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VENUS WINDS PROJECT (Tutorial #10) PREPARING VENUS IMAGES

USING PHOTOSHOP CS6 By Arthur C. Tarr

PREPARE MASK

Venus images in this application, stored as FITS files, are defined by a 512x512px square array that must be converted into image formats such as TIFF or JPEG. The convention is that the x-values range from 0-511 left to right and the y-values range from 0-511 top to bottom. The (x,y) center coordinate of the array is defined here as (255,255). It is essential that successive images of Venus be centered within the 512x512px domain corresponding to the dimensions of the imaging chip. It should always be assumed that the image is NOT centered. Centering is achieved by use of a mask, two examples of which are displayed in Fig. 1. The mask is a Photoshop layer whose background is empty (transparent) while the mask or circle is opaque. The image is manipulated under either the mask or circle so that the Venus image is aligned with the mask’s circular outline or circle.

CREATING THE MASKS 1. Create two 512x512px masks: A centered circular mask whose interior is transparent (Fig. 1, left) and a thin circle on a transparent background (Fig. 1, right). In these examples, the inner diameter is 332px.

Fig. 1 Left: A centered mask, 332px in diameter Right: A 332px diameter circle mask 2. Save both of the masks for alignment purposes: Save As Mask332px.psd Save As Circle332px.psd 3. Close both masks. WHY TWO MASKS? The thin circle is ideal for precisely centering the Venus image because the dark limb often appears irregular due to spatial variations in the IR emission along the limb. The centered mask may then be employed to mask out the unnecessary surrounding parts, such as glare, of the image.

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ANALYZING THE IMAGE 1. Open image layer File>Open>20040712>1600.tif See Fig. 2 The image’s dimensions are 512x512px. Note that the overexposed bright crescent encloses a totally black crescent shape whose pixel values have been arbitrarily set to 0 because, otherwise, they would exceed the maximum possible values of the camera chip. These black pixels are UNUSABLE! The brightest pixel values are mostly saturated at the maximum value (= xxxx) although some usable pixels do exist along the inner margin of the bright crescent. However, faint pixels to the left of the bright crescent glare and are not usable for our purposes so they will be masked out. Note that Venus is not centered in the image (Fig. 2) and, in addition, Venus is rotated about 12°CCW. Rotation is necessary to align the y-axis parallel to the N-S central meridian. After centering and rotation, the features are oriented properly (Fig. 3). In addition, a change in Brightness and Contrast may show more cloud features next to the inner crescent.

Fig. 2. Image 1600.tif from the 12 July 2004 sequence. Fig. 3. Image 1600.tif rotated 12° deg CW 2. Convert image to RGB color Image>Mode>RGB Color WHY RGB COLOR?: It is necessary to convert the cloud image to RGB color because the semi-transparent image may eventually overlay a color base map. The RGB Color mode does not affect the appearance of gray tones.

3. Label the image layer as 1600 rgb

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4. Duplicate the original layer to create a mask: Click cmd-j on the layer thumbnail and rename it as 1600 rgb mask. Refer to Fig. 4.

Fig. 4 – Photoshop display of rotated image (left) and enlarged image of layers window (right)

7. Deselect original layer (click visibility eye off) 8. Select the mask layer 1600 rgb rotated

ADJUSTING BRIGHTNESS AND CONTRAST 1. Open a Brightness/Contrast adjustment layer Layer>New Adjustment Layer>Brightness/Contrast 2. Use the Quick  Selection  Tool to paint the background (pure white and black areas); click Sample All Layers in Option bar. In addition, the Lasso  Tool is useful for outlining and deleting very fine detail. 3. To delete black areas set: Brightness = 150 and Contrast = -50 (Black areas become either hidden or oversaturated) Select area polygons; click Edit>Cut

4. To delete saturated white areas set: Brightness = -150 and Contrast = -50 (Pure white areas are bright sunlit areas) Select area polygons; click Edit>Cut

5. Clean up rest of background with Eraser  Tool  or  Lasso  Tool    6. Reset Brightness = 0, Contrast = -50 7. Load mask layer (cmd-click in the mask’s thumbnail; marching ants should appear) 8. Click mask layer’s visibility eye to hide it 9. Select original layer (e.g., 1600 rgb) in the Layers panel (turn on visibility eye)

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10. Click on Add layer mask button to select the original layer (at bottom of Layers panel, the button is the square icon with a circle in center) 11. Add a layer above the layer mask to invert pixels Layer>New Adjustment Layer>Invert

Click the visibility eye ON to invert, OFF to not invert.

IMPORTANT: The original image is, in effect, a negative. Bright patches in the original image are actually gaps in the high-altitude clouds that allows greater thermal radiation to shine through. 12. Click mask’s thumbnail; edit layer mask, if necessary 13. Click original image’s visibility eye off 14. Save adjusted image Save As 1600.adj.psd (Fig. 5) 15. Close

Fig. 5: Venus clouds (dark areas) in original image.

Fig. 6: Venus clouds (lighter areas) in the inverted and rotated image.

CREATE CLOUD LAYERS

1. Open image layer File>Open>20040712>ADJ>1600.adj.psd 2. Invert pixel values Layer>New Adjustment Layer>Invert (Fig. 6) 3. Save as .psd file File>Save As>20040712> CLOUDS>1600.adj.psd 4. Close

Note: A similar procedure would be used to create layers for gaps in the clouds. Eliminate Step 2 in the procedure above and Save the image in the GAPS folder. Remember: A cloud layer is the inverted version of the gap layer.