一份yuv转argb代码分析

gistfile1.cpp

JNIEXPORT void JNICALL Java_jp_co_cyberagent_android_gpuimage_GPUImageNativeLibrary_YUVtoARBG(JNIEnv * env, jobject obj, jbyteArray yuv420sp, jint width, jint height, jintArray rgbOut)
{
    int             sz;
    int             i;
    int             j;
    int             Y;
    int             Cr = 0;
    int             Cb = 0;
    int             pixPtr = 0;
    int             jDiv2 = 0;
    int             R = 0;
    int             G = 0;
    int             B = 0;
    int             cOff;
    int w = width;
    int h = height;
    /*数据格式为yuv420sp(NV21)  
    ex: YYYYYYYYUVUV
    即8个Y值排在一起，UV交错排列， 每四个Y共用一对UV
    +-----+-----+-----+-----+
    | Y0  | Y1  | Y2  |  Y3 |
    +-----+-----+-----+-----+
    | Y4  | Y5  | Y6  |  Y7 |
    +-----+-----+-----+-----+
    | Y8  | Y9  | Y10 | Y11 |
    +-----+-----+-----+-----+
    | Y12 | Y13 | Y14 | Y15 |
    +-----+-----+-----+-----+
    | U0  | V0  | U1  | V1  |
    +-----+-----+-----+-----+
    | U2  | V2  | U3  | V3  |
    +-----+-----+-----+-----+
    */
    sz = w * h;//Y的数据大小 ，用于定位UV的首地址， Y的数据量始终等于图像的宽乘以高
    jint *rgbData = (jint*) ((*env)->GetPrimitiveArrayCritical(env, rgbOut, 0));
    jbyte* yuv = (jbyte*) (*env)->GetPrimitiveArrayCritical(env, yuv420sp, 0);

    for(j = 0; j < h; j++) { //逐行处理
             pixPtr = j * w;
             jDiv2 = j >> 1;
             for(i = 0; i < w; i++) {
                     Y = yuv[pixPtr];
                     if(Y < 0) Y += 255;
                     //i & 0x1) != 1  用于快速判断是不是偶数的trick
                     //含义是只在x坐标为偶数时计算Cb，Cr,也就是说
                     if((i & 0x1) != 1) {  //逐个像素处理
                             //UV的矩阵的宽和高是Y矩阵的一半大小
                             //所以每个像素对应的UV值坐标是（j/2,i/2)
                             // (i>>2)*2 是去掉最低位1的trick，把数字转换为偶数
                             cOff = sz + jDiv2 * w + (i >> 1) * 2;
                             Cb = yuv[cOff];
                             if(Cb < 0) Cb += 127; else Cb -= 128;
                             Cr = yuv[cOff + 1];
                             if(Cr < 0) Cr += 127; else Cr -= 128;
                     }
                     
                     //ITU-R BT.601 conversion
                     //
                     //R = 1.164*(Y-16) + 2.018*(Cr-128);
                     //G = 1.164*(Y-16) - 0.813*(Cb-128) - 0.391*(Cr-128);
                     //B = 1.164*(Y-16) + 1.596*(Cb-128);
                     //
                     Y = Y + (Y >> 3) + (Y >> 5) + (Y >> 7);
                     R = Y + (Cr << 1) + (Cr >> 6);
                     if(R < 0) R = 0; else if(R > 255) R = 255;
                     G = Y - Cb + (Cb >> 3) + (Cb >> 4) - (Cr >> 1) + (Cr >> 3);
                     if(G < 0) G = 0; else if(G > 255) G = 255;
                     B = Y + Cb + (Cb >> 1) + (Cb >> 4) + (Cb >> 5);
                     if(B < 0) B = 0; else if(B > 255) B = 255;
                     rgbData[pixPtr++] = 0xff000000 + (B << 16) + (G << 8) + R;
             }
    }

    (*env)->ReleasePrimitiveArrayCritical(env, rgbOut, rgbData, 0);
    (*env)->ReleasePrimitiveArrayCritical(env, yuv420sp, yuv, 0);
}