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Step 3C. Implement the CheckTransform Method

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

This is step 3C of the tutorial Writing Transform Filters.

Note

This step is not required for filters that derive from CTransInPlaceFilter.

 

The CTransformFilter::CheckTransform method checks if a proposed output type is compatible with the current input type. The method is also called if the input pin reconnects after the output pin connects.

The following example verifies whether the format is RLE8 video; the image dimensions match the input format; and the palette entries are the same. It also rejects source and target rectangles that do not match the image size.

HRESULT CRleFilter::CheckTransform(
    const CMediaType *mtIn, const CMediaType *mtOut)
{
    // Check the major type.
    if (mtOut->majortype != MEDIATYPE_Video)
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }

    // Check the subtype and format type.
    FOURCCMap fccMap = FCC('MRLE'); 
    if (mtOut->subtype != static_cast<GUID>(fccMap))
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }
    if ((mtOut->formattype != FORMAT_VideoInfo) || 
        (mtOut->cbFormat < sizeof(VIDEOINFOHEADER)))
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }

    // Compare the bitmap information against the input type.
    ASSERT(mtIn->formattype == FORMAT_VideoInfo);
    BITMAPINFOHEADER *pBmiOut = HEADER(mtOut->pbFormat);
    BITMAPINFOHEADER *pBmiIn = HEADER(mtIn->pbFormat);
    if ((pBmiOut->biPlanes != 1) ||
        (pBmiOut->biBitCount != 8) ||
        (pBmiOut->biCompression != BI_RLE8) ||
        (pBmiOut->biWidth != pBmiIn->biWidth) ||
        (pBmiOut->biHeight != pBmiIn->biHeight))
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }

    // Compare source and target rectangles.
    RECT rcImg;
    SetRect(&rcImg, 0, 0, pBmiIn->biWidth, pBmiIn->biHeight);
    RECT *prcSrc = &((VIDEOINFOHEADER*)(mtIn->pbFormat))->rcSource;
    RECT *prcTarget = &((VIDEOINFOHEADER*)(mtOut->pbFormat))->rcTarget;
    if (!IsRectEmpty(prcSrc) && !EqualRect(prcSrc, &rcImg))
    {
        return VFW_E_INVALIDMEDIATYPE;
    }
    if (!IsRectEmpty(prcTarget) && !EqualRect(prcTarget, &rcImg))
    {
        return VFW_E_INVALIDMEDIATYPE;
    }

    // Check the palette table.
    if (pBmiOut->biClrUsed != pBmiIn->biClrUsed)
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }
    DWORD cbPalette = pBmiOut->biClrUsed * sizeof(RGBQUAD);
    if (mtOut->cbFormat < sizeof(VIDEOINFOHEADER) + cbPalette)
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }
    if (0 != memcmp(pBmiOut + 1, pBmiIn + 1, cbPalette))
    {
        return VFW_E_TYPE_NOT_ACCEPTED;
    }

    // Everything is good.
    return S_OK;
}

Pin Reconnections

Applications can disconnect and reconnect pins. Suppose an application connects both pins, disconnects the input pin, and then reconnects the input pin using a new image size. In that case, CheckTransform fails because the dimensions of the image no longer match. This behavior is reasonable, although the filter could also try reconnecting the output pin with a new media type.

Next: Step 4. Set Allocator Properties.

Reconnecting Pins

Source and Target Rectangles in Video Renderers

Writing DirectShow Filters