我的PNG IDAT块数据有什么问题？

Question

我正在尝试使用OpenGL手动创建PNG文件的学习目的 所有其他块都是OKK（IHDR，PHY，IEND）。

首先，我读了OpenGl的Pixels：

int s_width = glutGet(GLUT_SCREEN_WIDTH), s_height = glutGet(GLUT_SCREEN_HEIGHT);       
int pixelArraySize = s_width*s_height*_glColorChannels;
unsigned char *pixelsArrayInfo = (unsigned char*)malloc(pixelArraySize);               

glPixelStorei(GL_PACK_ALIGNMENT, 1);                                                   
glReadBuffer(GL_FRONT);
glReadPixels(0, 0, (unsigned short)s_width, (unsigned short)s_height, GL_RGB, GL_UNSIGNED_BYTE, pixelsArrayInfo);

然后，我创建了生成这样的扫描线的函数： “每个扫描线都是一个屏幕行中的RGB值数组，然后是'0'”

unsigned char *generateScanlines(unsigned char *pixels, int s_width, int s_height)
{
    int eachScanlineLength = 1 + s_width*3;
    unsigned char *finalOutput = (unsigned char*)malloc(s_height*eachScanlineLength);
    for(int i=0; i<s_height; i++){
        finalOutput[i*eachScanlineLength] = 0;
        copyElement(finalOutput, pixels, i*eachScanlineLength,  (i+1)*eachScanlineLength, i*eachScanlineLength+1);
    }
    return finalOutput;
}

void copyElement(unsigned char *dest, unsigned char *src, int src_debut, int src_fin, int dest_debut)
{
    for(int i=src_debut, j=dest_debut; i<src_fin; i++, j++){
        dest[j] = src[i];
    }
}

unsigned char *deflateDatas(unsigned char *pixels, int s_width, int s_height, int *deflatedDataLength)
{
    unsigned char *deflated = (unsigned char*)malloc(compressBound(s_height*(1 + s_width*3)));
    unsigned char *scanlines = invertArray(generateScanlines(pixels, s_width, s_height), s_height*(1 + s_width*3));

    z_stream defstream;
    defstream.zalloc = Z_NULL;
    defstream.zfree = Z_NULL;
    defstream.opaque = Z_NULL;
    defstream.avail_in = (uInt)(s_height*(1 + s_width*3));
    defstream.next_in = (Bytef *)scanlines;
    defstream.avail_out = (uInt)(compressBound(s_height*(1 + s_width*3)));
    defstream.next_out = (Bytef *)deflated;

    deflateInit(&defstream, 0);
    deflate(&defstream, Z_FINISH);
    deflateEnd(&defstream);

    *deflatedDataLength = compressBound(s_height*(1 + s_width*3));

    return deflated;
}

，然后看来它可以工作，但是当我对其进行测试时，我的OpenGL程序会得到：

[小PNG输出] [1]

我也创建了一个基本的BMP文件，它可以完美地奏效，

我尝试找到是否有任何错误，也许是在扫描线的生成或误解PNG文件格式中。

invertarray（）代码：

unsigned char *invertArray(unsigned char *myArray, int arrayEnd)
{    unsigned char *invertedtableau = (unsigned char*)malloc(arrayEnd*sizeof(unsigned char));    
for(int i=0 ; i<=arrayEnd ; i++)    
{        invertedtableau[i] = myArray[arrayEnd-i];    
}    
return invertedtableau;                           }

解决方案 我发现错误来自何处，AccordInd到Mark Adler，Scanlines Gemeration方法是错误的。 另外，由于OpenGL仅与左下角兼容，因此文件被倒置，但是PNG是左上方格式，然后我们需要在生成scanlines之前将像素缓冲液反转（EHAT我使用interverArray（）方法）。

最后一个错误是调用方法和存储温度的长度也是错误的。

整个放气代码：

    // generating scanline function
unsigned char *generateScanlines(unsigned char *pixels, int s_width, int s_height, int colorChannel)
{
    int eachScanlineLength = 1 + s_width * colorChannel, i = 1, j = 0;                            // one scanline length
    unsigned char *scanlines = (unsigned char *)malloc(s_height * eachScanlineLength); // memory allocation for the scanline output

    memset(scanlines, 0, s_height * eachScanlineLength * sizeof(char)); // we set all the output values to 0

