help! small bitmap pixels showing up on ts

Great article, the whole dpi v lpi is a different issue that i now undertsand and perhaps worded badly.
What im getting at here is this-
If I have an image created using colour index and there are very small single pixel areas at 300dpi of one colour will these show up and/or stick to the t shirt?
The same applies if i want to do an area of shading. For example i have a one color texture that i turn in to a halftone bitmap to get a fine one color gradient (what i'm asking has nothing to do with process color as i don't use it or anything to do with halftones being created by rip software.)The halftone has very small areas of fine dots to get the detail. What i want to know is , is there any rule of thumb for mesh count to dpi ratios to know what will and wont show up on a tee. There's no point in me spending ages in photoshop creating miniscule fading pixels or halftone dots if they wont even register on the t shirt. also if I'm doing 600 dpi for these tiny dots ... would it be better doing them at 300 dpi (is bigger better... obviously a better resolution and therefore a more intricate fade/gradient/halftone, but when does the resolution become obsolete and a waste of file size and time)I appreciate this is probably a grey area but any advice , or rules of thumb would be greatly appreciated. Andy

First of all are you doing the screen printing yourself or are you just supplying the design?

The reason I ask is because how you create this effect can be mandated by what type of equipment and/or process the screen printer will use in order to apply the intended effect. The last thing anyone wants to do is spend hours to attain a specific effect and find out the printer can not support the intended separation or half toning you are trying to obtain.

So if what you want to do is create a halftone dot pattern rather than an actual gradient you might want to try these steps...

1. On a new empty layer, enter Quick Mask mode (Q).
2. Create your gradient/fade with the Gradient tool, Brush, feathered Lasso, etc. This will show up in red (like Rubylith), even though your foreground color will be showing black.
3. Go to Filter>Pixelate>Color Halftone and set the Radius to 6px and Channel 1 angle to 45º. Click OK.
4. Hit Q again to exit Quick Mask, invert your selection (this may or may not be necessary depending on your selected Quick Mask options), and fill with your color of choice.

Since the mask is an alpha channel, you just get one channel of dots and don't have to worry about screen angles for the other unused channels. This works on Quick Masks, Alpha Channels, Layer Masks, and Spot Channels (which is what you'll use for t shirt art). These are just the basics, and everything else is just fine tuning your settings.

Hope this helps and good luck!

adfw76 said:

im doing halftones in phtotoshop to create very fine detail in a screenprinted tshirt design to give it a subtle vintage texture.(very fine a bit like sandpaper)

1.if i had a high mesh count screen how would that relate to dpi?is 300dpi high enough for very fine detail, would 400 -600 dpi be any better or would it just be a waste?
2. how small in pixel terms can dots/pixels be to show up/stick to tshirt?eg if ive got a 300dpi design and theres a halftone cirlce/dot of fading thats around 2 pixels across will this show up on the material?


thanks a lot everyone again, Andy

Click to expand...

Hi Andy,

You may be confusing two different requirments. Your mesh count is related to the "LPI" or Lines Per Inch", which is a setting for the frequency and size of your halftone dot.

The term "DPI" means Dots Per Inch. It is a leftover term, from the days of process cameras and halftone. It was adapted for use with laser printers. But it is often misused to describe the resolution of a raster (bitmap) image. The correct term for resolution is "PPI" or Pixels Per Inch.

In general, 300 ppi (or DPI if your more comfortable with that) is adequate, at the finish or print size. Whether you set the halftone dot in Photoshop or for the postscript printer, the rule of thumb is the mesh count should be 4 times the "LPI" or screen (again, which is the freqency and halftone dot size).

Choose the correct mesh for the job. But, if you are limited, you can use any screen to print halftones. You need only to adjust your halftone to the screen you want to use. For instance you can get a good halftone on a 110 mesh, with a 30 lpi setting (round 27.5 to 30). However you can get a good halftone on a 200 mesh with a 50 lpi halftone.

When printing by hand, though, it becomes challenging to print a good halftone above 45 lpi. But with practice some people do all the time. In general, 55 lpi pretty much is the limit.

The actual size of a pixel within the image varies and is not fixed. In other words, the size of a pixel is much smaller for a 200 ppi (or dpi) image at 1 inche than that same 200 ppi image at 10 inches. Although the resolution is exactly the same...200 ppi, the pixels are just larger. So it is relative. Thats why low res or small images get really bad when you just try to make them bigger.

The acutal pixel size becomes critical when you try to create a stochastic halftone or if you attempt to index an image. Your image should only be about 120 to 200 ppi. Much higher than that, and you will not be able to burn the dots on the screen.

I hope this helps.

Mike

Hi everyone thnaks for your informed answers.Im not a screenprinter im an artist but im trying to get as much control as possible in my arwtork so i can se what i can and cant acheive. Mike when you said....

The actual size of a pixel within the image varies and is not fixed. In other words, the size of a pixel is much smaller for a 200 ppi (or dpi) image at 1 inche than that same 200 ppi image at 10 inches. Although the resolution is exactly the same...200 ppi, the pixels are just larger. So it is relative. Thats why low res or small images get really bad when you just try to make them bigger.

i iunderstand most of that concept what im not sure about is...for example purposes this
I have a photograph i want to index. I choose 8 colours i think work best and do the usual process to obtain them. My image is at 100% percent and at 300ppi(i best us the correct terminology!) and 25cm x 25cm. I pick out each colour to make the differnt spot layers for the screens.On the white highlight layer there is a very small area which has very small pixels (say oa couple of pixels across). Because my image is at 100percent the pixels would be sent to the screen at that size(in relation to 300ppi) Now what would the optimum ppi be for all these pixels all to appear on screen. If i take it down to 200ppi i apreciate it would be beome more jaggedy but it would still be 25 x 25 cm. I gues what im saying is wheress the cut of point that it makes a differnce. Would 400dp 25 x 25 just be pointles overkill? whats th eoptimum resolution to have the small pixels apear safe on the tshirt and screens.
Final question- i would have thought index colour registrsation would be a nightmare as its like very very fine mosaique. if the registration was a tiny bit out , it would mess up the whole image and be very unforgiving unlike vector art which has larger areas.Is it best to layer some of the highlights so they dont misregister? by that i mean if i had a v.small area of red with a white fine highlight would it best to colour undrneath the white on the red layer(fill in the blanks basically) so it doenst miss and show some of the shirt underneath? I apreciate that the white wouldnt be as white on top of the red but if this didnt matter as part of the artisitc side of the design.? thanks andy

you also said .....
The acutal pixel size becomes critical when you try to create a stochastic halftone or if you attempt to index an image. Your image should only be about 120 to 200 ppi. Much higher than that, and you will not be able to burn the dots on the screen.

I dint really understand any of this ..sorry if its 120 to 200ppi is the highest you can burn to a screen ..why are people recomending 300dpi all the time?whats schoastic..? sorry about all the can of worms style questions, im done!! thanks a lot and really apreciate your time.

Georgies said:

i have a question...for a 4 color porecess print, we usually go with 40 to 50 LPI with ur regular round halftone.

Whats is the equivilance for a stoschastic half tone?

Thanks George.

Click to expand...

Hi George,

You will not get good results using stochastic screens for 4 color process. The illusion of "full color" created with the process, specifically requires halftone dots printed at specific angles. Stochastic screens are really only suitable for one color prints or "index" printing.

I'm really a big fan of indexing. Unlike printing 4 color process, the image is broken down into a limited number of "spot" colors. The more colors used, the better the image looks. Depending on the image, six to eight colors should be used (usually including a black and white screens). More colors are not uncommon, if you have a larger press. The advantage of indexing is that it is easier to control consistency and printing on dark colors is easier.

The art of indexing is choosing the right predominant colors in the image. However, Photoshop or CorelPAINT can do it automatically too. As mentioned earlier, begin the image at 300 PPI, than reduce the image down to about 120 PPI before indexing. To create a stochastic screen for a one color image, first convert it to a grayscale image. Then, convert it to a "Bitmap" and choose "Diffusion Dither" as the mode.

I hope this helps,
Mike