从存储在 SQLite 中的字符串重新转换时 UIImage 颠倒

Question

我从模拟器照片库中保存了一张图片。

然后我将图像保存为字符串，当用户单击保存时，它会从字符串重新转换为图像并放置在另一个控制器的表格视图中。出于某种原因，只有某些库存照片被颠倒了。

图片转字符串代码：

extension UIImage
{
    func toString() -> String?
    {
        let data: Data? = self.pngData()
        return data?.base64EncodedString(options: .lineLength64Characters)
    }
}

将字符串重新转换回图像的代码：

extension String
{
    func toImage() -> UIImage?
    {
        if let data = Data(base64Encoded: self, options: .ignoreUnknownCharacters)
        {
            return UIImage(data: data)
        }
    
        return nil
   }
}

转换是基于另一个 StackOverflow 页面的推导：Convert between UIImage and Base64 string

整个“家庭”的数据库存储代码：

    /**
INSERT operation prepared statement for family

- Parameters
    - family: Contains the data for each child and parents' name and image
*/
func insertFamily(family: Family)
{
    var insertStatement: OpaquePointer? = nil
    let result = sqlite3_prepare_v2(self.db, self.insertFamilyQuery, -1, &insertStatement, nil)
    
    if result == SQLITE_OK
    {
        // Bind values to insert statement
        sqlite3_bind_text(insertStatement, 1, (family.childName! as NSString).utf8String, -1, nil)
        sqlite3_bind_text(insertStatement, 2, (family.childImage! as NSString).utf8String, -1, nil)
        sqlite3_bind_text(insertStatement, 3, (family.parentOneName! as NSString).utf8String, -1, nil)
        sqlite3_bind_text(insertStatement, 4, (family.parentOneImage! as NSString).utf8String, -1, nil)
        sqlite3_bind_text(insertStatement, 5, (family.parentTwoName! as NSString).utf8String, -1, nil)
        sqlite3_bind_text(insertStatement, 6, (family.parentTwoImage! as NSString).utf8String, -1, nil)
        
        // Execute statement
        let result = sqlite3_step(insertStatement)
        sqlite3_finalize(insertStatement)   // Destroy statement to avoid memory leak
        
        if result == SQLITE_DONE
        {
            os_log("Inserted family for %s", log: self.oslog, type: .info, family.childName!)
        }
        else
        {
            os_log("Could not insert family", log: self.oslog, type: .error)
            os_log("Expected: %s Received: %s", log: self.oslog, type: .error, SQLITE_DONE, result)
        }
    }
    else 
    {
        os_log("INSERT statement could not be prepared", log: self.oslog, type: .error)
        os_log("Expected: %s Received: %s", log: self.oslog, type: .error, SQLITE_DONE, result)
    }
}

整个“家庭”的数据库读取代码：

    /**
Pull the family for the given childName if it exists

- Parameters
    - childName: The child's name associated with the family

- Returns: The family found
*/
func pullFamily(childName: String) -> Family?
{
    var family = Family()
    var readStatement: OpaquePointer? = nil
    let selectStatement = self.pullFamilyQuery + childName + "'"
    
    // Read
    if sqlite3_prepare_v2(self.db, selectStatement, -1, &readStatement, nil) == SQLITE_OK
    {
        if sqlite3_step(readStatement) == SQLITE_ROW
        {
            let childName = String(describing: String(cString: sqlite3_column_text(readStatement, 0)))
            let childImage = String(describing: String(cString: sqlite3_column_text(readStatement, 1)))
            let parentOneName = String(describing: String(cString: sqlite3_column_text(readStatement, 2)))
            let parentOneImage = String(describing: String(cString: sqlite3_column_text(readStatement, 3)))
            let parentTwoName = String(describing: String(cString: sqlite3_column_text(readStatement, 4)))
            let parentTwoImage = String(describing: String(cString: sqlite3_column_text(readStatement, 5)))
            
            family = Family(childName: childName, 
                                childImage: childImage, 
                                parentOneName: parentOneName, 
                                parentOneImage: parentOneImage, 
                                parentTwoName: parentTwoName, 
                                parentTwoImage: parentTwoImage)
        }
    }
    else 
    {
        os_log("Could not pull family by child name ", log: self.oslog, type: .error, childName)
    }
    
    sqlite3_finalize(readStatement) // Destroy statement to avoid memory leak
    return family
}

家庭初始化器

init(childName: String?, childImage: UIImage?,
     parentOneName: String?, parentOneImage: UIImage?,
     parentTwoName: String?, parentTwoImage: UIImage?)
    {
        self.childName = childName
        self.childImage = childImage
        self.parentOneName = parentOneName
        self.parentOneImage = parentOneImage
        self.parentTwoName = parentTwoName
        self.parentTwoImage = parentTwoImage
    }

Blob 写入的测试代码

// Child Image
result = (family.childImage!.unrotatedPngData()?.withUnsafeBytes { bufferPointer -> Int32 in
        sqlite3_bind_blob(insertStatement, 1, bufferPointer.baseAddress, Int32(bufferPointer.count), SQLITE_TRANSIENT) })!

