mesa: Make _mesa_get_compressed_formats match the texture compression specs

The implementation deviated slightly from the GL_EXT_texture_sRGB spec
and from other implementations.  A giant comment block was added to
justify the somewhat odd behavior of this function.

In addition, the interface had unnecessary cruft.  The 'all' parameter
was false at all callers, so it has been removed.

Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit b189d1635d89cd7d900e8f9a5eed88d7dc0b46cb)

3 files changed, 93 insertions, 30 deletions

diff --git a/src/mesa/main/get.c b/src/mesa/main/get.c
index 0492e1585c3..d32c68a53a4 100644
--- a/src/mesa/main/get.c
+++ b/src/mesa/main/get.c
@@ -1569,11 +1569,11 @@ find_custom_value(struct gl_context *ctx, const struct value_desc *d, union valu
       break;
 
    case GL_NUM_COMPRESSED_TEXTURE_FORMATS_ARB:
-      v->value_int = _mesa_get_compressed_formats(ctx, NULL, GL_FALSE);
+      v->value_int = _mesa_get_compressed_formats(ctx, NULL);
       break;
    case GL_COMPRESSED_TEXTURE_FORMATS_ARB:
       v->value_int_n.n = 
-	 _mesa_get_compressed_formats(ctx, v->value_int_n.ints, GL_FALSE);
+	 _mesa_get_compressed_formats(ctx, v->value_int_n.ints);
       ASSERT(v->value_int_n.n <= 100);
       break;
 
