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git-svn-id: http://svn.code.sf.net/p/netpbm/code/trunk@1 9d0c8265-081b-0410-96cb-a4ca84ce46f8

1 files changed, 68 insertions, 0 deletions

diff --git a/lib/pm_gamma.h b/lib/pm_gamma.h
new file mode 100644
index 00000000..92b34145
--- /dev/null
+++ b/lib/pm_gamma.h
@@ -0,0 +1,68 @@
+#ifndef _PM_GAMMA_H_
+#define _PM_GAMMA_H_
+
+#include "pm_config.h"
+
+#include <math.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+#if 0
+} /* to fake out automatic code indenters */
+#endif
+
+static __inline__ float
+pm_gamma709(float const intensity) {
+
+    /* Here are parameters of the gamma transfer function
+       for the Netpbm formats.  This is CIE Rec 709.
+       
+       This transfer function is linear for sample values 0 .. .018 
+       and an exponential for larger sample values.
+       The exponential is slightly stretched and translated, though,
+       unlike the popular pure exponential gamma transfer function.
+    */
+    float const gamma = 2.2;
+    float const oneOverGamma = 1.0 / gamma;
+    float const linearCutoff = 0.018;
+    float const linearExpansion = 
+        (1.099 * pow(linearCutoff, oneOverGamma) - 0.099) / linearCutoff;
+
+    float brightness;
+
+    if (intensity < linearCutoff)
+        brightness = intensity * linearExpansion;
+    else
+        brightness = 1.099 * pow(intensity, oneOverGamma) - 0.099;
+
+    return brightness;
+}
+
+
+
+static __inline__ float
+pm_ungamma709(float const brightness) {
+
+    /* These are the same parameters as in pm_gamma, above */
+
+    float const gamma = 2.2;
+    float const oneOverGamma = 1.0 / gamma;
+    float const linearCutoff = 0.018;
+    float const linearExpansion = 
+        (1.099 * pow(linearCutoff, oneOverGamma) - 0.099) / linearCutoff;
+    
+    float intensity;
+
+    if (brightness < linearCutoff * linearExpansion)
+        intensity = brightness / linearExpansion;
+    else
+        intensity = pow((brightness + 0.099) / 1.099, gamma);
+
+    return intensity;
+}
+
+#ifdef __cplusplus
+}
+#endif
+#endif