Android - 读取没有alpha的PNG图像并解码为ARGB_8888

Question

我尝试从sdcard(在模拟器中)读取图像,然后用.创建一个Bitmap图像

BitmapFactory.decodeByteArray

方法.我设置了选项:

options.inPrefferedConfig = Bitmap.Config.ARGB_8888
options.inDither = false

然后我将像素提取到ByteBuffer中.

ByteBuffer buffer = ByteBuffer.allocateDirect(width*height*4)
bitmap.copyPixelsToBuffer(buffer)

我在JNI中使用此ByteBuffer将其转换为RGB格式并想要计算它.

但总是我得到错误的数据 - 我在不修改ByteBuffer的情况下进行测试.我唯一要做的就是将它放入JNI的本机方法中.然后将其转换为a unsigned char*并将其转换回a,ByteBuffer然后将其返回到Java.

unsigned char* buffer = (unsinged char*)(env->GetDirectBufferAddress(byteBuffer))
jobject returnByteBuffer = env->NewDirectByteBuffer(buffer, length)

在显示图像之前,我得到了数据

bitmap.copyPixelsFromBuffer( buffer )

但其中有错误的数据.

我的问题是,如果这是因为图像内部转换为RGB 565或这里有什么问题？

.....

得到答案:

- >>>是的,它在内部转换为RGB565.

有人知道如何使用ARGB8888像素格式从PNG创建这样的位图图像吗？

如果有人有想法,那就太好了!

Answer 1

ARGB_8888位图(在Honeycomb前版本上)本身以RGBA格式存储.所以alpha通道最后移动了.在本地访问Bitmap的像素时,您应该考虑到这一点.

我假设您正在为低于3.2的Android版本(API级别<12)编写代码,因为从那时起方法的行为

BitmapFactory.decodeFile(pathToImage);
BitmapFactory.decodeFile(pathToImage, opt);
bitmapObject.createScaledBitmap(bitmap, desiredWidth, desiredHeight, false /*filter?*/);

已经改变.

在较旧的平台(API级别<12)上,BitmapFactory.decodeFile(..)方法在默认情况下尝试返回带有RGB_565配置的Bitmap,如果它们找不到任何alpha,这会降低iamge的质量.这仍然可以,因为您可以使用强制执行A​​RGB_8888位图

options.inPrefferedConfig = Bitmap.Config.ARGB_8888
options.inDither = false 

当图像的每个像素的alpha值为255(即完全不透明)时,会出现真正的问题.在这种情况下,即使您的Bitmap具有ARGB_8888配置,Bitmap的标志'hasAlpha'也会设置为false.如果您的*.png文件至少有一个真正的透明像素,则此标志将设置为true,您不必担心任何事情.

因此,当您想要使用创建缩放的位图时

bitmapObject.createScaledBitmap(bitmap, desiredWidth, desiredHeight, false /*filter?*/);

该方法检查'hasAlpha'标志是否设置为true或false,并且在您的情况下将其设置为false,这将导致获得缩放的Bitmap,该Bitmap自动转换为RGB_565格式.

因此,在API级别> = 12时,会有一个名为的公共方法

public void setHasAlpha (boolean hasAlpha);

哪个会解决这个问题.到目前为止,这只是对问题的解释.我做了一些研究,发现setHasAlpha方法已经存在了很长时间并且它是公开的,但是已被隐藏(@hide注释).以下是它在Android 2.3上的定义:

/**
 * Tell the bitmap if all of the pixels are known to be opaque (false)
 * or if some of the pixels may contain non-opaque alpha values (true).
 * Note, for some configs (e.g. RGB_565) this call is ignore, since it does
 * not support per-pixel alpha values.
 *
 * This is meant as a drawing hint, as in some cases a bitmap that is known
 * to be opaque can take a faster drawing case than one that may have
 * non-opaque per-pixel alpha values.
 *
 * @hide
 */
public void setHasAlpha(boolean hasAlpha) {
    nativeSetHasAlpha(mNativeBitmap, hasAlpha);
}

现在这是我的解决方案提案.它不涉及任何位图数据的复制:

1. 如果当前Bitmap实现具有公共'setHasAplha'方法,则在运行时使用java.lang.Reflect进行检查.(根据我的测试,它完全适用于API级别3,我没有测试过较低版本,因为JNI不起作用).如果制造商明确将其设为私有,受保护或删除,则可能会出现问题.

2. 使用JNI为给定的Bitmap对象调用'setHasAlpha'方法.即使对于私有方法或字段,这也可以完美地工作.官方认为JNI不会检查您是否违反了访问控制规则.来源:http://java.sun.com/docs/books/jni/html/pitfalls.html(10.9)这给了我们很大的力量,应该明智地使用.我不会尝试修改最终字段,即使它可以工作(只是举个例子).请注意这只是一个解决方法......

这是我对所有必要方法的实现:

JAVA PART:

// NOTE: this cannot be used in switch statements
    private static final boolean SETHASALPHA_EXISTS = setHasAlphaExists();

    private static boolean setHasAlphaExists() {
        // get all puplic Methods of the class Bitmap
        java.lang.reflect.Method[] methods = Bitmap.class.getMethods();
        // search for a method called 'setHasAlpha'
        for(int i=0; i<methods.length; i++) {
            if(methods[i].getName().contains("setHasAlpha")) {
                Log.i(TAG, "method setHasAlpha was found");
                return true;
            }
        }
        Log.i(TAG, "couldn't find method setHasAlpha");
        return false;
    }

    private static void setHasAlpha(Bitmap bitmap, boolean value) {
        if(bitmap.hasAlpha() == value) {
            Log.i(TAG, "bitmap.hasAlpha() == value -> do nothing");
            return;
        }

        if(!SETHASALPHA_EXISTS) {   // if we can't find it then API level MUST be lower than 12
            // couldn't find the setHasAlpha-method
            // <-- provide alternative here...
            return;
        }

