[Mesa-dev] [PATCH 1/2] mesa: Fix a decoding bug in ETC2 signed-{r11, rg11} formats

[Anuj Phogat]

This patch fixes two issues in decoding signed formats:
1. Handle a corner case when base code word value is -128. As per
OpenGL ES 3.0 specification -128 is not an allowed value and should
be truncated to -127.
2. Converting a decoded 16 bit signed data to 16 bit unsigned data by
adding 2 ^ 15 gives us an output which matches the decompressed image
(.ppm) generated by ericsson's etcpack tool. ericsson is also doing this
conversion in their tool because .ppm image files don't support signed
data. gles 3.0 specification doesn't suggest this conversion. That is
the reason etc2 signed format tests currently fails on gles3 branch.
We need to keep the decoded data in signed format. This fix makes both
signed format tests in gles3 conformance pass.

Signed-off-by: Anuj Phogat <anuj.phogat at gmail.com>
---
 src/mesa/main/texcompress_etc.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/mesa/main/texcompress_etc.c b/src/mesa/main/texcompress_etc.c
index 44bdb84..b98ff02 100644
--- a/src/mesa/main/texcompress_etc.c
+++ b/src/mesa/main/texcompress_etc.c
@@ -613,6 +613,10 @@ etc2_signed_r11_fetch_texel(const struct etc2_block *block,
    GLint modifier, idx;
    GLshort color;
    GLbyte base_codeword = (GLbyte) block->base_codeword;
+
+   if (base_codeword == -128)
+      base_codeword = -127;
+
    /* Get pixel index */
    idx = etc2_get_pixel_index(block, x, y);
    modifier = etc2_modifier_tables[block->table_index][idx];
@@ -637,9 +641,7 @@ etc2_signed_r11_fetch_texel(const struct etc2_block *block,
       color = (color << 5) | (color >> 5);
       color = -color;
    }
-   /* Add 2^15 to get a 16-bit unsigned value */
-   color = color + (1 << 15);
-   ((GLushort *)dst)[0] = color;
+   ((GLshort *)dst)[0] = color;
 }
 
 static void
-- 
1.7.7.6