[Mesa-dev] [PATCH 4/5] mesa: Add function for decoding ETC1 textures

[Chad Versace]

Add function _mesa_etc1_image_decode_to_abgr8888(). It is intended to be
used by glCompressedTexSubImage2D to translate ETC1 textures into ABGR.

Signed-off-by: Chad Versace <chad.versace at linux.intel.com>
---
 src/mesa/main/texcompress_etc.c | 82 +++++++++++++++++++++++++++++++++++++++++
 src/mesa/main/texcompress_etc.h |  6 +++
 2 files changed, 88 insertions(+)

diff --git a/src/mesa/main/texcompress_etc.c b/src/mesa/main/texcompress_etc.c
index e3b2e83..d84ee0e 100644
--- a/src/mesa/main/texcompress_etc.c
+++ b/src/mesa/main/texcompress_etc.c
@@ -178,3 +178,85 @@ _mesa_fetch_texel_2d_f_etc1_rgb8(const struct swrast_texture_image *texImage,
    texel[BCOMP] = UBYTE_TO_FLOAT(bgr888[2]);
    texel[ACOMP] = 1.0f;
 }
+
+static int
+align(int x, int m)
+{
+   return (m * x + m - 1) / m;
+}
+
+/**
+ * Decode texture data in format `MESA_FORMAT_ETC1_RGB8` to
+ * `MESA_FORMAT_ABGR8888`.
+ *
+ * The size of the source data must be a multiple of the ETC1 block size,
+ * which is 8, even if the texture image's dimensions are not aligned to 4.
+ * From the GL_OES_compressed_ETC1_RGB8_texture spec:
+ *   The texture is described as a number of 4x4 pixel blocks. If the
+ *   texture (or a particular mip-level) is smaller than 4 pixels in
+ *   any dimension (such as a 2x2 or a 8x1 texture), the texture is
+ *   found in the upper left part of the block(s), and the rest of the
+ *   pixels are not used. For instance, a texture of size 4x2 will be
+ *   placed in the upper half of a 4x4 block, and the lower half of the
+ *   pixels in the block will not be accessed.
+ *
+ * \param src_width in pixels
+ * \param src_height in pixels
+ * \param dst_rowstride in bytes
+ */
+void
+_mesa_etc1_image_decode_to_abgr8888(const uint8_t *src,
+                                    int src_width,
+                                    int src_height,
+                                    uint8_t *dst,
+                                    int dst_rowstride)
+{
+   /* An XRGB pixel is 4 bytes. */
+   const int pixel_bytes = 4;
+
+   /* A block contains 4x4 pixels. */
+   const int block_width = 4;
+   const int block_height = 4;
+
+   /* A block contains 8 bytes. */
+   const int block_bytes = 8;
+
+   /* The stride in units of blocks. */
+   const int block_stride = align(src_width, block_width) / block_width;
+
+   /* The block being decoded. */
+   int block_x;
+   int block_y;
+
+   /* Iterate over each block. */
+   for (block_y = 0; block_y * block_height < src_height; ++block_y) {
+      for (block_x = 0; block_x * block_width < src_width; ++block_x) {
+         struct etc1_block block;
+
+         /* The pixel's position relative to the block's base. */
+         int pixel_x;
+         int pixel_y;
+
+         const uint8_t *src_block_base =
+               src + block_bytes * (block_y * block_stride + block_x);
+
+         uint8_t *dst_block_base = dst
+                                 + block_y * block_height * dst_rowstride
+                                 + block_x * block_width * pixel_bytes;
+
+         etc1_parse_block(&block, src_block_base);
+
+         /* Iterate over each pixel in block. */
+         for (pixel_y = 0; pixel_y < block_height; ++pixel_y) {
+            for (pixel_x = 0; pixel_x < block_width; ++pixel_x) {
+               uint8_t *dst_texel = dst_block_base
+                                  + pixel_y * dst_rowstride
+                                  + pixel_x * pixel_bytes;
+
+               etc1_fetch_texel(&block, pixel_x, pixel_y, dst_texel);
+               dst_texel[3] = 0xff;
+            }
+         }
+      }
+   }
+}
diff --git a/src/mesa/main/texcompress_etc.h b/src/mesa/main/texcompress_etc.h
index 8e8427f..552a7bd 100644
--- a/src/mesa/main/texcompress_etc.h
+++ b/src/mesa/main/texcompress_etc.h
@@ -36,5 +36,11 @@ _mesa_texstore_etc1_rgb8(TEXSTORE_PARAMS);
 void
 _mesa_fetch_texel_2d_f_etc1_rgb8(const struct swrast_texture_image *texImage,
                                  GLint i, GLint j, GLint k, GLfloat *texel);
+void
+_mesa_etc1_image_decode_to_abgr8888(const uint8_t *src,
+                                    int src_width,
+                                    int src_height,
+                                    uint8_t *dst,
+                                    int dst_rowstride);
 
 #endif
-- 
1.7.11.1