libswscale bad dst 图像指针 cgo

Question

我正在尝试使用 libswscale 在使用 cgo 编码为 h264 之前缩放图像。这里我写了一个简单的demo（抱歉代码风格不好，我只是想快速验证一下）：

func scale(img []byte, scaleFactor int) {

    input, _, _ := image.Decode(bytes.NewReader(img))

    if a, ok := input.(*image.YCbCr); ok {

        width, height := a.Rect.Dx(), a.Rect.Dy()

        var format C.enum_AVPixelFormat = C.AV_PIX_FMT_YUV420P

        context := C.sws_getContext(C.int(width), C.int(height), format, C.int(width/scaleFactor), C.int(height/scaleFactor), 0, C.int(0x10), nil, nil, nil)

        in := make([]uint8, 0)

        in = append(in, a.Y...)

        in = append(in, a.Cb...)

        in = append(in, a.Cr...)

        stride := []C.int{C.int(width), C.int(width / 2), C.int(width / 2), 0}

        outstride := []C.int{C.int(width / scaleFactor), C.int(width / scaleFactor / 2), C.int(width / scaleFactor / 2), 0}

        out := make([]uint8, width*height/scaleFactor/scaleFactor*3/2)

        C.sws_scale(context, (**C.uint8_t)(unsafe.Pointer(&in[0])), (*C.int)(&stride[0]), 0,

            C.int(height), (**C.uint8_t)(unsafe.Pointer(&out[0])), (*C.int)(&outstride[0]))

        min := image.Point{0, 0}

        max := image.Point{width / scaleFactor, height / scaleFactor}

        output := image.NewYCbCr(image.Rectangle{Min: min, Max: max}, image.YCbCrSubsampleRatio420)

        paneSize := width * height / scaleFactor / scaleFactor

        output.Y = out[:paneSize]

        output.Cb = out[paneSize : paneSize*5/4]

        output.Cr = out[paneSize*5/4:]

        opt := jpeg.Options{

            Quality: 90,

        }

        f, _ := os.Create("img.jpeg")

        jpeg.Encode(f, output, &opt)

    }

}

每次我运行代码片段时，我都会收到一条错误消息bad dst image pointers，说我的代码有什么问题。我是cgo的新手，所以代码对你来说可能很愚蠢，我为此道歉。如果您有更优雅的方式来实现功能，我会全力以赴。任何建议将不胜感激。

Answer 1

swscale 需要一个二维数组。那是指向指针数组的指针。每个指针指向图像的不同平面 (y,u,v)。您正在制作一个缓冲区并将指针传递给该缓冲区的指针。没有指向 swscale 的 U 和 V 平面的指针。因此，不好的指针。

Answer 2

事实证明，我的输入和输出缓冲区都是错误的，它们都需要是@szatmary 提到的二维数组。这是工作代码func Scale(img []byte, outw, outh int) []byte {    input, _, _ := image.Decode(bytes.NewReader(img))    if a, ok := input.(*image.YCbCr); ok {        width, height := a.Rect.Dx(), a.Rect.Dy()        var format C.enum_AVPixelFormat = C.AV_PIX_FMT_YUV420P        context := C.sws_getContext(C.int(width), C.int(height), format, C.int(outw), C.int(outh), 0, C.int(0x10), nil, nil, nil)        y := (*C.uint8_t)(C.malloc(C.ulong(len(a.Y))))        C.memcpy(unsafe.Pointer(y), unsafe.Pointer(&a.Y[0]), (C.size_t)(len(a.Y)))        cb := (*C.uint8_t)(C.malloc(C.ulong(len(a.Cb))))        C.memcpy(unsafe.Pointer(cb), unsafe.Pointer(&a.Cb[0]), (C.size_t)(len(a.Cb)))        cr := (*C.uint8_t)(C.malloc(C.ulong(len(a.Cr))))        C.memcpy(unsafe.Pointer(cr), unsafe.Pointer(&a.Cr[0]), (C.size_t)(len(a.Cr)))        in := []*C.uint8_t{y, cb, cr}        stride := []C.int{C.int(a.YStride), C.int(a.CStride), C.int(a.CStride), 0}        outstride := []C.int{C.int(outw), C.int(outw / 2), C.int(outw / 2), 0}        paneSize := outw * outh        a := (*C.uint8_t)(C.malloc(C.ulong(paneSize)))        b := (*C.uint8_t)(C.malloc(C.ulong(paneSize >> 2)))        c := (*C.uint8_t)(C.malloc(C.ulong(paneSize >> 2)))        out := []*C.uint8_t{a, b, c}        C.sws_scale(context, (**C.uint8_t)(unsafe.Pointer(&in[0])), (*C.int)(&stride[0]), 0,            C.int(height), (**C.uint8_t)(unsafe.Pointer(&out[0])), (*C.int)(&outstride[0]))        min := image.Point{0, 0}        max := image.Point{outw, outh}        output := image.NewYCbCr(image.Rectangle{Min: min, Max: max}, image.YCbCrSubsampleRatio420)        C.memcpy(unsafe.Pointer(&output.Y[0]), unsafe.Pointer(a), (C.size_t)(paneSize))        C.memcpy(unsafe.Pointer(&output.Cb[0]), unsafe.Pointer(b), (C.size_t)(paneSize>>2))        C.memcpy(unsafe.Pointer(&output.Cr[0]), unsafe.Pointer(c), (C.size_t)(paneSize>>2))        opt := jpeg.Options{            Quality: 75,        }        var buf bytes.Buffer        w := bufio.NewWriter(&buf)        jpeg.Encode(w, output, &opt)        return buf.Bytes()    }    return nil}