在在C BMP文件，填充处理 [英] Dealing with padding in a BMP file in C

问题描述

我想编辑C. BMP文件我的code适用于无填充BMP文件，但我有处理填充的麻烦。

有，我看过，但大部分都使用其他语言如C＃和Java，所以我没有发现他们非常有用的BMP文件的几个其他问题。

下面是像素阵列看起来像什么，但更大的：

 字符bmpArray [MAX] = {B，G，R，B，G，R，B，G，R，0,0,0，
                      B，G，R，B，G，R，B，G，R，0,0,0，
                      B，G，R，B，G，R，B，G，R，0,0,0}
 

的零点是填充字节，使每行被4整除，这取决于图像的像素宽度。我所要做的是将这些填充字节他们是在同一个位置，只处理了B，G，R字节的方式。如果我申请编辑的填充值，产生的图像将被扭曲。

我提出，产生的填充字节的基础上的宽度的量的功能。
它使用这个公式 4 - （（宽* 3）％4）和它的作品，因为我与不同宽度的图像进行了测试。

我成功提取了B，G，BMP文件的R数据，并把它变成一个数组所以我只会张贴我有麻烦code的一部分。

  INT C = 0;
对于（int类型的= 0; A＆LT;高度; A ++）{
    对于（INT B = 0; B＆LT;宽* 3; b ++）{
        如果（bmpArray〔一个*（宽* 3）+ B]所述; 127）{
            bmpArray [一个*（宽* 3）+ B] = 0;
        }其他{
            bmpArray [一个*（宽* 3）+ B] = 255;
        }
        C ++;
    }
    对于（INT垫= 0;垫及LT;填充;垫++）{
        bmpArray [C ++] = 0×00;
    }
}
 

我所试图做的是画输出BMP文件的每一行，然后当我到达行末停产了，就是宽* 3，在那之后才去绘制填充字节下一行像素。

另外，有一种方法，我可以用一个for循环识别填充像素，if语句然后用不修改的填充像素？例如：

 为（int类型的= 0; A＆LT; bmpArraySize; A ++）{
    paddingBytes =？ //例如用于第一行
                       // paddingBytes是1 +宽* 3 + 1
                       //和i +宽* 3 + 2和i +宽* 3 + 3如果填充= 3
    如果（α= paddingBytes）{
        bmpArray [α] = 0×00;
    }
    否则，如果（bmpArray [A]＆LT; 127）{
        bmpArray [一个] = 0;
    }
    其他{
        bmpArray [一个] = 255;
    }
}
 

解决方案

问题是这部分：

  INT C = 0;
对于（int类型的= 0; A＆LT;高度; A ++）{
    对于（INT B = 0; B＆LT;宽* 3; b ++）{
        如果（bmpArray〔一个*（宽* 3）+ B]所述; 127）{
            bmpArray [一个*（宽* 3）+ B] = 0;
        }其他{
            bmpArray [一个*（宽* 3）+ B] = 255;
        }
    }
    对于（INT垫= 0;垫及LT;填充;垫++）{
        bmpArray [C ++] = 0×00; / *只有在这里是'C'更新！ * /
    }
}
 

在每行的末尾，你填写在填补开始ç，这在 0 开出等覆盖的第一行的前几个字节。然后，每个接下来的行被复制，但你继续从一开始就覆盖（其中 C 最初指向）。

的填充应添加的每行的。在循环中，你调整 A 和 B ，但你忘了调整的填充。

我建议更直接的code（未经测试！）：

 为（int类型的= 0; A＆LT;高度; A ++）{
    对于（INT B = 0; B＆LT;宽* 3; b ++）{
        如果（bmpArray [A *（宽* 3 +填充）+ B＆LT; 127）{
            bmpArray [一个*（宽* 3 +填充）+ B] = 0;
        }其他{
            bmpArray [A *（宽* 3 +填充）+ B] = 255;
        }
    }
    对于（INT垫= 0;垫及LT;填充;垫++）{
        bmpArray [A *（宽* 3 +填充）+ 3 *宽+ PAD] = 0×00;
    }
}
 

  

有没有办法，我可以找出填充像素。

是 - 我的循环上面进行填充的调整，它会自动跳过填充本身。您可以安全地在最后删除显式设置填充为0环。

I am trying to edit a BMP file in C. My code works for BMP files with no padding but I am having trouble dealing with padding.