guard result == SQLITE_OK else {
    logger.error("[\(#function); \(#line)] ERROR: Could not bind child image")
    logger.error("[\(#function); \(#line)] ERROR: Expected - \(SQLITE_OK), Received - \(result)")
    logger.error("[\(#function); \(#line)] [ERROR: \(self.getDatabaseError())")
    return

Blob 读取测试代码

while sqlite3_step(readStatement) == SQLITE_ROW
{
    guard
        let childName = sqlite3_column_text(readStatement, 0).flatMap({ String(cString: $0) }),
        let childImageBlob = sqlite3_column_blob(readStatement, 1)
        //let parentOneName = sqlite3_column_text(readStatement, 2).flatMap({ String(cString: $0) }),
        //let parentOneImageBlob = sqlite3_column_blob(readStatement, 3),
        //let parentTwoName = sqlite3_column_text(readStatement, 4).flatMap({ String(cString: $0) }),
        //let parentTwoImageBlob = sqlite3_column_blob(readStatement, 5)
    else {
        logger.error("[\(#function); \(#line)] Could not pull family")
        logger.error("\(String(cString: sqlite3_errmsg(self.db)))")
        return families
    }
            
    // Convert child image data to child image
    let childData = Data(bytes: childImageBlob, count: Int(sqlite3_column_bytes(readStatement, 1)))
    guard let childImage = UIImage(data: childData) else {
        logger.error("Could not convert child image for child name \(childName)")
        return families
    }

    ...
}

Answer 1

问题不在于字符串的 base-64 编码（也不在于数据库）。问题是pngData 不保留图像方向。如果我用手机在纵向模式下拍照，结果UIImage 的imageOrientation 是UIImage.Orientation.right，如果我在横向模式下拍摄，音量按钮在顶部，方向是.down，等等. 如果我直接使用那些图像，或者抓住他们的jpgData，他们就很好。但是，如果我使用 pngData 为这些图像提取 Data，则生成的资源将分别错误地旋转 90˚ 或 180˚。

所以你有几个选择：

1. 您可以使用jpgData，它确实尊重图像的方向。虽然像 0.7 或 0.8 这样的压缩值在适当的压缩下看起来相当不错，但请注意这是一种有损格式。

2. 您可以在提取pngData 之前重新渲染图像（必要时）。例如。这是一个 UIImage 扩展来检索未旋转的 PNG：

   extension UIImage {
       func unrotatedPngData() -> Data? {
           if imageOrientation == .up {
               return pngData()
           }
   
           let format = UIGraphicsImageRendererFormat()
           format.scale = scale
           return UIGraphicsImageRenderer(size: size, format: format).image { _ in
               draw(at: .zero)
           }.pngData()
       }
   }
   

应该注意，还有其他更隐蔽的方式可能会导致图像方向混乱，但pngData 可能是这里的罪魁祸首。如果你想测试这篇论文，试试jpgData，如果可行，确实是pngData的问题。但是，如果jpgData 也不起作用，那么问题就出现在您的管道中，但我们需要see that code 才能进一步诊断它。但聪明的钱在pngData。无论如何，base-64 编码和数据库都不是问题的根源。

---

话虽如此，我不建议将 base-64 编码图像存储在数据库中。这会更慢，并将已保存资产的大小增加 ⅓。我只是将图像保存为 BLOB：

var statement: OpaquePointer?

var result = sqlite3_prepare(db, "INSERT INTO images (filename, image) VALUES (?, ?)", -1, &statement, nil)

guard result == SQLITE_OK else {
    printError(in: db)
    return
}

defer { sqlite3_finalize(statement) }

result = sqlite3_bind_text(statement, 1, filename, -1, SQLITE_TRANSIENT)

guard result == SQLITE_OK else {
    printError(in: db)
    return
}

result = image.unrotatedPngData()?.withUnsafeBytes { bufferPointer -> Int32 in
    result = sqlite3_bind_blob(statement, 2, bufferPointer.baseAddress, Int32(bufferPointer.count), SQLITE_TRANSIENT)
}

guard result == SQLITE_OK else {
    printError(in: db)
    return
}

result = sqlite3_step(statement)

guard result == SQLITE_DONE else {
    printError(in: db)
    return
}

您也可以将图像作为 BLOB 获取：

var statement: OpaquePointer?

let result = sqlite3_prepare(db, "SELECT filename, image FROM images", -1, &statement, nil)
guard result == SQLITE_OK else { ... }
defer { sqlite3_finalize(statement) }

while sqlite3_step(statement) == SQLITE_ROW {
    guard
        let filename = sqlite3_column_text(statement, 0).flatMap({ String(cString: $0) }),
        let bytes = sqlite3_column_blob(statement, 1)
    else {
        printError(in: db)
        return
    }

    let data = Data(bytes: bytes, count: Int(sqlite3_column_bytes(statement, 1)))
    guard let image = UIImage(data: data) else { ... }

    // do whatever you want with the image
}

在哪里

func printError() {
    let message = sqlite3_errmsg(db).flatMap { String(cString: $0) } ?? "Unknown error"
    print(message)
}