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

Question

我在 Visual Studio 2015 年用 C 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 800x800 RGB 像素的图像并将其写入磁盘。当我在 tjCompress2 函数中使用 TJSAMP_GRAY 时，磁盘上生成的图像看起来不错：

当我使用 TJSAMP_444 时，图像看起来很奇怪：

但是，这两张图片都无法在 Adobe Photoshop 中打开，只能在 MS-Paint、Chrome 或 Internet Explorer 中打开。 TJSAMP_GRAY 以外的所有其他选项都会产生奇怪的图像，质量设置为 100 和 TJSAMP_GRAY 也会导致奇怪的输出。 （此 post 中的图像通过屏幕截图复制并保存为 png 以获取较小的数据。） 我还通过 P- 使用 turbojpeg.dll 库编写了一个 C#-程序调用。该程序正在生成有效输出。

提问：我错在哪里？

有问题的源代码：

#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;
}

为了完整起见，测试 C# 源代码，工作正常：

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

缺少 _wopen-行 _O_BINARY。因此，项目配置或 libjpeg-turbo TurboJPEG-API 没有错误。只是，写入光盘的输出被解释为编码文本，因此被 _write 函数或某些底层实例修改。

正确的源代码应该是：

#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;
}

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

TurboJPEG API from libjpeg-turbo-1.5.1-vc.exe produces wrong output

c

libjpeg-turbo

turbojpeg

visual-studio-2015