There are a few other questions on BMP files that I have read but most of them use other languages like C# and Java so I didn't find them very useful.

Here is what the pixel array looks like, but much larger:

char bmpArray[MAX] = {B,G,R,B,G,R,B,G,R,0,0,0,
                      B,G,R,B,G,R,B,G,R,0,0,0,
                      B,G,R,B,G,R,B,G,R,0,0,0}

The zeros are for padding bytes to make each row divisible by 4, it depends on the pixel width of the image. What I am trying to do is leave these padding bytes the way they are in the same position and only deal with the B,G,R bytes. If I apply edits to the padding values, the resulting image will be distorted.

I made a function that generates the amount of padding bytes based on the width. It uses this formula 4 - ((width * 3) % 4) and it works as I tested it with images with different width.

I successfully extracted the B, G, R data of the BMP file and put it into an array so I will only post the part of the code I am having trouble with.

int c = 0;
for (int a = 0; a < height; a++) {
    for (int b = 0; b < width*3; b++) {
        if (bmpArray[a*(width*3)+b] < 127) {
            bmpArray[a*(width*3)+b] = 0;
        } else {
            bmpArray[a*(width*3)+b] = 255;
        }
        c++;
    }
    for (int pad = 0; pad < padding; pad++) {
        bmpArray[c++] = 0x00;
    }
}

What I am trying to do is "draw" each row of the output BMP file and then stop as soon as I reach the end of the row, that is width*3, then after that draw the padding bytes before going to the next row of pixels.

Alternatively, is there a way I can identify the padding pixels using a single for loop and then use an if statement to not modify the padding pixels? For example:

for (int a = 0; a < bmpArraySize; a++) {
    paddingBytes = ??? //for example for the first row
                       // paddingBytes are i + width*3 + 1
                       // and i + width*3 + 2 and i + width*3 + 3 if padding = 3
    if (a = paddingBytes) {
        bmpArray[a] = 0x00;
    }
    else if (bmpArray[a] < 127) {
        bmpArray[a] = 0;
    }
    else {
        bmpArray[a] = 255;
    }
}

解决方案

The problem is in this part:

int c = 0;
for (int a = 0; a < height; a++) {
    for (int b = 0; b < width*3; b++) {
        if (bmpArray[a*(width*3)+b] < 127) {
            bmpArray[a*(width*3)+b] = 0;
        } else {
            bmpArray[a*(width*3)+b] = 255;
        }
    }
    for (int pad = 0; pad < padding; pad++) {
        bmpArray[c++] = 0x00;  /* ONLY HERE is 'c' updated! */
    }
}

At the end of each line, you fill out the padding starting at c, which starts out at 0 and so overwrites the first few bytes of the first line. Then, each next line gets copied but you continue overwriting from the start (where c initially pointed to).

The padding should be added on each line. In the loops, you adjust a and b but you forget to adjust for the padding.

I suggest the more straightforward code (untested!):

for (int a = 0; a < height; a++) {
    for (int b = 0; b < width*3; b++) {
        if (bmpArray[a*(width*3 + padding)+b] < 127) {
            bmpArray[a*(width*3 + padding)+b] = 0;
        } else {
            bmpArray[a*(width*3 + padding)+b] = 255;
        }
    }
    for (int pad = 0; pad < padding; pad++) {
        bmpArray[a*(width*3 + padding) + 3*width + pad] = 0x00;
    }
}

is there a way I can identify the padding pixels ..

Yes – in my loop above with adjustments for padding, it automatically skips the padding itself. You can safely remove the explicit 'set padding to 0' loop at the end.

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