Bimap getPixels方法研究

Android bitmap getPixels有俩个方法，这个俩个方法都是获取图片的pixel的。

1、getPixel(int x, int y)

2、getPixels(int[] pixels, int offset, int stride, int x, int y, int width, int height)

先来看第1个方法：

    /**     * Returns the {@link Color} at the specified location. Throws an exception     * if x or y are out of bounds (negative or >= to the width or height     * respectively). The returned color is a non-premultiplied ARGB value.     *     * @param x    The x coordinate (0...width-1) of the pixel to return     * @param y    The y coordinate (0...height-1) of the pixel to return     * @return     The argb {@link Color} at the specified coordinate     * @throws IllegalArgumentException if x, y exceed the bitmap's bounds     */    public int getPixel(int x, int y) {        checkRecycled("Can't call getPixel() on a recycled bitmap");        checkPixelAccess(x, y);        return nativeGetPixel(mNativeBitmap, x, y);    }

从方法执行上看，先判断这个bitmap是否被recycle掉。 

接着 checkPixelAccess(x, y);来检查x,y的值是否合法。0 < x < bitmap.getWidth(); y类似

然后就是native方法。

这个方法的作用就是：获取指定bitmap上x,y坐标上的点的像素。

接着看第2个方法：

     * Returns in pixels[] a copy of the data in the bitmap. Each value is     * a packed int representing a {@link 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.     * The returned colors are non-premultiplied ARGB values.     *     * @param pixels   The array to receive the bitmap's colors     * @param offset   The first index to write into pixels[]     * @param stride   The number of entries in pixels[] to skip between     *                 rows (must be >= bitmap's width). Can be negative.     * @param x        The x coordinate of the first pixel to read from     *                 the bitmap     * @param y        The y coordinate of the first pixel to read from     *                 the bitmap     * @param width    The number of pixels to read from each row     * @param 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.     * @throws ArrayIndexOutOfBoundsException if the pixels array is too small     *         to receive the specified number of pixels.     */    public void getPixels(int[] pixels, int offset, int stride,                          int x, int y, int width, int height) {        checkRecycled("Can't call getPixels() on a recycled bitmap");        if (width == 0 || height == 0) {            return; // nothing to do        }        checkPixelsAccess(x, y, width, height, offset, stride, pixels);        nativeGetPixels(mNativeBitmap, pixels, offset, stride,                        x, y, width, height);    }

与上面相同的方法就不再提了。

看接下来执行 checkPixelsAccess(x, y, width, height, offset, stride, pixels);方法：

首先x,y满足 大于0，小于bitmap.getWidth() 和getHeight()，x + width还需要小于bitmap.width()，y类同。否则会抛出IllegalArgumentException异常

offset参数不可为负，offset+width不能大于pixels[]数组的长度，

stride参数可正可负，但需要 小于传入的参数width，注意是width，不是bitmap.getWidth()

最终该方法将结果写入到pixels数组。

再来说说几个参数具体含义：

pixels      获取位图颜色值的数组 
offset       写入到pixels[]中的第一个像素索引值 
stride       pixels[]中的行间距个数值(必须大于等于位图宽度)。可以为负数 
x          　从位图中读取的第一个像素的x坐标值。 
y              从位图中读取的第一个像素的y坐标值 
width       从每一行中读取的像素宽度 
height     读取的行数 

stride = getWidth + padding 这个getWidth是图片的width,padding里面是附加信息。