[x264-devel] Re: [patch] VUI extended corrected

[Christian Heine]

Hi,

Mike Matsnev wrote:
> Christian Heine wrote:
> 
>> Before we going on, lets recap, and find a common ground again. Here 
>> are my three statements.
>>
>> - SAR never changes since the time the image is digitised, unless the 
>> it is resized.
>> - The cropping rectange feature can be seen a superset of the overscan 
>> feature. Overscan effectively results in an additional cropping rect.
>> - DAR always refers to the image contained in the cropping rectangle, 
>> that results from both the original cropping rectangle and the 
>> overscan cropping.
>>
>> I admit that the phrase "use a different SAR" is not clear, it should 
>> read "specify a different SAR". But what we were really arguing about 
>> are statements 2 and 3. But the spec is clear about this.
>>
>> My point boils down to this: _If_ the DAR refers to the image 
>> contained in the cropping rectangle, and the cropping rectangle 
>> changes (because of the overscan) but DAR should stay the same, then 
>> SAR must have been different in the first place.
> 
> I think you misunderstand overscan. All the flag does it tell the 
> decoder/display
> that it's ok to hide some part of the picture, nothing is said about 
> changing SAR,
> etc. The typical use for this feature would be an STB connected to the 
> TV set. It
> could have two modes of operation. By default it outputs a normal
> signal to TV, so it overscans. In the other mode it scales down the 
> picture and adds
> black borders, so even with TV overscan the entire image is visible. But 
> this
> scaling does not change SAR and DAR in any way.

I never claimed that overscan changes SAR (see statement 1). But I claim 
that images that result from an analog transfer and have an overscan 
area (the size of the area is specific to the video standard and 
digitizing equipment used) have a different SAR than pictures without an 
overscan area with the same resolution and that should be presented with 
the same DAR. The overscan flag tells the display whether the "active" 
region is the whole image or some part of it. The "some part of it" 
depends on the video standard the digitizing equipment uses.

An image with resolution 1024x768 SAR 1:1 (=> DAR is 4:3) is scaled to 
720x576 SAR 48:45 (=> DAR is still 4:3).

An PAL signal with DAR 4:3 is sampled at 13.5MHz. The digitized image 
has a resolution of 720x576, where a rectangle of 702x576 corresponds to 
the "active" region which in turn corresponds to DAR 4:3. The SAR is 
128/117. The DAR of the whole image (720x576) is 160:117 (~1.3675).

At presentation time the display adds top and bottom borders until the 
DAR of the extended image matches the DAR of the display. It then scales 
it proportionally so that either the whole image as well as the top and 
bottom borders it added (underscan mode) or only a part of the image is 
visible (overscan mode). What that part is depends on the display. If 
the display conforms to the same standard the original signal had before 
digitizing it exactly matches the "active" region of the image.

Regards,
Christian Heine

-- 
This is the x264-devel mailing-list
To unsubscribe, go to: http://developers.videolan.org/lists.html