, 0, 0]) // depth 8, truecolor, deflate, adaptive, no interlace
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png.append(chunk("IHDR", ihdr))
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// Scanlines: filter byte 0 + RGB triples.
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var raw = Data(capacity: frame.height * (1 + frame.width * 3))
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for y in 0..<frame.height {
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raw.append(0)
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for x in 0..<frame.width {
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let px = frame.pixels[y * frame.width + x]
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raw.append(UInt8(min(max(px.x, 0), 1) * 255 + 0.5))
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raw.append(UInt8(min(max(px.y, 0), 1) * 255 + 0.5))
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raw.append(UInt8(min(max(px.z, 0), 1) * 255 + 0.5))
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}
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}
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png.append(chunk("IDAT", zlibStored(raw)))
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png.append(chunk("IEND", Data()))
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return png
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}
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}
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