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Bitmap 之 getPixels() 的 stride

Posted on 2012-04-15 16:37 zljpp 閱讀(216) 評論(0) 編輯收藏

學習Graphics中遇到位圖(Bitmap)中getPixels()方法，對該方法的用法大體理解，但對其中的stride參數卻不明白具體的用法以及用意，現記述過程如下：

getPixels()方法的用處為獲取位圖(Bitmap)中的像素值(顏色值)，存入類型為int的pixels數組中，至于從RGB轉換為int數值的算法是什么，暫時不知，存疑！！

Android英文SDK中有關getPixels()方法的介紹如下：

public void getPixels (int[] pixels, int offset, int stride, int x, int y, int width, int height)

Returns in pixels[] a copy of the data in the bitmap. Each value is a packed int representing a Color. The stride parameter allows the caller to allow for gaps in the returned pixels array between rows. For normal packed results, just pass width for the stride value.

Parameters

pixels	The array to receive the bitmap's colors
offset	The first index to write into pixels[]
stride	The number of entries in pixels[] to skip between rows (must be >= bitmap's width). Can be negative.
x	The x coordinate of the first pixel to read from the bitmap
y	The y coordinate of the first pixel to read from the bitmap
width	The number of pixels to read from each row
height	The number of rows to read

Throws

IllegalArgumentException	if x, y, width, height exceed the bounds of the bitmap, or if abs(stride) < width.
ArrayIndexOutOfBoundsException	if the pixels array is too small to receive the specified number of pixels.

看完英文文檔仍然不甚明白，于是去搜了下中文Android文檔相應內容， getPixels()

public void getPixels (int[] pixels, int offset, int stride, int x, int y, int width, int height)

把位圖的數據拷貝到pixels[]中。每一個都由一個表示顏色值的int值來表示。幅度參數(stride)表明調用者允許的像素數組行間距。對通常的填充結果，只要傳遞寬度值給幅度參數。

參數

pixels 接收位圖顏色值的數組

offset 寫入到pixels[]中的第一個像素索引值

stride pixels[]中的行間距個數值(必須大于等于位圖寬度)。可以為負數

x 從位圖中讀取的第一個像素的x坐標值。

y 從位圖中讀取的第一個像素的y坐標值

width 從每一行中讀取的像素寬度

height 　　讀取的行數

　　異常

IllegalArgumentExcepiton 如果x，y，width，height越界或stride的絕對值小于位圖寬度時將被拋出。

ArrayIndexOutOfBoundsException 如果像素數組太小而無法接收指定書目的像素值時將被拋出。

看完后仍然對Stride解釋中的＂行間距＂不太明白，去查了下Stride在英語中的原義，Stride在柯林斯中的英英釋義如下：

1　If you stride somewhere, you walk there with quick, long steps.
　　stride意為"大踏步快速前進"
2　A stride is a long step which you take when you are walking or running.
　　stride在此做名詞，意為"大步"
3　Someone's stride is their way of walking with long steps.
　　指代某人具體邁大步的方式．

于是可以把stride理解為人行走過程中所邁大步的一段距離，而在此方法中可以理解為每行的像素數，至于用處是什么，還要繼續尋找答案．

然后去StackOverFlow去搜了搜"getPixels()　stride"關鍵字，查找到如下信息：

1 In most cases the stride is the same as the width. The stride is useful if you are trying to copy/draw a sub-region of a Bitmap. For instance, if you have a 100x100 bitmap and you want to draw the 50x50 top-right corner, you can use a width of 50px and a stride of 100px．(注：stride絕對值要大于等于位圖的寬度)

2 Stride is number of bytes used for storing one image row.

Stride can be different from the image width.

Most of the images are 4 byte aligned.

For ex. a 24 bit (RGB) image with width of 50 pixels. The total bytes required will be 150 (3(RGB)*50). As image will be 4 byte aligned, in this case the byte required will become 154.

So you will see stride as 154, width 50 and image alignment as 4 byte.

