5

I am trying to make a 3D model of a building using a series of photos.

This is a photo I took yesterday:

enter image description here

This one I took today, when I was trying to continue from that point:

enter image description here

Yesterday's sky was cloudy, while today it was completely gray and grim and dark.

This is probably what causes reduced contrast and blue tint in the second photo.

Structure From Motion algorithms are very sensitive to changes in colors like that. I need to find a way to fix the color palette of the second batch of photos to make it closer to the first.

How can I compensate for the difference in post processing?

  • 3
    Not sure if you've noticed, but on second image far right bottom window on the main house is half-opened :) – dev101 Jan 2 at 5:47
5

On fair days the world is illuminated by a combination of sunlight and skylight. On overcast days, the world is illuminated by skylight only. Now skylight is very blue thus images reproduce with a blueish tint. If you were using color slide film, the remedy is to mount a warming filter. The most popular is the Wratten 1A (Kodak trade name) commonly called a “Skylight”. We mount such a filter to warm our images. The 1A is pale pink and absorbs ultraviolet as it reduces excess bluishness. This filter is effective when the principle subject is in “open shade” under a clear blue sky.

In digital photography we can mount warming filters however, your camera features “white balance”. This is a camera setting designed to neutralize a color cast induced by the ambient light source color. Additionally clever use of “white balance” can be used to intentionally alter the color balance of an image.

To mediate the bluish cast of your images make sure “white balance” is turned on. If you review your camera manual’s “white balance” instructions, you will discover that you can use a white placard as a target. This method of use causes the camera’s logic to preview the color of the ambient light and subsequently apply a correction. You can use this method to effectively neutralize an off-color cast. Additionally, most image editing software allows post camera exposure corrections to the color balance of images. Peruse the instruction manual that covers the imaging software you are using. Take the tutorial, you can do this!

  • I also have white balance filter in image editors. But the problem is that I need to "balance" the photos to be the same, not just get the colors correct. The software I use requires that. When I try white balance in my camera or an editor, the photos do look better, but they still have wildly different color palletes, causing the 3D reconstruction to fail. – Tomáš Zato Jan 1 at 16:09
  • 1
    Image editors often have 'eyedroppers' that you can use to tell them what is white or neutral grey in the image. The editor will then balance the photo so that the reference object is white or neutral. In the case of your images, you could try to select a neutral that is common to all of your images- the white base of the far structure for instance. Go through each image and select that as your reference. All images should then be balanced relative to each other (what you say you need)- but not necessarily absolutely balanced. – BobT Jan 1 at 20:48
4

Spot on color balance is achieved by photographing a grey card or grey/white/black card or, if you want to get fancy, a color checker passport.

Use these as the baseline points in post processing to set your white bal for all the shots taken in the same lighting. (Or, one can usually set custom white bal in camera. Dealer’s choice)

Because your target remains the same, the white bal between shots in different lighting becomes much more stable.

  • Can I do the same approach with the photos I already have? I have two sets of 300 photos, each with different lighting and therefore colors. I just don't know how to find the "difference" that could be applied on the entire set. – Tomáš Zato Jan 1 at 18:24
  • @TomášZato It wouldn't be completely precise, but it might be close enough for your purposes to balance white against some particular part of your picture. Perhaps the ground floor window-frame color, or something like that? – Tripp Kinetics Jan 1 at 21:41
  • @TrippKinetics I tried that, among with other basic image editor functions, failed and then asked this question. – Tomáš Zato Jan 2 at 3:11
  • 1
    a blown pixel is a terrible choice, as it is per definition pure white in the image. find something that should be a neutral gray/white, but isn't maximally bright. that shed roof looks like a good candidate. – ths Jan 2 at 7:42
  • Working on a blown pixel might be a useful idea if trying to reverse the effect of mistaken post processing :) – rackandboneman Jan 2 at 12:21
3

If you have Photoshop, you can try Match Color tool.

simplified instructions:

  1. open source image
  2. open target image
  3. (in target image) Image > Adjustments > Match Color
  4. In Image Statistics > Source section select your reference (source) image (from step 1)

Fiddle with sliders/check-boxes until you are satisfied. Try to match compositions/scenes between images, don't try to match color of an image with predominantly grass and the one with predominantly house (for example). They have to be fairly similar for good/great results.

This is the result with Color Intensity set to 85 (other options are default): enter image description here

For the very reason of variable light conditions, these kind of shootings should be taken on the same day, with fairly constant light (clear sky or uniform cloudiness) in as short time interval as possible and manual WB camera settings (exposure difference is also important, e.g. on your images you have a very bright sky in the back).

Of course, if you have enough experience, you can manually match colors between photographs using other adjustments tools in PS (all are listed in the same Image > Adjustments menu):

  1. Color Balance
  2. Hue/Saturation
  3. Channel Mixer
  4. and many others

But they might be too laborious and complicated for a novice user.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.