GstBaseTransform's prepare_output_buffer and occasional passthrough

[Carlos Rafael Giani]

Hello,

in gstreamer-imx I have a "imxvpuvideotransform" element. This element 
uses the i.MX6 IPU for rotations, colorspace conversions, scaling, and 
deinterlacing. All of these operations can be done in one step.

As an optimization, I check if a frame really needs to be transformed, 
or if it can be passed through. Cases where pass through is possible:
- input and output width/height/pixel format are the same
- video cropping is enabled, but the crop rectangle contains the entire 
frame (thus not actually cropping anything)
- input frame is not interlaced
- input frame is interlaced, but no deinterlacing will be performed
- rotation is disabled

Since rotation can be set at any time (it is a property), video crop 
rectangles can in theory change during playback, and deinterlacing might 
be required only for some frames in mixed mode, I need to evaluate this 
per-frame. I perform the evaluation in the prepare_output_buffer() function.

Initially I called gst_base_transform_set_passthrough() in 
prepare_output_buffer(). However, this lead to problems in certain 
situations, such as when playbin tries to play next media. For this 
reason, I now call gst_base_transform_set_passthrough() once, during 
initialization, and set GstBaseTransform's passthrough flag to FALSE. I 
handle passthrough manually by setting the output buffer to the input 
buffer in prepare_output_buffer() if passthrough is required. 
transform() is still called, but does nothing then.


Here is the relevant part in the code: 
https://github.com/Freescale/gstreamer-imx/blob/master/src/ipu/videotransform/videotransform.c#L1295


My question now is: is this an acceptable solution? The documentation 
neither confirms nor denies this. It states: "Elements that only do 
analysis can return a subbuffer or even just return a reference to the 
input buffer (if in passthrough mode)." What if I do *outbuf = input; if 
I set GstBaseTransform's passthrough to FALSE?

It works right now. I want to make sure this isn't just a fluke, and in 
some future GStreamer version, somebody puts an assert that fires in 
this case.

cheers
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