来自libjpeg-turbo-1.5.1-vc.exe的TurboJPEG API产生错误的输出。

Question

我在Visual 2015中用C编写了一个测试程序，用于测试libjpeg-turbo TurboJPEG-API。

在这个测试程序中，我生成了一个800×800 RGB像素的映像，并将其写入磁盘。当我在TJSAMP_GRAY函数中使用tjCompress2时，磁盘上产生的映像看起来很好：

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当我使用TJSAMP_444代替时，图像看起来是头晕的：

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然而，这两种图像不能在中打开，而只能在MS、Chrome或Internet中打开。除TJSAMP_GRAY之外的所有其他选项都会产生一个白发图像，质量设置为100与TJSAMP_GRAY也会导致白头发输出。(这篇文章中的图片通过屏幕截图复制，并保存为png以获取更小的数据。)我还通过P-Invoke使用turbojpeg.dll库编写了一个C#程序。这个程序正在产生有效的输出。

问题:我的错在哪里？

所涉及的源代码：

#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <sys/stat.h>
#include "turbojpeg.h"

unsigned char buffer[800 * 800 * 3];

int main(int argc, char** argv)
{
    unsigned long size = 0;

    unsigned char* compressedImage = NULL;

    for (int x = 0; x < 800; x++)
        for (int y = 0; y < 800; y++)
        {
            switch ((x / 80) % 3)
            {
            case 0:
                buffer[(y * 800 + x) * 3] = 255;
                buffer[(y * 800 + x) * 3 + 1] = 0;
                buffer[(y * 800 + x) * 3 + 2] = 0;
                break;
            case 1:
                buffer[(y * 800 + x) * 3] = 0;
                buffer[(y * 800 + x) * 3 + 1] = 255;
                buffer[(y * 800 + x) * 3 + 2] = 0;
                break;
            case 2:
                buffer[(y * 800 + x) * 3] = 0;
                buffer[(y * 800 + x) * 3 + 1] = 0;
                buffer[(y * 800 + x) * 3 + 2] = 255;
                break;
            }
        }

    tjhandle compressor = tjInitCompress();

    compressedImage = tjAlloc(1024 * 1024 * 4);

    size = 1024 * 1024 * 4;

    tjCompress2(compressor, buffer, 800, 0, 800, TJPF_RGB, &compressedImage, &size, TJSAMP_444, 80, TJFLAG_NOREALLOC | TJFLAG_ACCURATEDCT);

    tjDestroy(compressor);

    int handle = _wopen(L"D:\\file.jpeg", _O_CREAT | _O_WRONLY | _O_TRUNC, _S_IREAD | _S_IWRITE);

    printf("LENGTH=%ld\n", size);

    if (_write(handle, compressedImage, size) != size)
        printf("Write Error.\n");

    _close(handle);

    tjFree(compressedImage);

    return 0;
}

为了完成测试，测试C#源代码，它工作得很好：

class Program
{
    [DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
    private static extern IntPtr tjInitCompress();

    [DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
    private static extern IntPtr tjAlloc(int bytes);

    [DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
    private static extern int tjCompress2(IntPtr handle, IntPtr srcBuf, int width, int pitch, int height, int pixelFormat, ref IntPtr jpegBuf, ref ulong jpegSize, int jpegSubsamp, int jpegQual, int flags);

    [DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
    private static extern int tjDestroy(IntPtr handle);

    [DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
    private static extern void tjFree(IntPtr buffer);

    static void Main(string[] args)
    {
        ulong size = 1024 * 1024 * 4;

        byte[] buffer = new byte[800 * 800 * 3];

        for (int x = 0; x < 800; x++)
            for (int y = 0; y < 800; y++)
            {
                switch ((x / 80) % 3)
                {
                    case 0:
                        buffer[(y * 800 + x) * 3] = 255;
                        buffer[(y * 800 + x) * 3 + 1] = 0;
                        buffer[(y * 800 + x) * 3 + 2] = 0;
                        break;
                    case 1:
                        buffer[(y * 800 + x) * 3] = 0;
                        buffer[(y * 800 + x) * 3 + 1] = 255;
                        buffer[(y * 800 + x) * 3 + 2] = 0;
                        break;
                    case 2:
                        buffer[(y * 800 + x) * 3] = 0;
                        buffer[(y * 800 + x) * 3 + 1] = 0;
                        buffer[(y * 800 + x) * 3 + 2] = 255;
                        break;
                }
            }

        IntPtr umBuffer = Marshal.AllocHGlobal(800 * 800 * 3);
        Marshal.Copy(buffer, 0, umBuffer, 800 * 800 * 3);

        IntPtr compressor = tjInitCompress();

        IntPtr compressedImage = tjAlloc(1024 * 1024 * 4);

        tjCompress2(compressor, umBuffer, 800, 0, 800, 0, ref compressedImage, ref size, 0, 80, 5 * 1024);

        byte[] mCPData = new byte[size];

        Marshal.Copy(compressedImage, mCPData, 0, (int)size);

        System.IO.File.WriteAllBytes("D:\\managed.jpeg", mCPData);

        tjFree(compressedImage);

        Marshal.FreeHGlobal(umBuffer);
    }
}

Answer 1

_wopen-Line正在丢失_O_BINARY。因此，在项目配置或libjpeg-turbo TurboJPEG-API中没有错误。只是，写入磁盘的输出被解释为编码文本，因此被_write-function或某些底层实例修改。

正确的源代码是：

#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <sys/stat.h>
#include "turbojpeg.h"

unsigned char buffer[800 * 800 * 3];

int main(int argc, char** argv)
{
    unsigned long size = 0;

    unsigned char* compressedImage = NULL;

    for (int x = 0; x < 800; x++)
        for (int y = 0; y < 800; y++)
        {
            switch ((x / 80) % 3)
            {
            case 0:
                buffer[(y * 800 + x) * 3] = 255;
                buffer[(y * 800 + x) * 3 + 1] = 0;
                buffer[(y * 800 + x) * 3 + 2] = 0;
                break;
            case 1:
                buffer[(y * 800 + x) * 3] = 0;
                buffer[(y * 800 + x) * 3 + 1] = 255;
                buffer[(y * 800 + x) * 3 + 2] = 0;
                break;
            case 2:
                buffer[(y * 800 + x) * 3] = 0;
                buffer[(y * 800 + x) * 3 + 1] = 0;
                buffer[(y * 800 + x) * 3 + 2] = 255;
                break;
            }
        }

    tjhandle compressor = tjInitCompress();

    compressedImage = tjAlloc(1024 * 1024 * 4);

    size = 1024 * 1024 * 4;

    tjCompress2(compressor, buffer, 800, 0, 800, TJPF_RGB, &compressedImage, &size, TJSAMP_444, 80, TJFLAG_NOREALLOC | TJFLAG_ACCURATEDCT);

    tjDestroy(compressor);

    int handle = _wopen(L"D:\\file.jpeg", _O_CREAT | _O_WRONLY | _O_TRUNC | _O_BINARY, _S_IREAD | _S_IWRITE);

    printf("LENGTH=%ld\n", size);

    if (_write(handle, compressedImage, size) != size)
        printf("Write Error.\n");

    _close(handle);

    tjFree(compressedImage);

    return 0;
}