        // using android.os.Build.VERSION.SDK to support API level 3 and above
        // use android.os.Build.VERSION.SDK_INT to support API level 4 and above
        if(Integer.valueOf(android.os.Build.VERSION.SDK) <= 11) {
            Log.i(TAG, "BEFORE: bitmap.hasAlpha() == " + bitmap.hasAlpha());
            Log.i(TAG, "trying to set hasAplha to true");
            int result = setHasAlphaNative(bitmap, value);
            Log.i(TAG, "AFTER: bitmap.hasAlpha() == " + bitmap.hasAlpha());

            if(result == -1) {
                Log.e(TAG, "Unable to access bitmap."); // usually due to a bug in the own code
                return;
            }
        } else {    //API level >= 12
            bitmap.setHasAlpha(true);
        }
    }

    /**
     * Decodes a Bitmap from the SD card
     * and scales it if necessary
     */
    public Bitmap decodeBitmapFromFile(String pathToImage, int pixels_limit) {
        Bitmap bitmap;

        Options opt = new Options();
        opt.inDither = false;   //important
        opt.inPreferredConfig = Bitmap.Config.ARGB_8888;
        bitmap = BitmapFactory.decodeFile(pathToImage, opt);

        if(bitmap == null) {
            Log.e(TAG, "unable to decode bitmap");
            return null;
        }

        setHasAlpha(bitmap, true);  // if necessary

        int numOfPixels = bitmap.getWidth() * bitmap.getHeight();

        if(numOfPixels > pixels_limit) {    //image needs to be scaled down 
            // ensures that the scaled image uses the maximum of the pixel_limit while keeping the original aspect ratio
            // i use: private static final int pixels_limit = 1280*960; //1,3 Megapixel
            imageScaleFactor = Math.sqrt((double) pixels_limit / (double) numOfPixels);
            Bitmap scaledBitmap = Bitmap.createScaledBitmap(bitmap,
                    (int) (imageScaleFactor * bitmap.getWidth()), (int) (imageScaleFactor * bitmap.getHeight()), false);

            bitmap.recycle();
            bitmap = scaledBitmap;

            Log.i(TAG, "scaled bitmap config: " + bitmap.getConfig().toString());
            Log.i(TAG, "pixels_limit = " + pixels_limit);
            Log.i(TAG, "scaled_numOfpixels = " + scaledBitmap.getWidth()*scaledBitmap.getHeight());

            setHasAlpha(bitmap, true); // if necessary
        }

        return bitmap;
    }

加载lib并声明本机方法:

static {
    System.loadLibrary("bitmaputils");
}

private static native int setHasAlphaNative(Bitmap bitmap, boolean value);

原生部分('jni'文件夹)

Android.mk:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := bitmaputils
LOCAL_SRC_FILES := bitmap_utils.c
LOCAL_LDLIBS := -llog -ljnigraphics -lz -ldl -lgcc
include $(BUILD_SHARED_LIBRARY)

bitmapUtils.c:

#include <jni.h>
#include <android/bitmap.h>
#include <android/log.h>

#define  LOG_TAG    "BitmapTest"
#define  Log_i(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define  Log_e(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)


// caching class and method IDs for a faster subsequent access
static jclass bitmap_class = 0;
static jmethodID setHasAlphaMethodID = 0;

jint Java_com_example_bitmaptest_MainActivity_setHasAlphaNative(JNIEnv * env, jclass clazz, jobject bitmap, jboolean value) {
    AndroidBitmapInfo info;
    void* pixels;


    if (AndroidBitmap_getInfo(env, bitmap, &info) < 0) {
        Log_e("Failed to get Bitmap info");
        return -1;
    }

    if (info.format != ANDROID_BITMAP_FORMAT_RGBA_8888) {
        Log_e("Incompatible Bitmap format");
        return -1;
    }

    if (AndroidBitmap_lockPixels(env, bitmap, &pixels) < 0) {
        Log_e("Failed to lock the pixels of the Bitmap");
        return -1;
    }


    // get class
    if(bitmap_class == NULL) {  //initializing jclass
        // NOTE: The class Bitmap exists since API level 1, so it just must be found.
        bitmap_class = (*env)->GetObjectClass(env, bitmap);
        if(bitmap_class == NULL) {
            Log_e("bitmap_class == NULL");
            return -2;
        }
    }

    // get methodID
    if(setHasAlphaMethodID == NULL) { //initializing jmethodID
        // NOTE: If this fails, because the method could not be found the App will crash.
        // But we only call this part of the code if the method was found using java.lang.Reflect
        setHasAlphaMethodID = (*env)->GetMethodID(env, bitmap_class, "setHasAlpha", "(Z)V");
        if(setHasAlphaMethodID == NULL) {
            Log_e("methodID == NULL");
            return -2;
        }
    }

    // call java instance method
    (*env)->CallVoidMethod(env, bitmap, setHasAlphaMethodID, value);

    // if an exception was thrown we could handle it here
    if ((*env)->ExceptionOccurred(env)) {
        (*env)->ExceptionDescribe(env);
        (*env)->ExceptionClear(env);
        Log_e("calling setHasAlpha threw an exception");
        return -2;
    }

    if(AndroidBitmap_unlockPixels(env, bitmap) < 0) {
        Log_e("Failed to unlock the pixels of the Bitmap");
        return -1;
    }

    return 0;   // success
}

而已.我们完了.我已经发布了整个代码用于复制和粘贴目的.实际的代码并不是那么大,但是进行所有这些偏执的错误检查会使它变得更大.我希望这对任何人都有帮助.