Align with KDFS 1.4.0 rounding update (#548)

Updates the codec rounding for sRGB to align with the KDFS 1.4.0
specification update, which reverts the alpha channel endpoint 
expansion behavior to match an earlier version of the specification.

This changes the alpha channel to use the same method 
((x << 8) | 0x80) as the RGB channels. The previous behavior for 
alpha was doing ((x << 8) | x). The impact of this change is 
differences in the LSB for sRGB images.

Fixes #547

Author: solidpixel

Docs/ChangeLog-5x.md

@@ -18,6 +18,10 @@ header.  We always recommend rebuilding your client-side code using the
 header from the same release to avoid compatibility issues.
 
 * **General:**
+  * **Change:** Changed sRGB alpha channel endpoint expansion to match the
+    revised Khronos Data Format Specification (v1.4.0), which reverts an
+    unintended specification change. Compared to previous releases, this change
+    can cause LSB bit differences in the alpha channel of compressed images.
   * **Feature:** Arm64 builds for Linux added to the GitHub Actions builds, and
     Arm64 binaries for NEON, 128-bit SVE 128 and 256-bit SVE added to release
     builds.

Source/astcenc_color_unquantize.cpp

@@ -925,15 +925,8 @@ void unpack_color_endpoints(
 			alpha_hdr = false;
 		}
 
-		vmask4 mask(true, true, true, false);
-
-		vint4 output0rgb = lsl<8>(output0) | vint4(0x80);
-		vint4 output0a = output0 * 257;
-		output0 = select(output0a, output0rgb, mask);
-
-		vint4 output1rgb = lsl<8>(output1) | vint4(0x80);
-		vint4 output1a = output1 * 257;
-		output1 = select(output1a, output1rgb, mask);
+		output0 = lsl<8>(output0) | vint4(0x80);
+		output1 = lsl<8>(output1) | vint4(0x80);
 	}
 	// An HDR profile decode, but may be using linear LDR endpoints
 	// Linear LDR 8-bit endpoints are expanded to 16-bit by replication

Source/astcenc_internal.h

@@ -1583,19 +1583,13 @@ static inline vmask4 get_u8_component_mask(
 	astcenc_profile decode_mode,
 	const image_block& blk
 ) {
-	vmask4 u8_mask(false);
-	// Decode mode writing to a unorm8 output value
-	if (blk.decode_unorm8)
+	// Decode mode or sRGB forces writing to unorm8 output value
+	if (blk.decode_unorm8 || decode_mode == ASTCENC_PRF_LDR_SRGB)
 	{
-		u8_mask = vmask4(true);
-	}
-	// SRGB writing to a unorm8 RGB value
-	else if (decode_mode == ASTCENC_PRF_LDR_SRGB)
-	{
-		u8_mask = vmask4(true, true, true, false);
+		return vmask4(true);
 	}
 
-	return u8_mask;
+	return vmask4(false);
 }
 
 /**

Test/Images/Small/astc_reference-main-avx2_fast_results.csv

@@ -319,7 +319,7 @@ def parse_command_line():
     parser.add_argument("--encoder", dest="encoders", default="avx2",
                         choices=coders, help="test encoder variant")
 
-    parser.add_argument("--reference", dest="reference", default="ref-5.0-avx2",
+    parser.add_argument("--reference", dest="reference", default="ref-main-avx2",
                         choices=refcoders, help="reference encoder variant")
 
     astcProfile = ["ldr", "ldrs", "hdr", "all"]