[Taconut]

Hi! I'm using Kodi with a projector, and the surface I'm projecting on is at a weird angle. I've been able to use the overscan calibration to correct most of this, but the screen still appears as a trapezoid on the screen. I'd like to stretch the top two corners of the screen to correct for this distortion.
I'm almost positive I could code in my own support for this distortion using OpenGL, but I'm not sure where or how I would start. Can anyone at least give me a push in the right direction?

Thanks!

[Memphiz]

Isn't the projector supposed to have an optical correction for that in the lense adjustment?
Doing this correction in software is not what you want - i am sure. Just imagine how the image would look (upper half interpolated and lower half downscaled).

[jjd-uk]

Yep any decent projector should have keystone adjustment built in

[Taconut]

Unfortunately my projector doesn't have keystone correction. Also, it would be nice to extend one corner by a different amount than another.

Memphiz, I'm 100% sure this will look nice. I've tried it under windows using a projection mapping program and it looks perfectly fine. You'd think it would be gross, but no matter how hard I look, I see no issue.

[skylarking]

Positioning the projector so it is horizontally centered on the screen, which is usually easy to accomplish in practice, removes horizontal keystone issues (side to side differences). Positioning the projector so it is vertically centered on the screen, which is also usually easy to accomplish in practice, removes vertical keystone issues (top to bottom differences). Such projector placement issues are usually easy to accomplish in a dedicated theatre room but less so in other multi use rooms.

IMO, removing keystone issues at the source via correct projector placement provides for the best picture quality.

However, many people simply do not want their projector hanging too far from their ceiling, so placement of the projector to remove vertical keystone may be more difficult dur to asthetic reasons. Just as in other areas, it seems "form over function" can also win out in home theatre dispite some people paying big $$ for projectors, ceiling mounts, screens, etc But that's why all projectors i have seen have some form of vertical keystone correction built in, either via lens shift (better method) or via software settings within the GUI (quality depends on algorithms & processing power). More costly projectors add horisontal keystone adjustment capabilities to their verical keystone adjustment capability, again via either lens shift or via software settings within the GUI or both.

Even though keystone correction within Kodi could be a cool feature, i'd guess it would cater to a very small user base and not that usefull to most.
Where within Kodi you would need to look, to have code process/scale video, i have no idea since i know nothing about the source code and the video pipeline
Hopefully others can provide more clues which module such a feature could reside within the sourc code.

But the question in my mind is, why can't you better place your projector to remove much of the keystone distortion in the first instance?
What projector do you have (that does not have any form of keystone adjustment) so others can take note and avoid purchasing such a hobbled product?

[edited to add following]
Just to clarify, to avoid keystone distortion, the projector should be perpendicular to the screen when centered horizontally and vertically.

[Taconut]

Skylarking,

Thanks for explaining it. You did a much better job describing my issue than I could have.

The projector is a Brookstone HDMI Pocket Projector Mobile (M#: 890574) that I won in a contest. It gets the job done for everything but keystoning and you really can't beat free.

My projector is horizontally centered, but the wall I'd like to project on is at a 45-degree angle directly above my bed. So, in order to position it correctly, the projector would have to occupy an area already occupied by the bed (or my head).

If I could only inject a shader into Kodi, I'd be all set. I've already built a calibration tool that generates the homography matrix.

[skylarking]

I updated my post to clarify the projector should be perpendicular to the screen when centered horizontally and vertically to avoid keystone distortion

In your case, since the screen is at 45 degrees, and presumably sloping down from your head towards your feet, you will never be able to vertically center the projector to the screen at anywhere near 90 degrees.
As such your vertical keystone distortion would be rather bad
I'd think even a high end projector would not correct such large amount of vertical distortion
Projector placement is key but in your case it seems elusive

Maybe you can place it on a hostital style bed table that you can place the projector and kodi box on and slide into the middle to the bed when watching a movie
Unfortunately free in $ can sometimes cost more in frustration and spent time

As mentioned, what code would be needed and where to place it within the code base is beyond my knowledge.
Maybe others could help with some advice on that front

[Memphiz]

Thats another try to squeeze a solution out of kodi because of a poor equipment situation. Granted you won the projector and didn't intent to have one in the first place i guess. But projecting at a 45 degree wall is so special. Distortion from the projection is one thing. Your viewing angel adds to it too i think. Any chance you can place the thingy below your bed at 45 degree angel?

[Taconut]

Well, to be honest, I'm kind of doing what the two of you said right now. My projector is at a 45-degree angle. But the top two corners are still pushed inwards. If I could just push them out a bit everything would be perfect.

[lazzarus]

you ever find a solution for this? I'm looking as well.
so far, I came up with only one solution that involves an old version of mpc-hc and ffdshow
sadly the older version of mpc-hc doesn't support 5.1 lpcm hdmi output