HDR - Church

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    Merging Photos to an HDR Image

    Creating an HDR image in Photoshop is very simple:

    y Click Merge to HDR... in the menu (File > Automate > Merge to HDR...)y Click Browse button and select all the photos you want to use (at least two). The

    photos can be for example in JPEG or RAW format.

    y Click OK and Photoshop merges the photos automatically. This operation might take

    some time If you are merging many high resolution raw photos. (According to my

    experience it can take -3 hours when Intel Core 2 Duo 1.8GHz merges nine 14-bit

    12 megapixel raw photos.)

    When the merging is done you see the preview window like in picture 2. On the left

    side you see the original photos, in the middle you see the HDR photo and on the right

    side you see the histogram. The red ticks in the histogram represent EV steps. Normal

    digital camera is able to capture 6-8 EVs of dynamic range. My HDR photo covers a

    dynamic range of almost 10EVs (picture 2). So there is more dynamic range than a

    single shot could capture. In my example the HDR image is made from 9 shots with

    1EV intervals. Some might think that 9 exposures is ridiculously too much. It's true that

    the dynamic range of 10EVs could be captured even with just two shots but the result

    would be of lower quality. I don't know the merging algorithms of HDR programs but

    presumably they take best parts of the photos and / or do some averaging between the

    pixels in different shots and therefore noise is reduced effectively if there are several

    exposures. So several photos with small exposure intervals is better than a few photos

    with large exposure intervals (at least what comes to noise). My example photo is noise

    free even in the darkest shadows.

    (One might wonder, what is the meaning of the slider below the histogram. That's just

    for viewing the HDR image at different exposures. All the HDR image data remains

    intact while using the slider.)

    Saving the HDR Image

    After clicking OK in the HDR tool, the image is normally opened in Photoshop. At first it

    should be saved. An HDR photo can be saved for example to PSD or OpenEXR format.

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    I personally use OpenEXR because it can be opened in several other programs (such

    as Photomatix) that support HDR imaging.

    Picture 2. Merge to HDR tool in Adobe Photoshop.

    Original HDR Image in Photoshop

    Inpicture 3 you see how the original HDR image looks like in Photoshop. Let's examine

    it a little and set some goals. There are obviously several problems: yellow cast,

    perspective distortion, some barrel distortion, and distracting elements on the left side.

    My goal is to fix all these and create a symmetrical and simple high contrast photo with

    completely black sky.

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    Picture 3. The original unprocessed HDR image in Photoshop.

    Basic Clean Up

    First, let's do some basic clean up to fix some problem areas. I adjusted the exposure

    slider (in the bottom of the HDR image) about +4EV to see the problem areas in the sky.

    My goal is to have completely black sky. If you look at the original image on the left

    (picture 4), you see several lens reflections and a few stars. I use Clone Stamp Tool

    and other copying methods to get rid of these (see the picture on the right). After clean

    up there are still some large faded lens reflections in the sky but these will be darkened

    to pure black later in the tone mapping process.

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    Picture 4. Basic clean up in Photoshop.

    Perspective Correction

    The next step is perspective correction. First I turn Grid on (View > Show > Grid) to be

    my guide. Then I use Perspective Tool (Edit > Transform > Perspective) to fix the

    perspective distortion. After that I notice that there is unfortunately some barrel

    distortion. Barrel distortion is somewhat tricky to fix but doable with careful use of the

    Warp Tool (Edit > Transform > Warp). To keep the maximum image quality, It's best to

    commit all transform operations at once. Inpicture 5you see the image after

    perspective correction.

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    Picture 5. The perspective is corrected.

    Removing Distractions and Creating Symmetry

    There are some distracting elements in the lower left corner ofpicture 5. Next I'll use

    Clone Stamp Tool and other copying methods to fix that area. Now the photo is cleaner

    and the effect of symmetry is stronger (picture 6).

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    Picture 6. Symmetrical and simple.

