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天色轨迹-冷冰川的柴火间

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[资料]Combining 2 Different Exposures  

2008-04-19 21:18:14|  分类: 作为资料留着 |  标签: |举报 |字号 订阅

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Combining 2 Different Exposures   
To Hold Detail in Objects with a Large Brightness Range
With a Layer Mask in Photoshop

Some astronomical objects exhibit a tremendous brightness range that is difficult to capture on a single exposure.

One solution to this problem is to make two exposures, a long exposure for the faint details, and a short exposure for the bright detail, and composite the best parts of each image together.

M42 Short Exposure
M42 Long Exposure
M42 Composite
5 Min Exposure
+
50 Min Exposure
=
Composite

This problem is commonly encountered on objects like M42, the Orion Nebula. Its core is extremely bright, which is why we can see so much detail, and even some color visually. Yet it also contains beautiful nebulosity in the faint outlying areas of the loop to the south of the Trapezium, seen here at the top of the long exposure.

At left above is a single 5 minute exposure which records detail in the bright core of the Trapezium region. At center is a single 50 minute exposure with my Astro-Physics 130 EDT at f/8 with gas-hypersensitized Kodak PJM, which, at my site, is the sky-fog limit. At right is a composite of the two exposures.

In a long exposure that is made to capture the faintest details, the bright core will be so overexposed it will have no detail as in the 50 minute exposure. Short exposures can record detail in the brightest areas without overexposure, but obviously will record nothing in the faint areas that require an exposure to the sky fog limit.

Like just about everything else in Photoshop, there is more than one way to composite two images together to combine shadow and highlight detail. This method will use Layer Masks.

In this example, the long exposure will become the base image and the short exposure will become the "layer" on top of it.

The mask will block out everything on the base image except for the overexposed area in the long exposure and allow the correctly exposed core of the Trapezium from the short exposure to blend in perfectly.

The dark portion of the mask protects the image under it. Where the mask is totally black, no portion of the image on top gets through. Any light or white areas are unmasked and all of the image on top gets through. Think of the white in the mask as a hole that lets the image on top through. Any gray transitions from black to white mask the image in a gradation depending on the density of the mask.

The secret to getting the blend perfect is that we will make the mask from the overexposed portion of the long exposure itself, and we will be able watch the changes in the composite as we adjust the mask.




Summary of the Steps
  1. Align the images in Picture Window
  2. Open them in Photoshop
  3. Copy the short exposure to the clipboard
  4. Paste the short exposure on top of the long exposure in a layer
  5. Make a mask
  6. Paste the long exposure into the mask
  7. Open a new view of the image to see effects of manipulating the mask
  8. Blur the mask
  9. Adjust the density and contrast of the mask with curves
  10. Tweak the color of the core to match the rest of the image
  11. Flatten the image (combine the layers)
  12. Apply final color and density adjustments to the composite



Step 1 - Align the Two Images

Rotate and shift the scans of the images so that all of the stars are perfectly aligned. A method of accomplishing this in Photoshop is described in detail on my Aligning Negatives Digitally page.

However, it is much easier to align two negatives in Jonathan Sack's program Picture Window.

After the two original negatives are aligned, save each under a different file name such as M42_L.TIF and M42_S.TIF for long exposure and short exposure respectively. Make copies of these two files with the DUPLICATE command in Photoshop, put the originals away, and work on the copies. I would advise working on low resolution scans to start with so that things will go quickly and you can familiarize yourself with the steps in the process.

Both copies should have their basic levels set, see the page on Photoshop Enhancement Techniques.




Step 2 - Open both images in Photoshop

The following illustrations show what the two images and layers palette will look like in Photoshop during the various steps.

M42 Short Exposure M42 Long Exposure Layers Palette
Short Exposure
Long Exposure
Layers Palette

Open the short and long exposure copies in Photoshop and place the short exposure on the left and the long exposure on the right.

Open the Layers Palette using the menu commands WINDOW > SHOW LAYERS.

The layer's palette in Photoshop will allow you to composite two images together in various different ways with the mask.

Next we will place the long exposure on top of the short exposure.




Step 3 - Copy the short exposure

  • Click on the short exposure image M42_S.TIF.

  • SELECT > ALL

  • EDIT > COPY

This copies the short exposure to the clipboard in the computer's memory.




Step 4 - Paste the short exposure on top of the long exposure in a layer

  • Click on the long exposure image M42_L.TIF to make it active.

  • EDIT > PASTE

M42 Short Exposure M42 Long Exposure with Short Exposure pasted on top Layers Palette
Short Exposure
Short Exposure on top
of long exposure
Layers Palette

Photoshop will then paste the short exposure on top of the long exposure, but the bottom image (the long exposure) will not be visible. It is still there though, as evidenced by the thumbnail image in the Layers Palette background layer. It is just hidden by the image in the layer on top.

If you click on the icon of the eye in the layers palette, you can toggle the eye on and off, and this will toggle the display of that particular layer on and off in the image display window.




Step 5 - Make the Mask

  • Click on Layer 1, the top layer in the layers palette, to make it active. The background of layer 1 in the layers palette will turn dark blue to indicate that that is the layer that is active.

    Create a Mask
    The Layer Mask

  • Click the Mask Icon in the Layers Palette to add a layer mask to Layer 1. It is the little circle in the square at the bottom of the Layer's Palette.

  • Now you will see a white box appear in the top layer in the layer palette. This is where you will paste the mask. The mask is linked to the layer as indicated by the chain link icon between the thumbnail icon of the image in the layer, and the blank white thumbnail icon of the mask.




