2 projectors, 1 wide image

[Ronald Murphy]

I've never done this, but I believe it is commonly done. I have two 1280 X 1024 projectors projecting side-by side images for a total of 2560 X 1024. I want to create 2560 X 1024 video and display it across both screens simultaneously. What is my approach? Do I create one 2560 X 1024 surface and assign a projector to each side? What about some pixel overlap? How do I direct the left half of the video to proj 1 and the right half to proj 2? I have a Matrox TH2G, with one output already used by a third independent projector. I will create a separate surface for it. Any advice would be much appreciated. 

[fishmonkey]

to achieve a 2560 x 1024 image you would have to precisely abutt the images of the two projectors, however in practice this is nearly impossible to achieve. even if you could manage to to perfectly align the two projected images, then ideally you would also want the colour balance, brightness, contrast, etc. of each projector to be the very similar; with two images hard up against each other, any differences between the projectors is thrown into sharp relief.

because of these issues, edge-blending is commonly used when combining the output of multiple projectors. here you overlap the images, and use the overlapping area to allow a more gradual transition between the projectors. you sacrifice some resolution to gain a more seamless transition. there is a good explanation here:

https://www.immersaview.com/resources/edge-blending/

it's pretty straightforward to do this in QLab 3. in your case, you will want to create a surface that is a bit smaller horizontally than the combination of the two projectors to allow for the overlap. the amount of overlap you need varies but i think it's usually in the range of 100–300 pixels. the number of pixels may also depend on the final aspect ratio that you are trying to achieve (in one show i used a 400 pixel overlap as i did not need any more horizontal resolution anyway). maybe someone else with more experience can clarify this further. once you have your surface, add your two projectors, and tile them with the overlap. in the bottom left corner of the surface editor window you will see a check box for edge blending (which i think defaults to on in this situation). the other important control is the blend gamma in the top right corner of the surface editor, which you can try tweaking if you find you have a dark or bright patch where the blend is (when displaying content, see below about projector black).

once you have your surface set up, you simply point your video cues at that surface. QLab takes care of splitting the image and blending it across the projectors.

i usually use the QLab grids to align the projectors with the blend. when you have them in good alignment the fine grid lines will be visible. after that i check with my content. in my experience the most difficult stuff to blend perfectly is text, as it has so many fine curves and edges.

if you need to do a quick re-alignment before a show and don't have time to mess with the actual projectors, you can try using the corner pins of the projectors in the surface editor.

oh, and one other issue with edge-blending to be aware of is the effect it has on projector black. if you do not have shutters or dowsers and you have very dark scenes in your show without video, then you may find that the blend area is visible because you have twice as much projector black hitting that area of the screen. for obvious reasons you cannot blend the projector black. this issue will be exacerbated if your projectors are mounted relatively close to the projection surface...

[Ronald Murphy]

Thank you for a thorough response! Sounds like if I use 200 px overlap, I should make my video 2360 wide rather than the full 2560 wide. I'll try this tonight!

---

From: ql...@googlegroups.com <ql...@googlegroups.com> on behalf of fishmonkey <fishmo...@gmail.com>
Sent: Wednesday, March 23, 2016 10:31 PM
To: QLab
Subject: [QLab] Re: 2 projectors, 1 wide image

 

to achieve a 2560 x 1024 image you would have to precisely abutt the images of the two projectors, however in practice this is nearly impossible to achieve. even if you could manage to to perfectly align the two projected images, then ideally you would also want the colour balance, brightness, contrast, etc. of each projector to be the very similar; with two images hard up against each other, any differences between the projectors is thrown into sharp relief.

because of these issues, edge-blending is commonly used when combining the output of multiple projectors. here you overlap the images, and use the overlapping area to allow a more gradual transition between the projectors. you sacrifice some resolution to gain a more seamless transition. there is a good explanation here:

https://www.immersaview.com/resources/edge-blending/

it's pretty straightforward to do this in QLab 3. in your case, you will want to create a surface that is a bit smaller horizontally than the combination of the two projectors to allow for the overlap. the amount of overlap you need varies but i think it's usually in the range of 100–300 pixels. the number of pixels may also depend on the final aspect ratio that you are trying to achieve (in one show i used a 400 pixel overlap as i did not need any more horizontal resolution anyway). maybe someone else with more experience can clarify this further. once you have your surface, add your two projectors, and tile them with the overlap. in the bottom left corner of the surface editor window you will see a check box for edge blending (which i think defaults to on in this situation). the other important control is the blend gamma in the top right corner of the surface editor, which you can try tweaking if you find you have a dark or bright patch where the blend is (when display content, see below about projector black).

once you have your surface set up, you simply point your video cues at that surface. QLab takes care of splitting the image and blending it across the projectors.

i usually use the QLab grids to align the projectors with the blend. when you have them in good alignment the fine grid lines will be visible. after that i check with my content. in my experience the most difficult stuff to blend perfectly is text, as it has so many fine curves and edges.

if you need to do a quick re-alignment before a show and don't have time to mess with the actual projectors, you can try using the corner pins of the projectors in the surface editor.

oh, and one other issue with edge-blending to be aware of is the effect it has on projector black. if you do not have shutters or dowsers and you have very dark scenes in your show without video, then you may find that the blend area is visible because you have twice as much projector black hitting that area of the screen. for obvious reasons you cannot blend the projector black. this issue will be exacerbated if your projectors are mounted relatively close to the projection surface...


On Thursday, March 24, 2016 at 4:37:53 AM UTC+11, Ronald Murphy wrote:

I've never done this, but I believe it is commonly done. I have two 1280 X 1024 projectors projecting side-by side images for a total of 2560 X 1024. I want to create 2560 X 1024 video and display it across both screens simultaneously. What is my approach? Do I create one 2560 X 1024 surface and assign a projector to each side? What about some pixel overlap? How do I direct the left half of the video to proj 1 and the right half to proj 2? I have a Matrox TH2G, with one output already used by a third independent projector. I will create a separate surface for it. Any advice would be much appreciated. 

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[Greg Leeper]

Just a quick add to the last reply- usually the blend overlap is a percentage between 10-20%.

You'll find 3 chip DLP projectors handle hiding the overlap best, then single chip DLP, then LCD. Color accuracy and black level are what make a difference.

I wouldn't worry so much about matching your media size pixel for pixel to the blended region. Make it the best resolution you have based on your sources then scale to fit in Qlab.

A great tool for playing with and calculating overlap is Blendz, both a desktop & iOs app.

[Ronald Murphy]

Thank you!

Ron

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From: ql...@googlegroups.com <ql...@googlegroups.com> on behalf of Greg Leeper <gr...@opticusvideo.com>
Sent: Friday, March 25, 2016 11:20 AM
To: QLab
Subject: [QLab] 2 projectors, 1 wide image

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