    // then we copy pixels elements in the output, skipping the fisrt output values, that should ever be 0
    for (i = 1, j = 0; i < s_height && j < s_height; i++, j++)
        memcpy(scanlines + 1 + (i - 1) * eachScanlineLength, pixels + j * (eachScanlineLength - 1), eachScanlineLength - 1);

    memcpy(scanlines + 1 + (i - 1) * eachScanlineLength, pixels + j * (eachScanlineLength - 1), eachScanlineLength - 1);

    return scanlines;
}

// deflating IDAT CHUNK data algorithm
unsigned char *deflateDatas(unsigned char *pixels, int s_width, int s_height, int colorChannel, int *deflatedLen)
{
    unsigned long inLen = s_height * (1 + s_width * colorChannel), tmpLen = 0;          // input len of scanlines datas
    unsigned char *scanlines = generateScanlines(pixels, s_width, s_height, colorChannel); // generating scanlines from the pixels

    unsigned char *deflatedDatas = NULL; // setting up the deflated datas output
    int result = 0;

    // initialising zlib
    z_stream defstream;
    defstream.zalloc = Z_NULL;
    defstream.zfree = Z_NULL;
    defstream.opaque = Z_NULL;
    defstream.avail_in = inLen;
    defstream.next_in = (Bytef *)scanlines;
    defstream.avail_out = 0;
    defstream.next_out = (Bytef *)deflatedDatas;

    if ((result = deflateInit(&defstream, Z_DEFAULT_COMPRESSION)) == Z_OK)
    {
        // calculate the actual length and update zlib structure
        unsigned long estimateLen = deflateBound(&defstream, inLen);
        deflatedDatas = (unsigned char *)malloc(estimateLen);
        if (deflatedDatas != NULL)
        {
            // updation zlib configuration
            defstream.avail_out = (uInt)estimateLen;
            defstream.next_out = (Bytef *)deflatedDatas;

            // do the compression
            deflate(&defstream, Z_FINISH);
            tmpLen = (unsigned char *)defstream.next_out - deflatedDatas;
        }
    }
    deflateEnd(&defstream);       // end of deflating algorithm
    *deflatedLen = tmpLen; // copying the defalted data length to the IDAT->length
    free(scanlines);

    return deflatedDatas;
}

左下到左上的PixelBuffer翻转代码：

void flipPixels(unsigned char *pixelsArray, int s_width, int s_heigth, int colorChannel)
{
    int totalLength = s_width * s_heigth * colorChannel;
    int oneLineLength = s_width * colorChannel;
    unsigned char *tmp = (unsigned char *)malloc(totalLength * sizeof(unsigned char));
    memcpy(tmp, pixelsArray, totalLength);
    for (int i = 0; i < s_heigth; i++)
        memcpy(pixelsArray + oneLineLength * i, tmp + totalLength - oneLineLength * (i + 1), oneLineLength);
    free(tmp);
}

  [1]: 

https://i.sstatic.net /5khcg.png

Answer 1

我们没有看到所有相关的代码，但是首先，您似乎通过逆转字节来故意损坏您的图像数据。不要那样做。删除对invertarray（）的调用。

其次，您没有返回压缩数据的大小。您正在返回对此的上限估计。

使用deflateBound（）（调用deflateInit（）），而不是compressbound（）。调用deflateBound（）一次（而不是三次），然后将答案保存在无签名的长期绑定中。返回压缩数据的大小，即BOND -defstream.avail_out。

最后，您没有压缩！ deflateInit（）的第二个参数应为z_default_compression或0以外的其他内容，这意味着没有压缩。

您的示例对idat块有不正确的CRC，因此您在构造该块的代码中没有其他问题。

即使我撤消invertarray（）（本身有另一个错误），我得到的仍然是混乱的图像，似乎在每行上删除了一个字节。

除非您提供代码的全部，否则我们无法帮助您。有很多错误，您不知道它们在哪里。