上面內容表示stride參數有兩種用處

第一種：
可以截取圖片中部分區域或者圖片拼接．

截圖：假設讀取像素值的原圖片寬為w,高為h,此時設置參數pixels[w*h], 參數stride為 w ,參數offset為0，參數x ,y為截圖的起點位置，參數width和height為截圖的寬度和高度，則此方法運行后，返回的pixels[]數組中從pixels[0]至pixels[width*height-1]里存儲的是從圖片( x , y )處起讀取的截圖大小為width * height的像素值．
示例：修改Android SDK自帶的AipDemo程序中BitmapDecode示例，更換圖像為自制四角四色圖：

圖像大小為100*100,想截取圖片右上1/4圖像(圖上黃色部分)修改程序部分代碼為：

	int[] pixels = new int[w*h];
	mBitmap2.getPixels(pixels, 0, w, 50, 0, w/2, h/2);
	mBitmap3 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_8888);
	mBitmap4 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_4444);
	String txt = String.valueOf(pixels[10]);
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);

運行結果：

I/myBitmapDecode( 660): w = 100; h = 100
I/myBitmapDecode( 660): pixels[0]-16777216; pixels[1] = -16777216; pixels[10] = -4352
I/myBitmapDecode( 660): pixels[w]-16777216; pixels[h] = -16777216; pixels[w*h-1] = 0

我們看到右邊兩副ARGB_8888,ARGB_4444圖像隱約只在左上角顯示原圖右上的1/4黃色部分，其余部分為背景色白色，那么問題又來了，此時ARGB_8888,ARGB_4444圖像大小為多少?還是原圖的大小(100*100)嗎，或者是(50*50)了，不然背景色為何是畫布的背景色呢(白色)?那么把 pixels[100*100]數組設初始值看下情況(通過Log.i()我查到了pixels中存儲的像素值為百萬左右的負整數(-16777216)，所以這里胡亂取個數-2578654做為初始值，顏色不太好，請見諒)，修改后代碼如下：

	int[] pixels = new int[w*h];
	for(int i=0; i<w*h; i++){
		pixels[i] = -2578654; 
	}
	mBitmap2.getPixels(pixels, 0, w, 50, 0, w/2, h/2);
	mBitmap3 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_8888);
	mBitmap4 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_4444);
	String txt = String.valueOf(pixels[10]);
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);

運行結果：

I/myBitmapDecode( 727): w = 100; h = 100
I/myBitmapDecode( 727): pixels[0] = -16777216; pixels[1] = -16777216; pixels[10] = -4352
I/myBitmapDecode( 727): pixels[w] = -16777216; pixels[h] = -16777216; pixels[w*h-1] = -2578654

我們可以看到結果了，如果pixels[]中的數值為int默認值(0)的話，圖片相應的部分就為背景色，如果設置為別的初始值而在運行中沒有被修改的話，背景色就是修改值對應的RGB顏色．

原圖位置（offset）
下面設置下getPixels[]方法中offset，使得黃色部分截圖出現在它在原圖中的位置，

offset = x + y*w ，本例代碼如下：

	int[] pixels = new int[w*h];
	for(int i=0; i<w*h; i++){
		pixels[i] = -2578654; 
	}
	mBitmap2.getPixels(pixels, 50, w, 50, 0, w/2, h/2;
	mBitmap3 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_8888);
	mBitmap4 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_4444);
	String txt = String.valueOf(pixels[10]);
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);

運行結果：

I/myBitmapDecode( 761): w = 100; h = 100
I/myBitmapDecode( 761): pixels[0] = -2578654; pixels[1] = -2578654; pixels[10] = -2578654
I/myBitmapDecode( 761): pixels[w] = -2578654; pixels[h] = -2578654; pixels[w*h-1] = -2578654

當然可以用這個方法進行更復雜的運算，諸如截取素材圖片修改目標圖片(已存儲至pixels數組中)的指定區域！！

背景色設置（pixels[]）

背景顏色與pixels[]初始值一致，如紅色RED（-65536 0xffff0000），黃色YELLOW（-256 0xffffff00），具體詳見下面附注

	int[] pixels = new int[w*h];
	for(int i=0; i<w*h; i++){
		pixels[i] = -65536; 	// Color.RED : -65536 (0xffff0000)
	}
	mBitmap2.getPixels(pixels, 50, w, 50, 0, w/2, h/2);
	mBitmap3 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_8888);	
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10] + "; pixels[50] = " + pixels[50]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);
	
	for(int i=0; i<w*h; i++){
		pixels[i] = -256;  		// Color.YELLOW : -256 (0xffffff00)
	}
	mBitmap2.getPixels(pixels, 50*100 + 50, w, 50, 50, w/2, h/2);
	mBitmap4 = Bitmap.createBitmap(pixels, 0, w, w, h, Bitmap.Config.ARGB_4444);
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10] + "; pixels[50] = " + pixels[50]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);

