• TechNotes
• Alpha Channel in Textures

Dwight Atkinson, author of the books "Illustration In ArchiCAD" and "LightWorks in ArchiCAD" (write to info.beginnernomore@telus.net for buying instructions) shares his knowledge and gives some hints about the use of alpha channels in textures and their use in creating more realistic photo renderings in ArchiCAD. Enjoy:

• Dwight Atkinson writes: "Since alpha channels are a mystery to most users and questions recur in my seminars (Be like Tom Vu - take my seminar and have beautiful women laying around all over your boat- right), let me take a moment to clarify what I'm talking about in reference to the "alpha channel" and the "bump map" as used in an ArchiCAD material texture. When making materials for PhotoRendering, a photographic image or made-up pattern is applied to a surface with underlying color. This part is visible and obvious. But the invisible (and optional) alpha channel layer can elevate the appearance of "flat" ArchiCAD materials. I allude to this in my book "Illustration In ArchiCAD", where, in Chapter 4: Decoration, a lengthy series of illustrations show the range of effects invisible patterns in the alpha channel can have on a surface - from transparency to tinted varnishing. Since ArchiCAD lets the alpha channel behave arbitrarily and in opposite ways to the underlying material, there's a lot of action to be had using it in creating shimmering and interesting surface behavior. What gets tricky is when the image (the bitmap) has surface characteristics that aren't uniform. For instance, parts of this image overlay can be instructed to behave in special ways - to be transparent, to reveal the underlying color, to reflect specific types of light (as understood by ArchiCAD - in its own universe) Working in a photo editor, any selected pixels can be used to mask parts of the background image - In PhotoShop this is known as an "Alpha Channel Mask." Performing this action creates an extra image channel - a fourth one - additional to the first three grayscale channels - for Red, Green and Blue - that define the visible parts of the image. A good example is when looking at a photo of a masonry wall where the joints are to act as shadowed places to enhance the relief of the wall surface - less like the trailer trash product "Insul-brick" where the bricks and joints are drawn on a material with the thickness of roof shingles - no relief there - and more like real bricks with deep mortar joints. In the photo editor, one selects the joints (of a different color than the adjacent masonry) and masks these areas. Using the image in ArchiCAD, the user tells the alpha channel to be a "bump map" - In a photo rendering, this bump map reacts to the sun angle, simulating a cast shadow. In some cases - the vertical siding case - in the photo editor one would draw a vertical line in the alpha channel after creating the channel. By making the line several pixels wide and slightly blurring it on one edge, a soft overlapping siding shadow (the bump) can be simulated in a photo-rendering. But nowhere else. This technique doesn't work in Artlantis since it understands alpha channels differently than does ArchiCAD (and with LightWorks rendering). Remember that what you do in the alpha channel is invisible - it only affects bump mapping, transparency or reflected light (as specified by you in the texture editor dialog of ArchiCAD)."

• Category3dModeling, CategoryRendering