Because the effects described on this page and the next are subtle, they do not show up on scaled-down thumbnail images. I therefore have not included such thumbnails after most of the command sequences on these two pages.

To add an atmosphere effect to the sheared image from the previous page, I am going to blend the curved edge of the image with a light blue color that increases in intensity as one moves radially across the edge and then fades to the black of space.

Add fifty pixels of black along the right edge of the mask image from the previous page and display the padded image with xv.

pnmpad -black -r50 mask.pgm | xv -

Use xv's "Algorithms->Blur..." function with a mask size of 29, and then save the result at normal size back to the same filename.

Now move the padded, blurred mask image fifteen pixels to the left.

pnmpad -black -r15 mask.pgm | pnmcut 15 0 1848 4685 > mask.shifted_left.pgm

Add fifty pixels of black along the right edge of the sheared true-color image.

pnmpad -black -r50 S1999244172740.L1A_HNSG.sheared.ppm > S1999244172740.L1A_HNSG.sheared.padded.ppm

Fade the curved edge of the sheared, true-color image by multiplying it with the left-shifted mask image produced above.

pnmarith -m S1999244172740.L1A_HNSG.sheared.padded.ppm mask.shifted_left.pgm \
          > S1999244172740.L1A_HNSG.sheared.padded.edge_faded.ppm

Make a light blue, sky image.

ppmmake rgb:20/6a/ae 1848 4685 | pnmdepth 255 > sky.ppm

Blend in the sky color at the curved "horizon" and fade the sky to black.

pnminvert mask.shifted_left.pgm \
| pnmarith -m - sky.ppm \
| pnmarith -a - S1999244172740.L1A_HNSG.sheared.padded.edge_faded.ppm \
| pnmarith -m - mask.pgm \
> S1999244172740.L1A_HNSG.sheared.padded.edge_faded.sky_added.ppm