Designing for Printing on Dark Apparel
Printing on dark apparel is different from printing on light apparel. The dark apparel printing is a multi-part process. Transparency is not required but everything that is not transparent will be printed. In most cases transparency will be required to make the image look good, to avoid the "square slapped on a shirt" look.
The process for dark apparel involves first putting down a white opaque layer then putting the image on top of that white layer. The opaque underlayer is put down everyplace where the image shows any color. It is not put down where the image is transparent. For dark apparel you need to have transparency where the shirt color should show through. Any area that shows as a color, even if nearly transparent, will be printed with the white underlayer first. It will be opaque, not semi-transparent, because the white layer is opaque. So you cannot use partial transparency to fade an image into the apparel. The white underlayer must be cured before the next step can proceed.
When the underlayer is ready the design is printed on top of the white underlayer.
The white underlayer is printed anyplace that is not transparent except areas of the image that are black. Areas of black in the image don't get the white underlayer. The black ink gets printed directly on the shirt. This is because printing black ink on the white underlayer makes it come out dark grey. I have no clue as to the tolerance on that or what the definition of "black" is for that situation. (See Is Black Just Black?) Variations of "black" may not be distinguishable because without white (which only comes in opaque) lighter shades are usually created by putting down less ink (more blank space between the dots of color) and when the background is dark this just results in darker, not lighter. Also when the ink is placed directly on the fabric it is partially absorbed blending the ink color with the fabric.
Very subtle differences in color may be lost, and the areas printed as if they are the same color. Again I don't know what the range is where that happens.
No partial transparency in your image
Every place that has any color, no matter how slight, will get a white underlayer. If you have semi-transparent pixels this can cause the white to appear in unintended areas. It is common around text, where something was erased, or where a fade or gradient was applied.
Anti-alias is a means of smoothing the visual transition between various colored objects. For example it makes the edges of text look smoother. It does this by putting in-between colored pixels on the edges. Whether this is a problem when the anti-alias transitions between the edge of the image and the transparent background is a good question.
The transitional pixels are only semi-transparent. Because they have some color there will be a white underlayer. That means there could be a white halo if the white underlayer shows and is not covered by opaque color. But the width of antialiasing is normally very small and should not be noticeable, especially if the underlayer is "choked back" by a pixel or two as part of the print process. Ordinary text anti-alias should not cause a problem.
You can reliably evaluate this with the process described below for checking for partial transparency. If you are the nervous sort you might want to turn off anti-alias. Don't worry about jagged edges as long as you are using at least 200 dpi resolution in sizing your image. At that resolution anti-alias is not necessary on fabric. The natural fabric texture will mask any edge roughness.
Some people like to use a black stroke around text and other objects to reduce the risk of the white halo. Other people choose to use a white stroke to make any white edges look intentional, and to make those edges consistent.
When designing for the dark print process don't use drop shadows, gradients or similar effects on or near the edges of transparency. You can transition from one opaque color to another opaque color, but not between an opaque color and transparency. Again, the process described below should be a good representation of the effect of the white underlayer.
Check your design for partial transparency
One way of checking the design is to use the magic wand select all pixels that are 100% transparent, inverse the selection, put a new layer at the bottom of the layers, and fill that inversed selection with white. That way you can make sure that any white that is showing is intended to be showing. See a video of this method. Before uploading an image remember to delete any test background layers you might use so they don't get printed.
- Select the magic wand.
- Set tolerance to 0. Uncheck anti-alias. Uncheck contiguous. Check Sample All Layers.
- Click the wand on a transparent area.
- Go to Select, choose Invert.
- Go to Layers, Add a layer, drag that layer to the bottom.
- Click on that new layer to select it.
- Click on the paint bucket. Make the color white.
- Fill the selection with white.
Design specifically for the darker product.
Even with the white underlayer there is some effect from the dark background. Compensate by brightening your image a bit. Follow the normal guidelines of using colors in the CMYK color space.
It is better to make your image for black/dark t-shirts from the beginning rather than than trying to adjust an existing image. It can work, but mostly results are better if you create for transparency in the beginning.
If you want to try using the same image on both black/dark and light items then start with a transparent background, get it all correct for black/dark, save to your file name for dark apparel, then adjust as needed for white/light items and save to a different file name for non-dark apparel items.
You will get the best design results where the design is smaller areas broken up with transparency in between. See the cardinal red shirt example. Large continuous areas of color that wil require a large continuous area of the white underlayer is a bit more subject to cracking. Both the designs in the black shirt example have that risk. The brown shirt example has smaller areas continuous color.
Mis-registration is where the underlayer and the design printed on top of the underalyer don't line up properly. This is a natural risk in such a two step process. The risk can be reduced somewhat by adding a black stroke around objects next to a transparent area. That is because the underlayer is not used under black (which ends up being dark grey if it is printed on the underlayer).
Use the right kind of transparency
Gif only comes with indexed transparency - don't use it. PNG can be either PNG8 (indexed) or PNG24 (alpha) transparency. Only the PNG24 works with CafePress® process (or at least in general - I'm not going to confuse by discussing the exceptions). If you are having trouble with transparency the best kind of help you can get will be in (a) the products help files (b) tutorials on that product or (c) user forums for that particular product. The POD forums might or might not have someone familiar enough with your particular graphics application to be helpful.
Check the CafePress® tutorials on Designing for Dark Garments.
White on light?
Eventually you can expect the process to be offered on light apparel as well as the dark, but there are trade-offs. It will add to the cost of the garment - maybe as much as $5 per print side. The process that allows white to be printed is more expensive, and more labor intensive, and simply because of the multiple steps it is more error prone than the standard print process.
The direct print process on light apparel bonds the ink directly with the fabric for a very durable image. The dark print process prints on top of a base layer that is prone to cracking and flaking over time. The base layer is opaque in part because of fibers and so it affects the texture of the image printed on top of it. There are, as noted above, a different set of design constraints on the dark process with the underlayer over the light process without it. For more see the article White on Light.
For a detailed description of various print processes used by custom garment producers see the article "What is Digital Garment Printing?"