TexPaint artefacts after projecting image with alpha #46750

New Issue

item412 · 2015-11-12T16:24:30+01:00

item412 commented

2015-11-12 16:24:30 +01:00

System Information
win7 64

Blender Version
Broken: 2.76 8c84a18

Short description of error
in some cases projected images with alpha cause white lines on borders

Exact steps for others to reproduce the error
open attached .blend
apply image from camera

projTest.blend

**System Information** win7 64 **Blender Version** Broken: 2.76 8c84a18 **Short description of error** in some cases projected images with alpha cause white lines on borders ![757fb4524426fcb5b37e4cd00c3d2eb7.gif](https://archive.blender.org/developer/F254157/757fb4524426fcb5b37e4cd00c3d2eb7.gif) **Exact steps for others to reproduce the error** open attached .blend apply image from camera [projTest.blend](https://archive.blender.org/developer/F254159/projTest.blend)

item412 commented

2015-11-12 16:24:30 +01:00

Changed status to: 'Open'

item412 commented

2015-11-12 16:24:30 +01:00

Added subscriber: @item412

Sergey Sharybin commented

2015-11-17 09:21:42 +01:00

Added subscribers: @Psy-Fi, @ideasman42, @Sergey

Sergey Sharybin commented

2015-11-17 09:21:42 +01:00

Seems like blend function does not take care alpha mode (premul vs. straight) correctly into account.

@ideasman42, @Psy-Fi, will one of you have time to look into this area?

Seems like blend function does not take care alpha mode (premul vs. straight) correctly into account. @ideasman42, @Psy-Fi, will one of you have time to look into this area?

Bastien Montagne commented

2016-01-22 15:23:00 +01:00

Added subscriber: @mont29

Bastien Montagne commented

2016-01-22 15:23:00 +01:00

Changed status from 'Open' to: 'Archived'

Bastien Montagne closed this issue

2016-01-22 15:23:00 +01:00

Bastien Montagne self-assigned this 2016-01-22 15:23:00 +01:00

Bastien Montagne commented

2016-01-22 15:23:00 +01:00

Meh… Took me two hours to discover the issue is related to how that picture (12.png) is created, being straight alpha, and how bicubic interpolation works!

Basically, when interpolating points that are just on the edge, we get a mix of 0.0 and 0.5 alpha, and a mix of white and green colors, which gives us a rather transparent whitish green, which is correct result from code point of view.

You get exact same result if you e.g. scale that image in compositor.

So no real bug here, it's just some oddly-colored image and resize algo tricks… Just fill it with green everywhere and you'll be OK!

Meh… Took me two hours to discover the issue is related to how that picture (`12.png`) is created, being straight alpha, and how bicubic interpolation works! Basically, when interpolating points that are just on the edge, we get a mix of `0.0` and `0.5` alpha, and a mix of `white` and `green` colors, which gives us a rather transparent whitish green, which is correct result from code point of view. You get exact same result if you e.g. scale that image in compositor. So no real bug here, it's just some oddly-colored image and resize algo tricks… Just fill it with green everywhere and you'll be OK!

Bastien Montagne commented

2016-01-22 15:47:49 +01:00

For the records, the 'solution' to fix would be to convert to premultiplied before doing bicubic interpolation, here is a 'totally not for master) quick and dirty hack:

diff --git a/source/blender/blenlib/intern/math_interp.c b/source/blender/blenlib/intern/math_interp.c
index b45d8b4..a628b3b 100644
--- a/source/blender/blenlib/intern/math_interp.c
+++ b/source/blender/blenlib/intern/math_interp.c
@@ -94,10 +94,7 @@ static void vector_from_byte(const unsigned char *data, float vector[4], int com
                vector[2] = data[2];
        }
        else {
-               vector[0] = data[0];
-               vector[1] = data[1];
-               vector[2] = data[2];
-               vector[3] = data[3];
+               straight_uchar_to_premul_float(vector, data);
        }
 }
 
@@ -239,10 +236,7 @@ BLI_INLINE void bicubic_interpolation(const unsigned char *byte_buffer, const fl
                        byte_output[2] = (unsigned char)(out[2] + 0.5f);
                }
                else {
-                       byte_output[0] = (unsigned char)(out[0] + 0.5f);
-                       byte_output[1] = (unsigned char)(out[1] + 0.5f);
-                       byte_output[2] = (unsigned char)(out[2] + 0.5f);
-                       byte_output[3] = (unsigned char)(out[3] + 0.5f);
+                       premul_float_to_straight_uchar(byte_output, out);
                }
        }
 }

But I really do not think we want to do this? This is not 'lossless' conversion at all.

For the records, the 'solution' to fix would be to convert to premultiplied before doing bicubic interpolation, here is a 'totally **not** for master) quick and dirty hack: ``` diff --git a/source/blender/blenlib/intern/math_interp.c b/source/blender/blenlib/intern/math_interp.c index b45d8b4..a628b3b 100644 --- a/source/blender/blenlib/intern/math_interp.c +++ b/source/blender/blenlib/intern/math_interp.c @@ -94,10 +94,7 @@ static void vector_from_byte(const unsigned char *data, float vector[4], int com vector[2] = data[2]; } else { - vector[0] = data[0]; - vector[1] = data[1]; - vector[2] = data[2]; - vector[3] = data[3]; + straight_uchar_to_premul_float(vector, data); } } @@ -239,10 +236,7 @@ BLI_INLINE void bicubic_interpolation(const unsigned char *byte_buffer, const fl byte_output[2] = (unsigned char)(out[2] + 0.5f); } else { - byte_output[0] = (unsigned char)(out[0] + 0.5f); - byte_output[1] = (unsigned char)(out[1] + 0.5f); - byte_output[2] = (unsigned char)(out[2] + 0.5f); - byte_output[3] = (unsigned char)(out[3] + 0.5f); + premul_float_to_straight_uchar(byte_output, out); } } } ``` But I really do not think we want to do this? This is not 'lossless' conversion at all.

Sign in to join this conversation.

No Label

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

TexPaint artefacts after projecting image with alpha #46750