Requirements
Mosaic requires images to be georeferenced. Images that have no coordinate system specified can still be mosaicked, however is assumed that all images being mosaicked have the same coordinate system with the same pixel size. This is because a transformation cannot be performed if no coordinate system is specified.
If the destination document contains a rotation, the input images must also contain the same angle of rotation, coordinate system, and pixel size, or else a transformation can’t be applied. Remove the rotation of the destination document to mosaic images with rotation.
If an image is flattened with white space where transparency should be, it is recommended to convert that background layer to a layer and remove the whitespace to ensure the mosaic will be seamless between the input images.
Input images with a different coordinate system than the destination document will be transformed to match the destination document.
Mosaic Dialog Box
Open all the georeferenced image files (at least two) required to make the mosaic. The document that is active/selected will be the destination document, all other documents can be mosaicked into the destination. Click the Mosaic button in the Geographic Imager panel.
Mosaic button
Destination Document, Available Documents, and Mosaic Documents
The Mosaic dialog box displays the destination document and destination coordinate system at the top. The Destination Document displays the active document. The Available Documents list displays all open documents that can be included in the mosaic. Images that do not satisfy the mosaic conditions of the destination document (e.g. non-georeferenced image) cannot be included in the mosaic and are displayed in gray with the reason why it is incompatible.
Images that meet the mosaic conditions of the destination document can be added to the Mosaic Documents list using the transfer buttons - Transfer All or Transfer Right. At least two documents are needed to create a mosaic image (the destination document and at least one document in the Mosaic Documents list). All documents in the Mosaic Documents list will be used in the mosaic. To remove a document from the Mosaic Documents list, click the Transfer Left button. Mosaic documents can be reordered by using the Up and Down buttons. The document at the top of the Mosaic Documents list will be the first layer at the top of the Adobe Photoshop Layers panel after the mosaic is created.
Layer Options
Layer Organization |
Group source document layers: Groups layers into folders. Merge source document layers: Layers from the source document are flattened into a single layer and kept as a single layer in the destination document. Merge all: Merges all layers into a single layer. |
Place Mosaic Layers Above Destination Layer |
Input mosaic documents will be positioned above the destination document. Mosaic and destination documents order can be viewed in the Layers panel. |
Mosaic Options
Layer Blending Mode |
Apply an Adobe Photoshop blending mode for the mosaic. Learn more about Photoshop blend modes. |
Mosaic Alpha Channels |
Enable to include all alpha channels in the mosaic process. |
Crop to Destination Extents |
Enable to crop to the extent of the destination document. When this option is enabled, any documents outside the destination extents are disabled from being included in the mosaic. |
Advanced Transformation Options
The Advanced transformation options frame is enabled when a transformation during Mosaic is required (the destination document has a coordinate system that differs from the mosaic documents).
Layer Pre-processing
There are several options to handle layers of the mosaic document’s layers during their transformation into the mosaic:
Leave intact |
Maintains the original layer structure of mosaic documents being mosaicked. |
Merge raster layers |
Combines only raster layers while maintaining vector layers, for example, text and transparency. Choose this option to guarantee seamless edges between different layers, especially if they don’t have overlapping margins.
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Merge layers |
Combines all layers into one layer, maintaining transparency.
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Resampling Methods
There are several options for resampling the image during the transformation into the mosaic:
Bicubic (smooth gradients) |
Takes a weighted average value of the sixteen pixels closest to the transformed location in the source image. |
Bilinear |
Takes a weighted average value of the four pixels closest to the transformed location in the source image. This method results in a smoother image than the nearest neighbor method but at the expense of more processing time. |
Nearest Neighbor (hard edge) |
Takes the value of the pixel that is closest to the transformed location in the source image. This is the fastest method in terms of processing time and is the method to use to preserve a colour panel during image transformation. It is also the best method to preserve original colours in some non-RGB colour modes, such as CMYK, by avoiding the internal conversion to/from RGB. |
Automatic |
Adobe Photoshop chooses the resampling method based on the document type and whether the document is scaling up or down. |
Preserve Details (enlargement) |
A method for enlarging images that attempts to smooth out noise. |
Bicubic Smoother (enlargement) |
A method for enlarging images based on Bicubic interpolation but designed to produce smoother results. |
Bicubic Sharper (reduction) |
A method for reducing the size of an image based on Bicubic interpolation with enhanced sharpening. This method maintains the detail in a resampled image. If Bicubic Sharper oversharpens some areas of an image, try using Bicubic. |
A coordinate system transformation will resample an image, therefore any image analysis or classification should be performed prior to a transformation.
Precision and Strip width
To provide necessary transformation performance, Geographic Imager transforms one horizontal strip of the image at a time. Change the Precision to adjust the strip width being transformed:
Maximum (slow; best for World Projections) |
Recommend when performing transformations on images with the extent of the world. |
High |
Uses a strip width of 10 pixels—high-quality results and slightly slower transformation performance. |
Normal |
The default setting using a strip width of 50 pixels—a reasonable value for the majority of raster imagery combining good quality results and high transformation performance. |
Medium (fast) |
Uses a strip width of 250 pixels—good quality results and faster transformation performance. |
Custom |
Set a custom strip width. |
Some images may be optimized by setting a custom strip width, which may be especially true when working with very small/world-scale data, high latitude/polar area imagery, or projections that introduce severe spatial distortions. If the result contains visible horizontal stripping artifacts or transparent gaps, choose a smaller strip width or the Maximum precision setting. Choosing a smaller strip width will result in a higher-precision transformation, but the transformation process may be slower. Alternatively, for low-deformation, large-scale data, or for a quick preview of transformation results, a larger strip width can be used.
For example, if an image being transformed displays a spherical curve, a smaller strip width may be necessary to maintain a smooth image edge. If a larger strip width is used there may be a stepped edge to the image.
Mosaic Results
When the Group source document layers option is chosen, the mosaic documents will be grouped into image-specific subfolders (the subfolder is named after the image file name). The Layer Pre-processing Merge layers setting in the Advanced Transformation frame (when a transformation is performed) is only respected when the Layer Organization is set to either Merge source document layers or Merge all. When the Place Mosaic Layers Above Destination Layer option is checked, the input mosaic layers are placed above the destination document layer.
Mosaic example showing a completed mosaic with grouped layers. The top-right layer's visibility is hidden.
The mosaic can be treated like any other spatial image (e.g. toggle visibility, lock layers, adjust colours). Other Geographic Imager tools can be used on the mosaic such as Georeference, Transform, Tile, and GeoCrop. However, some functions may merge the layers, while others will keep the mosaic layers intact.
Important Notes and Limitations
Unsupported image colour modes for the destination document are Indexed Colour, Bitmap, and Adobe Photoshop Multichannel. The colour mode of the resulting mosaic is the colour mode of the destination document.
Images with multiple channels will maintain them mosaicked. However, channels will be merged regardless of the channel type. Alpha channels will always maintain order when merged (e.g. the alpha channel in the first position will be merged). When mosaicking images that contain mismatched alpha channels, it is recommended to manage the alpha channels prior to mosaic.
It is recommended to perform any coordinate system transformations as early as possible during the image editing process to minimize the potential adverse effects on a complex layer structure.
Shaded relief effects will be merged when a transformation is performed using the Merge raster layers option.
Images that are mosaicked or combined using the Adobe Photoshop Photomerge feature will not retain georeferencing information.
When mosaic documents don't have any overlap and a transformation is being performed in the mosaic, gaps are likely to be seen.
Related topics
Opening Images with Advanced Import
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