[Bf-committers] Proposal: Remove fixed upper bound on number of render layers and pave the way for stacked image layers

[Konrad Kleine]

Hi,

I've been wondering if there's a particular reason why the Image
struct defined in DNA_image_types.h has an upper bound on the number
of render layers (#define IMA_MAX_RENDER_SLOT 8). If it's possible to
have "n" render layers (e.g. use ListBase), then we might actually be
able to use the render layers as "image" layers and not only as
"render" layers.

A while ago when I proposed to introduce a new layered ImBuf stack to
the basic Image struct, people objected because it would have been a
pain to integrate. Whereas the render layers are already accessible
through the compositor and so forth. I must admit that I totally agree
with this objection.

One might not need more than 8 image layers when rendering but nearly
everybody needs more than 8 layers when it gets down to painting
images in Gimp or Photoshop, right?

Here's my proposal and some implications that came to my mind:

1. Code-wise replace the render layers array with a List of render
layers (whole lotta work, I know)
2. Enrich the render layer struct with information more related to 2D
graphics software (e.g. opacity, mask, etc.)
3. Associate the order of elements in the render layers list with the
order the images are stacked upon each other (e.g. first element in
list is the front image)
4. Build UI that can handle layering of images on top of each other
(e.g. http://wiki.blender.org/index.php/File:Soc-2010-kwk_image_layers.png)
5. Somehow allow compositing the image as a whole (e.g. through
blender's compositor or OpenGL texture blending)
6. Change the preview mode for images (blender-wide?)

General open questions:
- How to deal with image layers in e.g. textures? Is only a layer used
or the composite?

Here's the link to my first hacky attempt to implement image layers in
Blender: http://wiki.blender.org/index.php/User:Kwk/Gsoc2010/ImageLayers

-Konrad