The quality if an image changes as it is stored in a mosaic,

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01-21-2014 09:34 PM
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by Anonymous User
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Original User: Jamal432@gmail.com

The quality if an image changes as it is stored in a mosaic,

I???m wondering why the quality of an image changes as it is stored in a mosaic as shown in the screenshot below.
As the original image is stored in a mosaic, it gets darker.

[ATTACH=CONFIG]30690[/ATTACH], [ATTACH=CONFIG]30691[/ATTACH]

What might be the issue here?

Thank you

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Jamal
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by Anonymous User
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Original User: mboeringa2010

Jamal,

There are many different ways to translate or render a pixel value (e.g. RGB - Red, Green, Blue values as expressed in values ranging from 0-255) into a display "on screen" in ArcGIS. The reason for this is that it is often possible to "optimize" the display of an image by "stretching" out the pixels values across the entire range your computer screen can handle.

We are actually entering the field of color theory, gamuts and color proofing here also...

What seems to happen is that the rendering / stretch type is changed when adding the data to the Mosaic. Please note the stretch mechanisms DON'T affect the pixel values of the orginal image, they are merely for display in ArcGIS (unless you deliberately use some geoprocessing tool for images that writes out new data / pixel values based on a certain stretch).

You should be able to "restore" you image to the original display, by setting a similar "stretch" on the Mosaic, as on the original image. You can find the stretch options under the Symbology TAB of the layer properties. Choose RGB Composite / Stretch in the dialog, and start experimenting with the options.

For further reading, I really advice you to read the following Help pages:
Renderers used to display raster data
Raster rendering behavior
Improving the display of raster data and especially the section Enhancing the appearance of the raster data

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by Anonymous User
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Original User: mboeringa2010

Jamal,

There are many different ways to translate or render a pixel value (e.g. RGB - Red, Green, Blue values as expressed in values ranging from 0-255) into a display "on screen" in ArcGIS. The reason for this is that it is often possible to "optimize" the display of an image by "stretching" out the pixels values across the entire range your computer screen can handle.

We are actually entering the field of color theory, gamuts and color proofing here also...

What seems to happen is that the rendering / stretch type is changed when adding the data to the Mosaic. Please note the stretch mechanisms DON'T affect the pixel values of the orginal image, they are merely for display in ArcGIS (unless you deliberately use some geoprocessing tool for images that writes out new data / pixel values based on a certain stretch).

You should be able to "restore" you image to the original display, by setting a similar "stretch" on the Mosaic, as on the original image. You can find the stretch options under the Symbology TAB of the layer properties. Choose RGB Composite / Stretch in the dialog, and start experimenting with the options.

For further reading, I really advice you to read the following Help pages:
Renderers used to display raster data
Raster rendering behavior
Improving the display of raster data and especially the section Enhancing the appearance of the raster data

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by Anonymous User
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Original User: Jamal432@gmail.com

Jamal,

There are many different ways to translate or render a pixel value (e.g. RGB - Red, Green, Blue values as expressed in values ranging from 0-255) into a display "on screen" in ArcGIS. The reason for this is that it is often possible to "optimize" the display of an image by "stretching" out the pixels values across the entire range your computer screen can handle.

We are actually entering the field of color theory, gamuts and color proofing here also...

What seems to happen is that the rendering / stretch type is changed when adding the data to the Mosaic. Please note the stretch mechanisms DON'T affect the pixel values of the orginal image, they are merely for display in ArcGIS (unless you deliberately use some geoprocessing tool for images that writes out new data / pixel values based on a certain stretch).

You should be able to "restore" you image to the original display, by setting a similar "stretch" on the Mosaic, as on the original image. You can find the stretch options under the Symbology TAB of the layer properties. Choose RGB Composite / Stretch in the dialog, and start experimenting with the options.

For further reading, I really advice you to read the following Help pages:
Renderers used to display raster data
Raster rendering behavior
Improving the display of raster data and especially the section Enhancing the appearance of the raster data



Thank you very much Marco for the very useful answer.

It is looks fine now as I manage to match the settings of the �??RGB composite�?� in both images. However, the Mosaic is still a bit darker.


[ATTACH=CONFIG]30696[/ATTACH], [ATTACH=CONFIG]30697[/ATTACH]

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Jamal
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New Contributor III
I am having these same issues with my data and Marco's explanation has solved one of my problems.  The other is that when you open the original raster you can see the pixel size.  With the Mosaic Dataset those pixels have been smoothed out.  Please see my attached images.  Is there a way for you to have the Mosaic Dataset leave the pixel size the way it is in the original raster?

As for changing the stretch type and applying the gamma stretch, can you set this for the Mosaic Dataset in ArcCatalog so it doesn't need to be changed every time I open it in ArcMap?
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Original User: mboeringa2010

I am having these same issues with my data and Marco's explanation has solved one of my problems.  The other is that when you open the original raster you can see the pixel size.  With the Mosaic Dataset those pixels have been smoothed out.  Please see my attached images.  Is there a way for you to have the Mosaic Dataset leave the pixel size the way it is in the original raster?


ArcGIS doesn't actually change the pixel size of the original dataset, but it does change the display on the fly if you use the default settings, which apply a smoothing based on a Bilinear Interpolation. To "fix" this, you need to set the Default Resampling Method to Nearest Neighbour. There are two places where you can do this:

- In ArcCatalog as part of the Mosaic Dataset properties: Right click the dataset, choose Properties / Defaults TAB, and than open the Image Properties, now you will see the Default Resampling Method, set it to Nearest Neighbour.

- Under the Layer properties in ArcMap. Click on the + sign before the Mosaic Dataset in the ArcMap TOC to expand it and show Footprints, Boundary and Image. Right click the Image and choose Properties / Display TAB. Now set the "Resample during display using" option to Nearest Neighbour.

ArcMap will no longer "smooth" your images, but show the original pixel size (albeit still snapped to screen pixels, but this is completely unavoidable considering screens don't have infinite resolution).

As for changing the stretch type and applying the gamma stretch, can you set this for the Mosaic Dataset in ArcCatalog so it doesn't need to be changed every time I open it in ArcMap?


The settings you refer to are Layer properties, not Mosaic Dataset properties. Hence it is not possible to set them on the Mosaic Dataset in ArcCatalog. What you can do though, is to save out a Layer file (*.lyr) from within ArcMap once you have chosen your desired settings, and than add the *.lyr Layer file instead of the Mosaic Dataset directly. To do this, simply right click the layer in the ArcMap TOC and choose Save As Layer File. This way, you will always have the proper settings applied.

By the way, this is a recommended workflow for any dataset added to ArcMap, so also for Feature Classes etc., as it allows you to predefine crucial settings like allowable minimum-maximum display, legend/symbology properties and labeling.
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New Contributor III
Great! Thanks for the information.

Will
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