diff --git a/src/mesa/main/texcompress.c b/src/mesa/main/texcompress.c
index 040be943e82..42bd1eee5ca 100644
--- a/src/mesa/main/texcompress.c
+++ b/src/mesa/main/texcompress.c
@@ -131,16 +131,101 @@ _mesa_gl_compressed_format_base_format(GLenum format)
  * Return list of (and count of) all specific texture compression
  * formats that are supported.
  *
+ * Some formats are \b not returned by this function.  The
+ * \c GL_COMPRESSED_TEXTURE_FORMATS query only returns formats that are
+ * "suitable for general-purpose usage."  All texture compression extensions
+ * have taken this to mean either linear RGB or linear RGBA.
+ *
+ * The GL_ARB_texture_compress_rgtc spec says:
+ *
+ *    "19) Should the GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ *        GL_COMPRESSED_TEXTURE_FORMATS queries return the RGTC formats?
+ *
+ *        RESOLVED:  No.
+ *
+ *        The OpenGL 2.1 specification says "The only values returned
+ *        by this query [GL_COMPRESSED_TEXTURE_FORMATS"] are those
+ *        corresponding to formats suitable for general-purpose usage.
+ *        The renderer will not enumerate formats with restrictions that
+ *        need to be specifically understood prior to use."
+ *
+ *        Compressed textures with just red or red-green components are
+ *        not general-purpose so should not be returned by these queries
+ *        because they have restrictions.
+ *
+ *        Applications that seek to use the RGTC formats should do so
+ *        by looking for this extension's name in the string returned by
+ *        glGetString(GL_EXTENSIONS) rather than
+ *        what GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ *        GL_COMPRESSED_TEXTURE_FORMATS return."
+ *
+ * There is nearly identical wording in the GL_EXT_texture_compression_rgtc
+ * spec.
+ *
+ * The GL_EXT_texture_rRGB spec says:
+ *
+ *    "22) Should the new COMPRESSED_SRGB_* formats be listed in an
+ *        implementation's GL_COMPRESSED_TEXTURE_FORMATS list?
+ *
+ *        RESOLVED:  No.  Section 3.8.1 says formats listed by
+ *        GL_COMPRESSED_TEXTURE_FORMATS are "suitable for general-purpose
+ *        usage."  The non-linear distribution of red, green, and
+ *        blue for these sRGB compressed formats makes them not really
+ *        general-purpose."
+ *
+ * The GL_EXT_texture_compression_latc spec says:
+ *
+ *    "16) Should the GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ *        GL_COMPRESSED_TEXTURE_FORMATS queries return the LATC formats?
+ *
+ *        RESOLVED:  No.
+ *
+ *        The OpenGL 2.1 specification says "The only values returned
+ *        by this query [GL_COMPRESSED_TEXTURE_FORMATS"] are those
+ *        corresponding to formats suitable for general-purpose usage.
+ *        The renderer will not enumerate formats with restrictions that
+ *        need to be specifically understood prior to use."
+ *
+ *        Historically, OpenGL implementation have advertised the RGB and
+ *        RGBA versions of the S3TC extensions compressed format tokens
+ *        through this mechanism.
+ *
+ *        The specification is not sufficiently clear about what "suitable
+ *        for general-purpose usage" means.  Historically that seems to mean
+ *        unsigned RGB or unsigned RGBA.  The DXT1 format supporting alpha
+ *        (GL_COMPRESSED_RGBA_S3TC_DXT1_EXT) is not exposed in the list (at
+ *        least for NVIDIA drivers) because the alpha is always 1.0 expect
+ *        when it is 0.0 when RGB is required to be black.  NVIDIA's even
+ *        limits itself to true linear RGB or RGBA formats, specifically
+ *        not including EXT_texture_sRGB's sRGB S3TC compressed formats.
+ *
+ *        Adding luminance and luminance-alpha texture formats (and
+ *        certainly signed versions of luminance and luminance-alpha
+ *        formats!) invites potential comptaibility problems with old
+ *        applications using this mechanism since old applications are
+ *        unlikely to expect non-RGB or non-RGBA formats to be advertised
+ *        through this mechanism.  However no specific misinteractions
+ *        with old applications is known.
+ *
+ *        Applications that seek to use the LATC formats should do so
+ *        by looking for this extension's name in the string returned by
+ *        glGetString(GL_EXTENSIONS) rather than
+ *        what GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ *        GL_COMPRESSED_TEXTURE_FORMATS return."
+ *
+ * There is no formal spec for GL_ATI_texture_compression_3dc.  Since the
+ * formats added by this extension are luminance-alpha formats, it is
+ * reasonable to expect them to follow the same rules as
+ * GL_EXT_texture_compression_latc.  At the very least, Catalyst 11.6 does not
+ * expose the 3dc formats through this mechanism.
+ *
  * \param ctx  the GL context
  * \param formats  the resulting format list (may be NULL).
- * \param all  if true return all formats, even those with  some kind
- *             of restrictions/limitations (See GL_ARB_texture_compression
- *             spec for more info).
  *
  * \return number of formats.
  */
 GLuint
-_mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats, GLboolean all)
+_mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats)
 {
    GLuint n = 0;
    if (ctx->Extensions.TDFX_texture_compression_FXT1) {
@@ -152,24 +237,15 @@ _mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats, GLboolean a
          n += 2;
       }
    }
-   /* don't return RGTC - ARB_texture_compression_rgtc query 19 */
+
    if (ctx->Extensions.EXT_texture_compression_s3tc) {
       if (formats) {
          formats[n++] = GL_COMPRESSED_RGB_S3TC_DXT1_EXT;
-         /* This format has some restrictions/limitations and so should
-          * not be returned via the GL_COMPRESSED_TEXTURE_FORMATS query.
-          * Specifically, all transparent pixels become black.  NVIDIA
-          * omits this format too.
-          */
-         if (all)
-             formats[n++] = GL_COMPRESSED_RGBA_S3TC_DXT1_EXT;
          formats[n++] = GL_COMPRESSED_RGBA_S3TC_DXT3_EXT;
          formats[n++] = GL_COMPRESSED_RGBA_S3TC_DXT5_EXT;
       }
       else {
          n += 3;
-         if (all)
-             n += 1;
       }
    }
    if (ctx->Extensions.S3_s3tc) {
@@ -183,19 +259,6 @@ _mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats, GLboolean a
          n += 4;
       }
    }
-#if FEATURE_EXT_texture_sRGB
-   if (ctx->Extensions.EXT_texture_sRGB) {
-      if (formats) {
-         formats[n++] = GL_COMPRESSED_SRGB_S3TC_DXT1_EXT;
-         formats[n++] = GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT;
-         formats[n++] = GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT;
-         formats[n++] = GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT;
-      }
-      else {
-         n += 4;
-      }
-   }
-#endif /* FEATURE_EXT_texture_sRGB */
    return n;
 
 #if FEATURE_ES1 || FEATURE_ES2
diff --git a/src/mesa/main/texcompress.h b/src/mesa/main/texcompress.h
index 922da00912d..375cf90c8a2 100644
--- a/src/mesa/main/texcompress.h
+++ b/src/mesa/main/texcompress.h
@@ -37,7 +37,7 @@ extern GLenum
 _mesa_gl_compressed_format_base_format(GLenum format);
 
 extern GLuint
-_mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats, GLboolean all);
+_mesa_get_compressed_formats(struct gl_context *ctx, GLint *formats);
 
 extern gl_format
 _mesa_glenum_to_compressed_format(GLenum format);