Stitch the HDR images

The dilemma: HDR or panorama first?

Now we are at a crossroad with three options. We can make HDRs out of our single images and stitch the HDRs, or make 9 panoramas for each exposure and merge them to HDR. The last option is to import all LDR images at once in your stitcher, and directly stitch a HDR panorama. I will explain the first option as this allows for the most adjustment and corrections afterwards, and it makes it alot easier to remove the ugly tripod we took an extra shot for.

  • I like to use Photomatix to batch process all the single images to HDRs, it’s a real time-saver. It lets you specify how many images make up a HDR image (in this case 9), and works its way through an entire folder until all images are merged to HDR. Make sure to uncheck the align feature, and check the average feature. This keeps all HDRs at the same exposure level instead of optimizing it for display on a LDR monitor. You will have darker nadir shots if you don’t average.


Importing the images in the stitching software

You might have to rotate the images to portrait orientation, so the stitcher does not give an error. My favorite stitcher is PTGui, because it allows for extensive manual adjustments, as well as a 3-clicks-of-a-mouse-workflow. It’s as complicated as you want to. Let’s start with importing the images to PTGui. When you have a modest computer, you might want to stitch with the median exposure JPGs first, then save it as a template and apply this to the HDR images. Warping 32-bit HDR images real-time can be pretty demanding on your system. I will continue my story using HDR images.

  • When the HDR images are imported, you will see a popup screen asking for lens and sensor parameters (crop factor or sensor size). If you don’t know these you have to import a single JPG first, and note the values shown in PTGui because with the JPG, these values are stored within the EXIF data and displayed on your screen.


Preparing the stitch

  • When all HDR images are loaded in, click “Align Images” and let the computer generate the control points. These control points are generated in the overlapping parts of the images, and they link the images together. Be careful with repeating patters like a paved floor, the control points are calculated by color values, and not by counting the tiles.
  • Now click on Tools - Control Point Table, or the small table icon in the toolbar, or press Alt-B to open the Comtrol Point Table. Here we can delete the control points exceeding the value of 4. Don’t worry if this is half of your total control points, we’ve got enough.

    

 

  • Click the Advanced button in the upper right corner to unhide the advanced controls and click the Optimizer tab, select "Medium (recommended)" from the "Minimize lens distortion" pulldown menu and optimize the project.

 

  • If you have any stitching errors, or un-aligned images like shown a bit down the page in the before-after window, you have to find out which images go wrong and add some manual control points in the Control Points tab. in the Control Points tab we have two small screens for the linked images. when we start with image 0 in the left screen, we can see that on the other screen, image number 1 and 7 are bold. this means that image 0 is linked to image 1 and 7 by control points. when we select image 1 on the right screen, we can see image 2 and 0 are bold now in the left screen, and we can see the control points generated by PTGui. we search this way until we find an image with no control points.

    

 

  • When we arrive at image 8, we see it has no control points, its because the image is almost purely blue. Let's fix the sky! Select image 0 in the left, and 8 in the right screen. on the lower left corner of image 8 we can see the top of the red tree, search a recognisable shape we can link to the same tree in image 0. when we click in the right screen and hold the mouse-button. the mouse slows down automatically so we can fine-adjust the position of this control point. Now create a control point in the same way in image 0 to link the images together. Do the same with image 4 and 8. you can use the middle of the sun as target. be sure to be as accurate as possible. every image needs at the very least 2 control points for alignment. since the sky is uniform in color a small shift won't be visible. when you need more accuracy, you will need 3 control points per image at the least.

 

  • Now when we optimize again after we've added the control points, we will see the sky is in its usual position: at the top. Below is the result of our added control points.

 

  • Now there is only the floorshot (nadir) left to be fixed. Select image 9 and 10 in the Control Point tab screens. Lets add some control points. Remember the tripod is rotated 180 degrees in these 2 images, so don't put control points on the tripod or shadows! I've used the stones at the side of the road, they have a nice and even spacing where we can place the control points in the opening between them.

 

  • Lets see what the newly added control points do when they are optimized, got to the optimizer tab and hit optimize!

 

  • In this case this may work fine, because the asphalt has an overall colour and no clear texture. If there would be pavement for example, the stitch would most likely be shifted. This is because the two bottom shots were not shot from the same point of view as all the other images, wich the software assumes. We have to tell PTGui to shift these viewpoints of the floorshots, wich is done also in the Optimizer Tab, only in the advanced section. If we open this up wee see the possibility to align only the two floorshots by viewpoint shift. Check and uncheck the options as shown in the image below. If you optimize again now, the two floorshots will align much better.

 

As you see wee now have a pretty clean stitch, there are still some small misalignments like the telephone wire. They can be fixed with the further tweaking of control points, but the text would get kinda long. I guess by now you get the idea behind the control points and I would say you give it a go yourself! You will learn alot more by finding it out youself, then you would do when I talk you through every tiny bit of the process.

 

Leveling and centering your panorama

Regardless if you have leveled you panorama correctly before shooting, the stitcher can still distort the straight horizon. If this is the case we have to straighten the panorama by hand. This is done with control points also, only now we make 2 control points in the same image using Vertical Line control points. the lines between these 2 points are straightened out by the stitcher. You will get the best result if you use 2 images under a 40-130 degree angle to put your vertical lines in. Find an image with a straight vertical line like the edge of a house, or a lamppost, in our case the telephone pole in images 2 and 7 will have to do.

  • Under the Control Points tab, select image 2 in both screens. You can zoom in to get a closer view of the pole. Put one control point in the top of the pole, and in the other screen, add another control point at the bottom of the pole. When you click optimize again the result should improve. If not, you have to add another two vertical control points in another image. Just make sure not to use the opposite side of the panoramic view, it can have undesired results. When optimized now the panorama should be leveled. If there are no vertical lines available, you can also level manually by editing the yaw, pitch and roll values in the numeric transform screen found in the panorama editor.

 

  • In this same numerical transform screen you can center your panorama using the Yaw type-in field. fill in the amount of degrees it shoud be shifted, and click Apply to see if it is correct.

 

Stitching the panorama

  • Go to the “create panorama” tab, and click “Set optimum size” or enter a rounded value yourself. Choose your HDR output file type and specify a name for the panorama. Also make sure you both check the "Individual HDR layers" option, and the "HDR Panorama" option in the HDR output section. LDR output is useless. The single layers will be used to mask out the ugly tripod at the bottom in the next steps. Click "Create Panorama" and take a minute...

 

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