    Removing Yellow / Red Cast

    Now is the time to remove the yellow / red cast from the HDR photo. I do that by

    applying Hue / Saturation adjustment filter. I decrease the saturation of reds and yellows

    (picture 7).

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    Picture 7. Yellow cast has been removed with the Hue / Saturation tool.

    More Light and Less Light

    There are some very dark areas (top of the domes and stairs) and some very bright

    areas (light poles). I can predict that these are going to problems in the tone mapping

    process. For example, in the tone mapping process, when I'd be adjusting the toning

    curve to darken the sky, I'd spoil the domes and the stairs. Similarly when I'd be

    brightening the church, I'd spoil the lights. The solution to my problem is local exposure

    adjustment that can be done manually:More Light to Domes and Stairs

    y I apply Exposure (+1,5) adjustment layer to make the domes and stairs bright

    enough

    y I fill the mask of the adjustment layer with black color (to remove the effect from the

    whole image)

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    y I choose white color and paint the mask on areas I want to be brighter (domes and

    stairs)

    Less Light to Light Poles

    y I apply Exposure (-3) adjustment layer to make the lights dimmer

    y I fill the mask of the adjustment layer with black color (to remove the effect from the

    whole image)

    y I choose white color and paint the mask on areas I want to be darker (domes and

    stairs)

    Inpicture 8you can see the image after exposure adjustments. The domes and stairs

    are brighter and the lights are dimmer. This is the beauty of properly made HDR photos.

    I can apply +1.5 exposure adjustment to dark areas without revealing any noise!

    (One tone mapping method is to expose the whole image manually and then just do the

    actual tone mapping with Exposure and Gamma method. However, I won't need more

    manual exposure control with my example photo.)

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    Picture 8. Domes and stairs are made brighter and lights are made dimmer.

    Tone Mapping

    Now the image is ready for tone mapping. I did a lot of Photoshop processing before the

    actual tone mapping. The reason is the fact that when working on HDR photo, one can

    execute radical color correction operations without losing any information (because of

    the high dynamic range). I recommend doing all color correction before tone mapping

    and also to save the image just before tone mapping.

    In Photoshop, the tone mapping process is started by converting the image mode to 8-

    bit or 16-bit:

    y

    I click: Image > Mode > 16B

    its/Channel. This opens Photoshop's HDR Conversiontool.

    y There are four different tone mapping methods available in Photoshop. I choose

    Local Adaptation method because that's the only one that let's me manually adjust

    the toning curve (I click on the small arrow to see the toning curve).

    Inpicture 9 you see the HDR image with the default settings of Local Adaptation

    method.

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    Picture 9. The default settings of local adaptation.

    Local Adaptation and the Toning Curve

    Take a look atpicture 10. On the left you see the default toning curve which

    producespicture 9 and on the right you see adjusted toning curve that

    producespicture 11. How to modify the toning curve? Well let's inspect the histogram

    for a while. The steep mountain on the left represents the pixels of the sky and the

    mountains in the middle represents the pixels of the church. I want to achieve two

    things: black sky and good contrast to the church. The curve on the right does just that.

    All the pixels on the left side of the left control point become completely black. The right

    control point is moved to the left just where the "mountains" start. This means that the

    brightest pixels of the HDR photo will become almost white. These two adjustments also

    make the curve really steep which equals to high contrast.

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    Picture 10. On the left: The default toning curve. On the right: Toning curve adjusted for

    high contrast.

    Inpicture 11 you see the the tone mapped image. Notice that at this stage the photo

    isn't an HDR image anymore. It's just a 16-bit image.

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    Picture 11. 16-bit image after Photoshop HDR conversion.

    Sharpening

    The last thing to do is sharpening. I convert the image to a smart object and apply

    Smart Sharpen filter to it. Inpicture 12you see the final sharpened photo.

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    Picture 12. The Final sharpened photo.