Question

我有一个应用程序，允许用户拍照。拍摄完照片后，用户可以将其发送到我的网络服务器。但在我这样做之前，它需要调整位图的大小，因为我希望将一致的大小发送到我的网络服务器。

无论如何，我用来将位图加载到内存然后操作它的代码似乎占用了大量内存。目前正在使用此代码：

    /*
     *  This method is used to calculate image size.
     *  And also resize/scale image down to 1600 x 1200
     */
    private void ResizeBitmapAndSendToWebServer(string album_id) {

        Bitmap bm = null;
                    // This line is taking up to much memory each time..
        Bitmap bitmap = MediaStore.Images.Media.GetBitmap(Android.App.Application.Context.ApplicationContext.ContentResolver,fileUri);

                    /*
                     * My question is : Could i do the next image manipulation
                     * before i even load the bitmap into memory?
                     */
        int width = bitmap.Width;
        int height = bitmap.Height;

        if (width >= height) { // <-- Landscape picture

            float scaledWidth = (float)height / width;

            if (width > 1600) {
                bm = Bitmap.CreateScaledBitmap (bitmap, 1600, (int)(1600 * scaledWidth), true);
            } else {
                bm = bitmap;
            }
        } else {

            float scaledHeight = (float)width / height;

            if (height > 1600) {
                bm = Bitmap.CreateScaledBitmap (bitmap, (int)(1600 * scaledHeight), 1600 , true);
            } else {
                bm = bitmap;
            }

        }
                    // End of question code block.

        MemoryStream stream = new MemoryStream ();
        bitmap.Compress (Bitmap.CompressFormat.Jpeg, 80, stream);
        byte[] bitmapData = stream.ToArray ();
        bitmap.Dispose ();

        app.api.SendPhoto (Base64.EncodeToString (bitmapData, Base64Flags.Default), album_id);

    }

解决此类记忆问题的好方法是什么？

编辑1：

在阅读其他帖子后，我清楚地知道我正在使用我的代码做一些低效的事情。这是我一直在做的步骤：

将完整位图加载到内存中。
决定它是否是风景。
然后使用正确的尺寸创建新的位图。
然后将此位图转换为字节数组
处理初始位图。（但永远不要删除内存中的缩放位图）。

我真正应该做的事情：

确定实际的位图尺寸，而不将其加载到内存中：

private void FancyMethodForDeterminingImageDimensions() {

    BitmapFactory.Options options = new BitmapFactory.Options();
    options.InJustDecodeBounds = true;

    BitmapFactory.DecodeFile(fileUri.Path, options);

    // Now the dimensions of the bitmap are known without loading
    // the bitmap into memory.
    // I am not further going to explain this, i think the purpose is
    // explaining enough.
    int outWidth = options.OutWidth;
    int outHeight = options.OutHeight;

}

如果设置为true，解码器将返回null（无位图），但是 out ...字段仍将设置，允许调用者查询位图，而不必为其像素分配内存。

现在我知道了真正的尺寸。所以我可以在将其加载到内存之前对其进行下采样。</ li>
（在我的例子中）将位图转换为base64字符串并发送它。
处理所有内容，以便清除内存。

我目前无法测试这个，因为我不在我的开发机器上。如果这是正确的方法，任何人都可以给我一些反馈吗？我们将不胜感激。

Answer 1

        private void ResizeBitmapAndSendToWebServer(string album_id) {

            BitmapFactory.Options options = new BitmapFactory.Options ();
            options.InJustDecodeBounds = true; // <-- This makes sure bitmap is not loaded into memory.
            // Then get the properties of the bitmap
            BitmapFactory.DecodeFile (fileUri.Path, options);
            Android.Util.Log.Debug ("[BITMAP]" , string.Format("Original width : {0}, and height : {1}", options.OutWidth, options.OutHeight) );
            // CalculateInSampleSize calculates the right aspect ratio for the picture and then calculate
            // the factor where it will be downsampled with.
            options.InSampleSize = CalculateInSampleSize (options, 1600, 1200);
            Android.Util.Log.Debug ("[BITMAP]" , string.Format("Downsampling factor : {0}", CalculateInSampleSize (options, 1600, 1200)) );
            // Now that we know the downsampling factor, the right sized bitmap is loaded into memory.
            // So we set the InJustDecodeBounds to false because we now know the exact dimensions.
            options.InJustDecodeBounds = false;
            // Now we are loading it with the correct options. And saving precious memory.
            Bitmap bm = BitmapFactory.DecodeFile (fileUri.Path, options);
            Android.Util.Log.Debug ("[BITMAP]" , string.Format("Downsampled width : {0}, and height : {1}", bm.Width, bm.Height) );
            // Convert it to Base64 by first converting the bitmap to
            // a byte array. Then convert the byte array to a Base64 String.
            MemoryStream stream = new MemoryStream ();
            bm.Compress (Bitmap.CompressFormat.Jpeg, 80, stream);
            byte[] bitmapData = stream.ToArray ();
            bm.Dispose ();

            app.api.SendPhoto (Base64.EncodeToString (bitmapData, Base64Flags.Default), album_id);

        }

用于android的单声道内存高效位图处理

1 个答案: