CODEWITHSUNDEEP
imagec#-4.0image-processingdirectshowdirectshow.net
dan-
dan-

Reputation: 301

Where does directshow get image dimensions from?

We are using a directshow interface to capture images from a video stream. These images are presented in a fixed size window.

Once we have captured an image we store it as a bitmap. Downstream we have the ability to add annotation to the image, for example letters in a fixed size font.

In one of our desktop environments, the annotation has started appearing at half the size that it normally appears at. This implies that the image we are merging the text onto has dimensions that are maybe twice as large.

The system that this happens on is a shared resource as in some unknown individual has installed software on the system that differs from our baseline.

We have two approaches - the 1st is to reimage the system to get our default text size behaviour back. The 2nd is to figure out how directshow manages image dimensions so that we can set the scaling on the image correctly.

A survey of the directshow literature indicates that the above is not a trivial task. The original work was done by another team that did not document what they did. Can anybody point us in the direction of what directshow object we want to deal with to properly size the sampled image?

Upvotes: 1

Views: 1209

Answers (2)

CPlusSharp
CPlusSharp

Reputation: 891

When filters connect in DirectShow, they have an AM_MEDIA_TYPE. Here you will find a VIDEOINFOHEADER with a BITMAPINFOHEADER and this header has a biWidth and biHeight. Try to build the FilterGraph manually (with GraphEdit or GraphStudioNext) and inspect these fields.

Upvotes: 2

Roman Ryltsov
Roman Ryltsov

Reputation: 69632

DirectShow - as a framework - does not deal with resolutions directly. Your video source (such as capture hardware) is capable of providing video feed in certain resolution which you possibly can change. You normally use IAMStreamConfig as described in Configure the Video Output Format in order to choose capture resolution.

Sometimes you cannot affect capture resolution and you need to resample the image in whatever dimensions you captured it. There is no stock filter for this, however Media Foundation provides a suitable Video Resizer DSP which does most of the task. Unfortunately it does not fit DirectShow pipeline smoothly, so you need fitting and/or custom filter for resizing.

Upvotes: 2

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