Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~levay/color/PosterHandout.pdf Дата изменения: Wed Oct 27 23:51:00 2004 Дата индексирования: Tue Oct 2 13:29:48 2012 Кодировка: Поисковые слова: comet


Composite Colors: Image Layers
Grayscale Colorized

Screen layer blend mode, analagous to a projected transparency Global Levels adjustment layer applies to all layers below
Hue: 130 (green) Saturation: +100% Lightness: ­50%
H F656N

[N II] F658N

Colorize

[O III] F502N

Hue/Saturation dialog for the F656N (green) layer of the Eskimo Nebula color composite converts grayscale layer into single-hue color layer. Hue/Saturation adjustment layer applies to the He II (purple) image layer Levels adjustment layer, grouped with the He II (purple) image, applies to that layer only

The Photoshop Layers palette for the color composite image.
He II F469N

Resulting color composite image

· Import separate grayscale imges into separate Photoshop layers or a single RGB TIFF image with exposures composited into color channels. · Any number of filters may be included, rendered in any color, to produce a color composite. In practice, fewer filters/layers produce more color separation. · Three layers rendered in red, green and blue are are equivalent to one RGB layer with the same images in red, green and blue channels. · If the filter and rendered colors match the eye's response then a natural or "true" color image results. Otherwise, "false" colors, arbitrarily close to reality, will result. · Colors may be "representative" if the rendered colors do not match the filter colors but are consistent with their wavelength order, ionization level, etc.

Eskimo Nebula NGC 2392 · Hubble Space Telescope WFPC2 NASA, A. Fruchter and the ERO Team (STScI)


Combining Differently-Processed Versions of an Image
Different "upstream" processing steps may be used to enhance certain features or attributes of an image. Square root, log, or other mathematical operations on the data may compress dynamic range, increase contrast in a particular brightness range, etc. These versions may be combined in Photoshop, ideally to take advantage of the benefits of each processing method. While these mathematics may be accomplished within Photoshop, it is preferable to per form them on the full dynamic range data to prevent quantization artifacts.

Color composite from exposures divided by a radially-symmetric profile to compress the dynamic range, suppress the bright galaxy nucleus and increase the contrast in the spiral arms.

"Straight" color composite from log-scaled exposures represents the brightness of the nucleus relateive to the spiral arms of the galaxy.

The Photoshop Layers Palette for the combined image. The upper layer uses "Screen" layer mode to "project" this image, combined with the lower layer.

M51 · HST/WFPC2 Hubble Heritage Team (STScI/AURA) and N. Scoville (Caltech)

M51 NGC 5194 · Hubble Space Telescope WFPC2 NASA and the Hubble Heritage Team (STScI/AURA)

A combination of the two composites, showing the stronger delineation of the arms from the upper image, while preserving the larger brightness difference between the nuclueus and fainter spiral arms represented by the lower image.


Combining multiple images
Multiple images of the same field may be seamlessly (at least arbitrarily so) combined in Photoshop. This is useful to embed a higher resolution image into a wider-field view. Each image is in a separate layer. Semi-transparetn layer masks permit regions of the upper layer to be rendered transparent, allowing the lower image to show through. Judicious feathering and painting of the layer mask permits merging the two images arbitrarily closely. WFPC2 mosaic NOAO image

Mask for WFPC2 image layer

Composite image The layers palette for the NOAO/WFPC2 composite. A feathered, semi-transparent layer mask is applied to the upper ( WFPC2) layer, which softly combines the two images.

Horsehead Nebula · Hubble Space Telescope WFPC2, NOAO NASA, NOAO, and the Hubble Heritage Team (STScI/AURA)