ios - 将非常大的图像缩小到 MTLTexture
<p><p>我关注了<a href="https://developer.apple.com/library/content/samplecode/LargeImageDownsizing/Introduction/Intro.html" rel="noreferrer noopener nofollow">this example </a>从 Apple 缩小一个非常大的图像我是 <a href="https://stackoverflow.com/questions/42459312/load-a-remote-image-using-mtktextureloader" rel="noreferrer noopener nofollow">downloading from a remote location</a> .我已经用 Swift 重写了代码。它显然有效，但是当我调用 <code>MTKTextureLoader.newTexture</code> 时，应用程序会因 <code>_loadCGImage</code> 中的 <code>EXC_BAD_ACCESS</code> 而崩溃。没有其他提示，但我怀疑图像数据已经发布或什么...</p>
<p>任何提示为什么它会在没有任何正确错误消息的情况下崩溃？</p>
<p>这是顶层代码，</p>
<pre><code>// this is an extension of MTKTextureLoader
// [...]
if let uiImage = UIImage(contentsOfFile: cachedFileURL.path) {
let maxDim : CGFloat = 8192
if uiImage.size.width > maxDim || uiImage.size.height > maxDim {
let scale = uiImage.size.width > maxDim ? maxDim / uiImage.size.width : maxDim / uiImage.size.height
if let cgImage = MTKTextureLoader.downsize(image: uiImage, scale: scale) {
return self.newTexture(with: cgImage, options: options, completionHandler: completionHandler)
} else {
anError = TextureError.CouldNotDownsample
}
} else {
return self.newTexture(withContentsOf: cachedFileURL, options: options, completionHandler: completionHandler)
}
}
</code></pre>
<p>这是缩小尺寸的方法，</p>
<pre><code>private static func downsize(image: UIImage, scale: CGFloat) -> CGImage? {
let destResolution = CGSize(width: Int(image.size.width * scale), height: Int(image.size.height * scale))
let kSourceImageTileSizeMB : CGFloat = 40.0 // The tile size will be (x)MB of uncompressed image data
let pixelsPerMB = 262144
let tileTotalPixels = kSourceImageTileSizeMB * CGFloat(pixelsPerMB)
let destSeemOverlap : CGFloat = 2.0 // the numbers of pixels to overlap the seems where tiles meet.
let colorSpace = CGColorSpaceCreateDeviceRGB()
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)
guard let destContext = CGContext(data: nil, width: Int(destResolution.width), height: Int(destResolution.height), bitsPerComponent: 8, bytesPerRow: 0, space: colorSpace, bitmapInfo: bitmapInfo.rawValue) else {
NSLog("failed to create the output bitmap context!")
return nil
}
var sourceTile = CGRect()
sourceTile.size.width = image.size.width
sourceTile.size.height = floor( tileTotalPixels / sourceTile.size.width )
print("source tile size: \(sourceTile.size)")
sourceTile.origin.x = 0.0
// the output tile is the same proportions as the input tile, but
// scaled to image scale.
var destTile = CGRect()
destTile.size.width = destResolution.width
destTile.size.height = sourceTile.size.height * scale
destTile.origin.x = 0.0
print("dest tile size: \(destTile.size)")
// the source seem overlap is proportionate to the destination seem overlap.
// this is the amount of pixels to overlap each tile as we assemble the output image.
let sourceSeemOverlap : CGFloat = floor( ( destSeemOverlap / destResolution.height ) * image.size.height )
print("dest seem overlap: \(destSeemOverlap), source seem overlap: \(sourceSeemOverlap)")
// calculate the number of read/write operations required to assemble the
// output image.
var iterations = Int( image.size.height / sourceTile.size.height )
// if tile height doesn't divide the image height evenly, add another iteration
// to account for the remaining pixels.
let remainder = Int(image.size.height) % Int(sourceTile.size.height)
if remainder > 0 {
iterations += 1
}
// add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.
let sourceTileHeightMinusOverlap = sourceTile.size.height
sourceTile.size.height += sourceSeemOverlap
destTile.size.height += destSeemOverlap
print("beginning downsize. iterations: \(iterations), tile height: \(sourceTile.size.height), remainder height: \(remainder)")
for y in 0..<iterations {
// create an autorelease pool to catch calls to -autorelease made within the downsize loop.
autoreleasepool {
print("iteration \(y+1) of \(iterations)")
sourceTile.origin.y = CGFloat(y) * sourceTileHeightMinusOverlap + sourceSeemOverlap
destTile.origin.y = ( destResolution.height ) - ( CGFloat( y + 1 ) * sourceTileHeightMinusOverlap * scale + destSeemOverlap )
// create a reference to the source image with its context clipped to the argument rect.
if let sourceTileImage = image.cgImage?.cropping( to: sourceTile ) {
// if this is the last tile, its size may be smaller than the source tile height.
