Question

我正在使用本机C ++，而没有从头开始构建图像处理库，目前仅用于位图。

例如，在将流的位置设置为正确的位置之后，我使用以下方法从位图提取像素数据。

fread(pixelDataBuffer, sizeof(unsigned char), (height * width), streamIn);

其中 height 和 width 是位图图像的像素尺寸。我认为这可以工作并且对于每像素8位的图像有意义，但是它仍然不起作用（该函数返回0）。即使height和width的值均为256，并且pixelDataBuffer初始化如下，也会发生这种情况：

unsigned char pixelDataBuffer[height * width]; 
// also tried:
unsigned char pixelDataBuffer[65536]; // which is 256*256

谢谢！

为清晰起见添加更多代码：

bool isTrial = true;
FILE *streamIn;
FILE *outputFile;

int main(int argc, const char * argv[]) {

    if (isTrial) {
        streamIn = fopen("Images/cameraman.bmp", "rb");
        outputFile = fopen("Images/cameraman_copy.bmp", "wb");
    } else {
        streamIn = fopen("Images/flag.bmp", "rb");
        outputFile = fopen("Images/flag_copy.bmp", "wb");
    }

    unsigned char header[54];
    unsigned char colourTable[1024];

    if (streamIn == nullptr) {
        printf("null pointer");
    } else {

        for (int i = 0; i < 54; i++) {
            header[i] = getc(streamIn);
        }

        unsigned int width = *(int*)&header[18];
        cout << "width: " << width << "\n"; // = 256

        unsigned int height = *(int *)&header[22];
        cout << "height: " << height << "\n"; // = 256

        unsigned short bitDepth = *(short *)&header[28];
        cout << "bitDepth: " << bitDepth << "\n"; // = 8

        unsigned int fileSize = *(int *)&header[2];
        cout << "fileSize: " << fileSize << endl; // 66614

        if (1 < bitDepth && bitDepth <= 8) {
            short count = fread(colourTable, sizeof(unsigned char), 1024, streamIn); 
            if (count == 1024) {
                printf("colourTable read\n"); // colourTable gets read
            } else {
                printf("colourTable NOT read properly");
            }
        } else {
            printf("bitsPerPixel / bitDepth is more than 8");
        }


        { // getting pixelData, at this point I assumed that the stream's position starts where pixel data starts (i.e. after Headers & ColourTable)

            unsigned int pixelDataSize = height * width * (bitDepth/8); // = 65536
            cout << "pixelDataSize: " << pixelDataSize << endl;

            unsigned char pixelDataBuffer[pixelDataSize]; // also tried initializing like unsigned char pixelDataBuffer[height*width]

            short counter = fread(pixelDataBuffer, sizeof(unsigned char), pixelDataSize, streamIn);
            cout << "counter: " << counter << endl; // = 0 THIS IS THE ISSUE. Documentation says "If either size or count is zero, the function returns zero and both the stream state and the content pointed by ptr remain unchanged." But I think I have size and count >0 correctly?

        }


        { // writing our header onto the outputFile
            short count = fwrite(header, sizeof(unsigned char), 54, outputFile);
            if (count == 54) {
                printf("header written\n"); // Header gets written
            } else {
                printf("header not written");
            }
        }
    }
    return 0;
}

Answer 1

您的问题是声明fread的结果简短。

尝试始终始终使用自动。使用：

auto counter = fread(pixelDataBuffer, sizeof(unsigned char), pixelDataSize, streamIn);

使其起作用。问题是65536不能简短地表示，实际上它溢出了它的数量足以使结果看起来是0。

如果您使用的是C ++的旧版本（在C ++ 11之前），并且不能使用自动使用size_t（这是fread的实际返回类型）。

我还建议您学习将代码分解为较小的功能，并避免编写巨大的if块，以提高将来的可读性和可维护性。

从位图提取像素数据时出现问题

1 个答案: