[Bf-committers] REVIEW: Predivide for premultiplied images patch. ATTN: Brecht

[Troy Sobotka]

In the ever ongoing deluxe saga of trying to get Blender to correctly
ingest premultiplied alpha images such as TIFFs, the following patch
is offered for review. Special thanks go to Brecht for his extreme
patience and sagely advice.

ATTN: This patch also adds in the three lines of code to properly
update the window region when the update signal is set. This code was
supplied by Brecht.

Based on feedback from Brecht, the patch now is purely artist driven.
The artist toggles are available via the Image Input node and the UV
Image propertiers pane for ingestion. For output, if Color Management
is selected, a Predivide box is offered in the Properties box in the
Shading region.

Q: What does it do?
A: The patch predivides the RGB values when the artist driven toggles
are selected. This permits accurate sRGB to linear and vice versa
conversions.

Q: Does it allow me to load premultiplied alpha TIFF images correctly
or use my premultiplied Blender renders in the VSE?
A: Yes. In the case of the latter, you will need to unmangle the
existing renders via the following technique.

Q: Does it allow me to unmangle or reuse legacy renders I have made
with Blender using premultiplied alpha?
A: Yes. In the case of previously mangled alpha images, the correct
process would be to leave Predivide off for ingestion and turn
Predivide on for output.

Q: What is the correct workflow involved with premultiplied data
ingestion into a linear environment?
A: The correct workflow is to select Predivide for all sRGB
premultiplied alpha images and only select Predivide if you are saving
with a premultiplied alpha.

Q: Is there a set of images to test this with?
A: Yes. In the attached bug report there is a well crafted
premultiplied alpha foreground plate and a complimentary fully opaque
background. This pattern designed by Guillermo Espertino will quickly
reveal inconsistencies in sRGB premultiplied conversions. It would be
excellent if Blender experts of their respective domains would use the
two images to test where we need to extend this technique.

http://projects.blender.org/tracker/?func=detail&atid=498&aid=28085&group_id=9

Technical details:

1) Adds one flag to ImBuf and the corresponding iff for plugins.
2) Adds one variable to Image to hold the cm_flags.
3) Adds one bitwise flag define for use with the color_mgt_flag in
RenderData for Scene access.
4) Modifies the IMB_linear_from_rect and IMB_rect_from_linear to honor
the input and output flags.
5) Modifies pipeline.c to honor the Scene setting flag.
6) A few adjustments within image.c to copy the new cm_flags in the
Image struct to the ImBuf struct.
7) Updates the update window manager code according to instructions by Brecht.

With respect,
TJS