Bitmaps at wrong angles in PhotoShop CS

I am using PhotoShop CS, in OS X 10.3.4, on a Dual 1.8 GHz PowerPC G5. I use bitmap images to create special effects for poster art. The problem is: several of the angles I choose for the bitmaps do not appear as the correct angles.

For example- take an RGB color image (72dpi), and split the channels. Select the blue channel, go to Image>Mode>Bitmap. In the Bitmap dialog box leave the Output at 72 pixels/inch. For Method select Use: Halftone Screen. Click OK.

In the Halftone Screen dialog box, set the Frequency to 3 lines/inch. Set the Angle to 85 degrees. Set the Shape to Line. Click OK.

Now select the green channel (of the original image). Go to Image>Mode>Bitmap. The Output remains at 72 pixels/inch, the Method remains Halftone Screen. Click OK.

In the Halftone Screen dialog box set the Frequency to 5 lines/inch. And leave the Angle at 85 degrees, and the Shape set on Line. Click OK.

Compare these two bitmap line images and you will see that the angles of the lines are different.

I have found at certain Frequencies per inch that the Angles are never correct. Can anyone explain this? Am I doing something wrong?

This question has come up quite a bit lately. We have several other people with the same name as yours asking the same question.

Your question will be answered in the order in which it was received.

Where are you seeing the mis-alignment: on the screen, on a proof, or on the printed poster? How do you intend to print these posters? Offset?

If seeing the problem onscreen, are you viewing the image at 100%?

Bitmap images are only viewed accurately at 100% (or an even multiple of 100%).

The mis-alignment appears on screen, and on the print from my inkjet printer. On screen I have viewed this at 100%, 200% and 300%. The final output is from a large-format inkjet printer.

I would say that part of the problem here is the 72ppi.

Confining screen generation to such a coarse dpi (in this case it is more like printer resolution) limits the angles that are possible.

However, in messing around with this feature, even generating higher resolution bitmaps which should provide more accuracy, does not seem to render the specified angle as measured with the measure tool.

Odd.

The other thing that I notice is that the actual line frequency that is generated varies with the resolution of the bitmap you generate, so it is not really the correct lpi (printed) that you specify either.

So it doesn't get the angle OR the lpi correct, but it DOES make lines you gotta give it that... OK well it doesn't do that all that well either. Nothing like a real straight line halftone screen from the old days.

If you want real straight-line line screens, check out the Series 3 Screens plug-ins from Andromeda:

<http://www.andromeda.com/>

but they ain't cheap….