Step 6 - Paste the long exposure image into the mask

  • Click on the long exposure M42_L.TIF image to make it active.

  • Click on the background layer in the layer's palette to make it active.

  • SELECT > ALL and EDIT > COPY. this copies the long exposure to the clipboard.

M42 Short Exposure M42 Long Exposure with blank layer mask on top Layers Palette
Short Exposure
Long Exposure with blank
layer mask on top
Layers Palette

  • Make the mask active in the Layers Palette - hold down the ALT key and click on blank white mask box in Layer 1. The image display window will now turn blank showing you what the mask looks like, it is white or blank because you haven't pasted anything in it yet.

M42 Short Exposure Mask from long exposure image Layers Palette
Short Exposure
Long Exposure
with Mask on top
Layers Palette

  • Paste the long exposure into the mask. Edit > Paste.

The long exposure will be pasted in as a black and white mask. This is done automatically, since this mask is technically an alpha channel which is a black and white channel only.

The right hand window, which displays the mask, was blank white but will now display the black and white mask.

Note that your working composite now consists of three parts, the long exposure which resides on the background, the short exposure which resides on the layer on top of the background, and the black and white mask, which is seen in the layers palette along with the top layer, but which is really a filter between the two color images.

You can now close the M42_S.TIF image on the left as we do not need it anymore.




Step 7 - Open a New View of the image

  • VIEW > NEW VIEW will open up a copy of the composite. Place it on the left.

  • Click on the right hand copy to make it active.

  • The mask should be displayed in the right hand window. As you make changes to the mask, their effects on the composite will be seen in the left hand window.

Mask from long exposure image Layers Palette
Copy of composite
in left window
Layer Mask
Layers Palette
You will notice that the left image, the window that shows what the composite looks like as we work on it, does not look right now. This is because the mask needs to be adjusted. The bright portions of the mask in other areas of the image besides the Trapezium are letting portions of the short exposure into the long exposure where we don't want it. We only want the core of the Trapezium area from the short exposure to composite into the burned out area of the long exposure. We must adjust the mask




Step 8 - Blur the mask

  • Click on the right hand image to make sure it is active.

  • Click on the mask in the layers palette to make it active.

Mask from long exposure image Layers Palette
Copy of composite
Blurred layer mask
Layers Palette

  • Apply a Gaussian Blur - FILTER > BLUR > GAUSSIAN BLUR. You may have to experiment here with the size of the radius of the blur as this will depend on the resolution of the image that you are working on. Low resolution images will require a smaller pixel radius and high resolution images will need a larger radius. For a 1 megabyte image, try a blur of 10 pixels. For a 10 megabyte image, try a blur of 50 pixels.

Now the composite is really beginning to look more like what we are trying to accomplish, but we can tweak the mask even more.

The background of the mask is still a dark gray, which means some of the short exposure is still getting through. We have to make the sky background of the mask black.




Step 9 - Adjust the curves of the mask

  • Click on the right hand image to make the mask active.

  • Open the Curves dialog box - IMAGE > ADJUST > CURVES

  • Drag the black point at the bottom left of the curve a little to the right until the sky background reads 0 in the information palette. This will make the gray sky background of the mask black.

Mask from long exposure image Layers Palette
Copy of composite
Curved adjusted layer mask
Layers Palette

The curve for the mask can now be adjusted in many different ways to achieve the exact result you are looking for. Experiment by changing the black and white points and adding a point in the mid-tones and adjusting it. Watch the results in the left hand window.




Step 10 - Tweak the color of the core to match the rest of the image

  • You can now close the left hand duplicate copy of the composite.

  • Click on the icon of the short exposure in layer 1 in the layers palette.

    Trazezium Core Original Color Trapezium Core Color Corrected
    Original Color
    Color Corrected

  • Adjust the color with Levels or Curves. Since the mask is present, the corrections you make will only be applied to the core area of the composite.

    To my eye, the core of the Trapezium looks a little bit yellow compared to the rest of the image, so I will add some blue to the mid-tones of the shorter exposure portion of the composite. I also added a bit of magenta by pulling down the green mid-tone.

    Likewise, since the mask is still there, you can click on the background layer in the layers palette and make adjustments to the rest of the image, everything except the core area.




Step 11 - Flatten the Image (Combine the Two Layers)

  • Save this image with both the layers and the mask as a file with the Photoshop file format (.psd). Then you can experiment with it again at a later date without having to go through all of the pervious steps.

  • When both areas have been adjusted so that they blend together perfectly naturally, combine the two layers permanently by selecting FLATTEN IMAGE in the layers dialog box (under the right pointing triangle at the upper right of the Layers Dialog Box).

  • The image should now have a full range of detail from the bright core of the Trapezium to the outer loop.




Step 12 - Apply final color and density adjustments to the composite

All that is left to do is a final adjustment of the black and mid-tone points of the composited image in the CURVES dialog. Drag the black point a bit to the right from 0 to 15 and open up the mid-tones from 128 to 160.




The final composite below shows the effects of a slightly different, lighter mask. Since the mask is lighter, it lets more of the shorter exposure through, making the core of the Trapezium in the composite darker.

Short Exposure Layer Mask
Short Exposure
Layer Mask
Long Exposure Final Composite
Long Exposure
Final Composite

转载地址:http://www.astropix.com/HTML/J_DIGIT/LAYMASK.HTM
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