Alpha map

steve6

Could someone explain alpha maps for me and how they can be used in Sketchup (step-by-step tutorial?).

Thanks!

Frederik

Hi Steve,

It seem as if you never use the SCF Forum search...

You could also check the Wikipedia Alpha-channel explanation...

And I believe the Using clipmaps with Materials thread will also help you understand this...

steve6

I've seen all those. What I'm looking for is how exactly they are to be used in Sketchup (or Kerkythea), and a step-by-step guide. Don't forget, there are people out here sincerely trying to learn these programs who are entirely new to these programs!

remus

SU doesnt really do alpha maps, although it does support the alpha channel found in PNG files (and GIFs, i think.)

If you want to learn how to use alpha maps, as in a seperate image that acts to set the alpha channel for a certain image, youll want to check out the kerkythea site. Specificaly: http://www.kerkythea.net/joomla/index.php?option=com_remository&Itemid=42&func=fileinfo&id=9

Alan Fraser

To answer the original question. You don't need to do anything specific with images containing transparency; you just need to import them and they work.
Strictly speaking, pngs don't have alpha transparency; they just don't have any background. Tiffs do have alpha transparency because they have an actual alpha channel. Both can be prepared in Photoshop or something similar. Tiffs are easier to work with in Kerkythea.

To produce a transparent png, you just take an ordinary image...like a jpg...open it in PShop and do one of two things. Either start deleting the parts you don't want using the background eraser, or use the layers menu to create a Layer from Background, then you can use the standard eraser. There are variations on this, like creating a temporary layer on top of the image and painting on that...either the parts you want to keep or delete then using the select tool, switching to the image layer and deleting the bits you don't want. Save the result as a png and import into SU.
A tiff is somewhat similar, but instead of deleting parts of the image (so you can see the checker pattern underneath) you go to the Channels tab on the Layers palette (bottom right) and create a new channel...which will automatically be called alpha. Anything on this channel that is black will cause those parts of the other RGB channels to be transparent. Anything that is white will be solid. How you actually achieve this is a matter of personal preference. The easiest way is probably to again use a temporary layer to paint on, differentiating between the opaque and transparent bits, then copy this onto the alpha channel

GreyHead

@unknownuser said:

Strictly speaking, pngs don't have alpha transparency; they just don't have any background.

I believe that pngs have the capability of a full alpha channel, though it may not always be used. It's GIFs that only support a single 'transparent color' mode.

Bob

@unknownuser said:

tRNS contains transparency information. For indexed images, it stores alpha channel values for one or more palette entries. For truecolor and greyscale images, it stores a single pixel value that is to be regarded as fully transparent. http://en.wikipedia.org/wiki/Portable_Network_GraphicsWikipedia

ridix

With Corel Photopaint, all you need to do is to open an image, add masks where area needs to be transparent, than save as PNG file with “Transparency, Masked Area” checked. Than apply this PNG file as texture in SK.

ridix

Here is my test on transparent png file; the archway is made transparent in this case.

It is transparent of sorts, but as far as lighting and shadow is concerned shadow is generated by the plane the texture is sitting on. The shadow projection will not create the arch shadow. Well alpha map generate “correct” shadow pattern in Sketchup?

Gaieus

This function is not supported yet in SU (wish SU 7 fixed it)

Alan Fraser

We are unlikely to see transparent images casting shadows in SU. SU's shadows are vector based, therefore they only sense actual geometry. They are completely blind to whether a pixel on an image map is transparent or not. Of course Google may be able to figure a way, but I'm not holding my breath