Android的bitmap.setPixel（X，Y，颜色）在值设置传递 [英] Android bitmap.setPixel(x,y,color) is setting passed in value

问题描述

创建使用Bitmap.create（25，25，Config.ARGB_8888）位图

Create a bitmap using Bitmap.create(25, 25, Config.ARGB_8888)

设置像素具有α值小于或等于0xA9的结果没有被设定什么传入的像素我读到另一个堆栈溢出问题，说要setHasAlpha（真），我在我的测试一样 - 但这仍然没有解决问题。

Setting a pixel with an alpha value less than or equal to 0xA9 results in the pixel not being set with what's passed in. I read another stack overflow question saying to setHasAlpha(true), which I did in my test -- but that still didn't fix the issue.

下面是显示我的问题，我的Andr​​oid测试用例：

Here's my android test case showing my issue:

    public void testSettingBitmaps() {
    Bitmap bitmap = Bitmap.createBitmap(25, 25, Config.ARGB_8888);
    bitmap.setHasAlpha(true);

    int color = 0x00fefefe;
    int x= 0;
    int y = 0;

    for(int alpha = 0xFF000000; alpha != 0x00000000; alpha = alpha - 0x01000000) {
        int colorPlusAlpha = color + alpha;
        bitmap.setPixel(x, y, colorPlusAlpha);  

        //
        // This test succeeds if the bitmap let us set the pixel.
        //
        assertEquals(String.format("Current alpha value: %x, Expected pixel value: %x, Actual pixel value: %x", alpha, colorPlusAlpha, bitmap.getPixel(x, y)), 
                colorPlusAlpha, bitmap.getPixel(x, y)); 

    }
}

这code失败，出现以下的输出：junit.framework.AssertionFailedError：当前alpha值：a9000000，预计像素值：a9fefefe，实际像素值：a9fdfdfd预期：其中，-1442906370>却被：LT; - 1442972163>

This code fails with the following output:junit.framework.AssertionFailedError: Current alpha value: a9000000, Expected pixel value: a9fefefe, Actual pixel value: a9fdfdfd expected:<-1442906370> but was:<-1442972163>

推荐答案

这实际上是按预期工作。

This actually works as intended.

的问题是，位图被存储在premultiplied阿尔法格式。这意味着，当你设置一个像素值0xa9fefefe，存储的值实际上是0xa9a8a8a8（0xa9 * 0xFE的/ 255 = 0xa8）。然后当你调用与getPixel（），存储的值是非premultiplied。与舍入误差您获得0xa9fdfdfd。

The problem is that Bitmaps are stored in premultiplied alpha format. This means that when you set a pixel value to 0xa9fefefe, the stored value is actually 0xa9a8a8a8 (0xa9*0xfe/255=0xa8.) Then when you call getPixel(), the stored value is "un-premultiplied." With rounding errors you get 0xa9fdfdfd.

下面是一个故障，传递0xa9fefefe的ARGB值。每个RGB字节将由alpha值在存储前成倍增加。为了简化，我们将看看只有红色字节：

Here is a breakdown, you pass an ARGB value of 0xa9fefefe. Each RGB byte will be multiplied by the alpha value before being stored. To simplify we'll look at the red byte only:

R = 169 *255分之254
R = 168（或0xa8）

R=169*254/255 R=168 (or 0xa8)

然后当你调用与getPixel（），将RGB字节由阿尔法分为：

Then when you call getPixel(), the RGB bytes are divided by the alpha:

R = 255 *169分之168
R = 253（或者是0xFD）

R=255*168/169 R=253 (or 0xfd)

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