TLDR: Not JPGs, BMPs more likely.

I recall when the first LCD TVs came out and I wanted to get a cheap 1080 one. The problem was that almost every TV said 1080 even if it was just 720 upscaled.

I put a uniform image in BMP on a memory stick - every column being R, then G, then B repeated. As the image was exactly 1080, viewing it in full-screen on a 1080 screen gave a sort of uniform grey color. On an upscaled monitor you could see very visible banding.

This would not have worked had the image been JPG.

▲

rahimnathwani a day ago | parent | next [-]

This would also help to detect overscan. I remember when I was shopping for my first 1080p TV, many (most?) had overscan and no way to turn it off.

▲

jonathanlydall a day ago | parent [-]

Overscan is where it cuts off the edges of the image right?

I’m sure an LLM could offer an explanation, but I genuinely have never understood why overscan is even a thing on HDMI ports.

Most times I plug my AppleTV or laptop into a TV using HDMI by default it’s cutting off the edge of the image. When is it ever a good thing to cut off the edges of the incoming digital image and why does this seem to be the default behaviour on most TVs?

Edit: I asked an LLM and it says some content includes junk on the edge of images because it was never meant to be visible, so TVs enable it by default to cut down complaints/support requests. Apparently, even though HDMI supports a way to signal to TVs to disable overscan, many TVs ignore it.

I’m in my 40’s and have yet to personally encounter content where overscan should be used.

Maybe there were particularly crappy and prevalent analogue to digital converters in the early days of HDMI, but TV manufactures just never stopped doing it even though it’s almost never an issue any more. So we probably have a situation now where probably everyone has either manually changed their TV from its default or more likely are seeing an over scanned and thus non-pixel aligned image.

	▲	Kirby64 20 hours ago \| parent \| next [-]
		> I genuinely have never understood why overscan is even a thing on HDMI ports. Remember that an HDMI port may be an input for another device where overscan is necessary to prevent the “garbage” at the edges of the screen. An example of this would be a DVR that outputs HDMI to your TV but perhaps records video that needs to have overscan set. Or VHS as an input. Those are just examples.
	▲	phate 15 hours ago \| parent \| prev \| next [-]
		I'm also in my 40's and I haven't seen a reason for a TV to overscan in AGES. I only dealt with overscan when it was with old Analog signals like from an old VCR where timecode and close captioning data is on top most lines of the NTSC signal or when using a really old VGA to RCA adapter where you'd want the over scan to "stretch" your computer's desktop to fit the screen. I think your assessment is right, it's a hold over from before the Analog->Digital conversion and there hasn't been a big enough complaint to remove it.
	▲	rahimnathwani a day ago \| parent \| prev [-]
		Maybe having some image processing circuitry that deals with both the analogue-originated signals and the HDMI-originated signals reduced cost and/or time-to-market.

▲

Sesse__ a day ago | parent | prev [-]

Why do you think it wouldn't work for a JPEG? I just made one like that, and it worked just fine.

▲

lelanthran a day ago | parent [-]

> Why do you think it wouldn't work for a JPEG? I just made one like that, and it worked just fine.

JPGs are lossy encodings; my use-case would definitely not work with JPGs, not matter how high you put the quality.

For testing a display (like displaymate does) you literally want to light up specific pixels with specific colors. You can't do that with JPGs.

▲

derefr a day ago | parent [-]

You can do that with JPEGs, just not the obvious way (i.e. by exporting the JPEG at the target resolution.)

Instead, you need to export the JPEG at a resampled resolution that's a multiple of your target resolution, such that each pre-transform source-image pixel gets mapped to its own entirely-independent JPEG color block.

Most obvious (though perhaps not optimal?) approach: nearest-neighbour upscale your image by 8x, and then save as JPEG with 100% quality (which will create 8x8 blocks with 4:4:4 subsampling.)

	▲	lelanthran 7 hours ago \| parent [-]
		> You can do that with JPEGs, just not the obvious way (i.e. by exporting the JPEG at the target resolution.) Yeah, I don't think GP did it that way.