Added support for floating point depth buffers on Savage4-based

hardware. By also reversing the depth range this can compensate the loss
of accuracy of far objects caused by the projective transformation.
Software fallbacks work but are slightly slower since floats in a custom
(non IEEE) format have to be encoded and decoded. I havn't done anything
about polygon offsets yet. There doesn't seem to be an easy way do get
it right except making the offset unit as big as the lowest resolution
of depth values. For now float depth is disabled by default but can be
enabled through driconf (though I have seen only positive effects so
far).

1 files changed, 98 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/savage/savagespan.h b/src/mesa/drivers/dri/savage/savagespan.h
index 35247b4706..cb3a1b52fd 100644
--- a/src/mesa/drivers/dri/savage/savagespan.h
+++ b/src/mesa/drivers/dri/savage/savagespan.h
@@ -27,4 +27,102 @@
 
 extern void savageDDInitSpanFuncs( GLcontext *ctx );
 
+/*
+ * Savage 16-bit float depth format with zExpOffset=16:
+ *   4 bit unsigned exponent, 12 bit mantissa
+ *
+ * The meaning of the mantissa is different from IEEE floatint point
+ * formats. The same number can't be encoded with different exponents.
+ * So no bits are wasted.
+ *
+ * exponent | range encoded by mantissa | accuracy or mantissa
+ * ---------+---------------------------+---------------------
+ *       15 | 2^-1 .. 1                 | 2^-13
+ *       14 | 2^-2 .. 2^-1              | 2^-14
+ *       13 | 2^-3 .. 2^-2              | 2^-15
+ *      ... | ...                       |
+ *        2 | 2^-14 .. 2^-13            | 2^-27
+ *        1 | 2^-15 .. 2^-14            | 2^-27
+ *        0 | 2^-16 .. 2^-15            | 2^-28
+ *
+ * Note that there is no encoding for numbers < 2^-16.
+ */
+static __inline GLuint savageEncodeFloat16( GLdouble x )
+{
+    GLint r = (GLint)(x * 0x10000000);
+    GLint exp = 0;
+    if (r < 0x1000)
+	return 0;
+    while (r - 0x1000 > 0x0fff) {
+	r >>= 1;
+	exp++;
+    }
+    return exp > 0xf ? 0xffff : (r - 0x1000) | (exp << 12);
+}
+static __inline GLdouble savageDecodeFloat16( GLuint x )
+{
+    static const GLdouble pow2[16] = {
+	1.0/(1<<28), 1.0/(1<<27), 1.0/(1<<26), 1.0/(1<<25),
+	1.0/(1<<24), 1.0/(1<<23), 1.0/(1<<22), 1.0/(1<<21),
+	1.0/(1<<20), 1.0/(1<<19), 1.0/(1<<18), 1.0/(1<<17),
+	1.0/(1<<16), 1.0/(1<<15), 1.0/(1<<14), 1.0/(1<<13)
+    };
+    static const GLdouble bias[16] = {
+	1.0/(1<<16), 1.0/(1<<15), 1.0/(1<<14), 1.0/(1<<13),
+	1.0/(1<<12), 1.0/(1<<11), 1.0/(1<<10), 1.0/(1<< 9),
+	1.0/(1<< 8), 1.0/(1<< 7), 1.0/(1<< 6), 1.0/(1<< 5),
+	1.0/(1<< 4), 1.0/(1<< 3), 1.0/(1<< 2), 1.0/(1<< 1)
+    };
+    GLuint mant = x & 0x0fff;
+    GLuint exp = (x >> 12) & 0xf;
+    return bias[exp] + pow2[exp]*mant;
+}
+
+/*
+ * Savage 24-bit float depth format with zExpOffset=32:
+ *   5 bit unsigned exponent, 19 bit mantissa
+ *
+ * Details analogous to the 16-bit format.
+ */
+static __inline GLuint savageEncodeFloat24( GLdouble x )
+{
+    int64_t r = (int64_t)(x * ((int64_t)1 << (19+32)));
+    GLint exp = 0;
+    if (r < 0x80000)
+	return 0;
+    while (r - 0x80000 > 0x7ffff) {
+	r >>= 1;
+	exp++;
+    }
+    return exp > 0x1f ? 0xffffff : (r - 0x80000) | (exp << 19);
+}
+#define _1 (int64_t)1
+static __inline GLdouble savageDecodeFloat24( GLuint x )
+{
+    static const GLdouble pow2[32] = {
+	1.0/(_1<<51), 1.0/(_1<<50), 1.0/(_1<<49), 1.0/(_1<<48),
+	1.0/(_1<<47), 1.0/(_1<<46), 1.0/(_1<<45), 1.0/(_1<<44),
+	1.0/(_1<<43), 1.0/(_1<<42), 1.0/(_1<<41), 1.0/(_1<<40),
+	1.0/(_1<<39), 1.0/(_1<<38), 1.0/(_1<<37), 1.0/(_1<<36),
+	1.0/(_1<<35), 1.0/(_1<<34), 1.0/(_1<<33), 1.0/(_1<<32),
+	1.0/(_1<<31), 1.0/(_1<<30), 1.0/(_1<<29), 1.0/(_1<<28),
+	1.0/(_1<<27), 1.0/(_1<<26), 1.0/(_1<<25), 1.0/(_1<<24),
+	1.0/(_1<<23), 1.0/(_1<<22), 1.0/(_1<<21), 1.0/(_1<<20)
+    };
+    static const GLdouble bias[32] = {
+	1.0/(_1<<32), 1.0/(_1<<31), 1.0/(_1<<30), 1.0/(_1<<29),
+	1.0/(_1<<28), 1.0/(_1<<27), 1.0/(_1<<26), 1.0/(_1<<25),
+	1.0/(_1<<24), 1.0/(_1<<23), 1.0/(_1<<22), 1.0/(_1<<21),
+	1.0/(_1<<20), 1.0/(_1<<19), 1.0/(_1<<18), 1.0/(_1<<17),
+	1.0/(_1<<16), 1.0/(_1<<15), 1.0/(_1<<14), 1.0/(_1<<13),
+	1.0/(_1<<12), 1.0/(_1<<11), 1.0/(_1<<10), 1.0/(_1<< 9),
+	1.0/(_1<< 8), 1.0/(_1<< 7), 1.0/(_1<< 6), 1.0/(_1<< 5),
+	1.0/(_1<< 4), 1.0/(_1<< 3), 1.0/(_1<< 2), 1.0/(_1<< 1)
+    };
+    GLuint mant = x & 0x7ffff;
+    GLuint exp = (x >> 19) & 0x1f;
+    return bias[exp] + pow2[exp]*mant;
+}
+#undef _1
+
 #endif