// adjust the dest tile size to account for that difference.
ify == iterations - 1 && remainder > 0 {
var dify = destTile.size.height
destTile.size.height = CGFloat( sourceTileImage.height ) * scale
dify -= destTile.size.height
destTile.origin.y += dify
}
// read and write a tile sized portion of pixels from the input image to the output image.
destContext.draw(sourceTileImage, in: destTile, byTiling: false)
}
// !!! In the original LargeImageDownsizing code, it released the source image here !!!
// ;
// !!! I assume I don't need to do that in Swift?? !!!
/* while CGImageCreateWithImageInRect lazily loads just the image data defined by the argument rect,
that data is finally decoded from disk to mem when CGContextDrawImage is called. sourceTileImageRef
maintains internally a reference to the original image, and that original image both, houses and
caches that portion of decoded mem. Thus the following call to release the source image. */
// http://en.swifter.tips/autoreleasepool/
// drain will be called
// to free all objects that were sent -autorelease within the scope of this loop.
}
// !!! Original code reallocated the image here !!!
// we reallocate the source image after the pool is drained since UIImage -imageNamed
// returns us an autoreleased object.
}
print("downsize complete.")
// create a CGImage from the offscreen image context
return destContext.makeImage()
}
</code></pre>
<p>编辑:</p>
<p>每次我尝试用 <code>CGImage</code> 初始化 <code>MTLTexture</code> 时它都会崩溃，即使图像很小并且适合内存。所以它似乎与调整大小无关......这段代码也崩溃了，</p>
<pre><code>func newTexture(with uiImage: UIImage, options: ? = nil, completionHandler: @escaping MTKTextureLoaderCallback) {
if let cgImage = uiImage.cgImage {
return self.newTexture(with: cgImage, options: options, completionHandler: completionHandler)
} else {
completionHandler(nil, TextureError.CouldNotBeCreated)
}
}
</code></pre>
<p>在 ImageIO <code>copyImageBlockSetWithOptions</code>.</p>
<p>编辑2:
基于 warrenm 的回答的解决方法:调用 <code>newTexture(with: cgImage)</code> 同步而不是异步。比如上面的函数变成了，</p>
<pre><code>func newTexture(with uiImage: UIImage, options: ? = nil, completionHandler: MTKTextureLoaderCallback) {
if let cgImage = uiImage.cgImage {
let tex = try? self.newTexture(with: cgImage, options: options)
completionHandler(tex, tex == nil ? TextureError.CouldNotBeCreated : nil)
} else {
completionHandler(nil, TextureError.CouldNotGetCGImage)
}
}
</code></pre>
<p>(请注意，我已经删除了 <code>@escaping</code>，因为现在我实际上调用了同步方法。)</p>
<p>缩小代码是正确的。现在可以使用此解决方法:)</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>这似乎是用于创建纹理的 <code>CGImage</code> 生命周期的问题，并且可能是 MetalKit 中的错误。</p>
<p>从本质上讲，上下文返回的 <code>CGImage</code> 只保证存在到创建它的自动释放池范围的末尾。当你调用异步 <code>newTexture</code> 方法时，<code>MTKTextureLoader</code> 将创建纹理的工作移到后台线程上，并在 <code>CGImage</code> 外部进行操作在其后备存储已被释放后，其封闭自动释放池的范围。</p>
<p>您可以通过在完成处理程序中捕获图像(这将导致 ARC 延长其生命周期)或使用相应的同步纹理创建方法 <code>newTexture(with:options:) 来解决此问题</code>，在纹理完全初始化之前，它将保持在相关范围内。</p></p>
<p style="font-size: 20px;">关于ios - 将非常大的图像缩小到 MTLTexture，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/42567140/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/42567140/
</a>
</p>
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