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。