運行結果：

I/myBitmapDecode( 1671): w = 100; h = 100
I/myBitmapDecode( 1671): pixels[0] = -65536; pixels[1] = -65536; pixels[10] = -65536; pixels[50] = -16777216
I/myBitmapDecode( 1671): pixels[w] = -65536; pixels[h] = -65536; pixels[w*h-1] = -65536
I/myBitmapDecode( 1671): w = 100; h = 100
I/myBitmapDecode( 1671): pixels[0] = -256; pixels[1] = -256; pixels[10] = -256; pixels[50] = -256
I/myBitmapDecode( 1671): pixels[w] = -256; pixels[h] = -256; pixels[w*h-1] = -16735513

圖片拼接:

假設兩張圖片大小都為 w * h ，getPixels()方法中設置參數pixels[2*w*h],參數offset = 0，stride = 2*w讀取第一張圖片，再次運行getPixels()方法，設置參數offset = w，stride = 2*w，讀取第二張圖片，再將pixels[]繪制到畫布上就可以看到兩張圖片已經拼接起來了．

示例如下：

	int w = mBitmap2.getWidth();
	int h = mBitmap2.getHeight();
	int[] pixels = new int[2*w*h];
	for(int i=0; i<2*w*h; i++){
		pixels[i] = -2578654; 
	}
	mBitmap2.getPixels(pixels, 0, 2*w, 0, 0, w, h);
	mBitmap2.getPixels(pixels, w, 2*w, 0, 0, w, h);
	mBitmap3 = Bitmap.createBitmap(pixels, 0, 2*w, 2*w, h, Bitmap.Config.ARGB_8888);
	String txt = String.valueOf(pixels[10]);
	Log.i("myBitmapDecode", "w = " + w + "; h = " + h);
	Log.i("myBitmapDecode", "pixels[0] = " + pixels[0] + "; pixels[1] = " + pixels[1] + "; pixels[10] = " + pixels[10]);
	Log.i("myBitmapDecode", "pixels[w] = " + pixels[w] + "; pixels[h] = " + pixels[h] + "; pixels[w*h-1] = " + pixels[w*h-1]);
	Log.i("myBitmapDecode", "pixels[2*w-1] = " + pixels[2*w-1] + "; pixels[2*w] = " + pixels[2*w] + "; pixels[2*w*h-1] = " + pixels[2*w*h-1]);

運行結果：

I/myBitmapDecode( 989): w = 100; h = 100
I/myBitmapDecode( 989): pixels[0] = -16777216; pixels[1] = -16777216; pixels[10] = -16777216
I/myBitmapDecode( 989): pixels[w] = -16777216; pixels[h] = -16777216; pixels[w*h-1] = -16777216
I/myBitmapDecode( 989): pixels[2*w-1] = -3328; pixels[2*w] = -16777216; pixels[2*w*h-1] = -16735513

第二種：　
stride表示數組pixels[]中存儲的圖片每行的數據，在其中可以附加信息，即
stride = width + padding，如下圖所示

這樣可以不僅僅存儲圖片的像素信息，也可以儲存相應每行的其它附加信息．

最后，stride參數的意義及用處總結如下：

1　用來表示pixels[]數組中每行的像素個數，用于行與行之間區分，絕對值必須大于參數width，但不必大于所要讀取圖片的寬度w（在width < w 時成立）．(stride負數有何作用不知，存疑)．另，pixels.length >= stride * height,否則會拋出ArrayIndexOutOfBoundsException異常

2　stride > width時，可以在pixels[]數組中添加每行的附加信息，可做它用．

附注（Color顏色對應值）：

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