[qtAliceVision] FloatImageViewer: Clamp color values in fragment shader

Clamp negative values in the FloatImageViewer fragment shader before
performing the successive `pow`. The result of `pow` is undefined when
it is provided negative values. On Linux platforms, it still gives the
expected result, but on Windows this leads to issues.

Author: cbentejac

src/qtAliceVision/FloatImageViewer.frag

@@ -3,8 +3,8 @@ layout(location = 0) in vec2 vTexCoord;
 layout(location = 0) out vec4 fragColor;
 
 layout(std140, binding = 0) uniform buf {  // warning: matches layout and padding of FloatImageViewerMaterial::uniforms!
-    mat4 qt_Matrix; // offset 0
-    float qt_Opacity; // offset 64
+    mat4 qt_Matrix;  // offset 0
+    float qt_Opacity;  // offset 64
     float _padding1;
     float _padding2;
     float _padding3;
@@ -21,12 +21,19 @@ layout(binding = 1) uniform sampler2D tex;
 void main()
 {
     vec4 color = texture(tex, vTexCoord);
-    color.rgb = pow(pow((color.rgb * vec3(gain)), vec3(1.0/gamma)), vec3(1.0 / 2.2));
+
+    // Clamp negative values after applying the gain as the
+    // result of 'pow' is undefined when provided with negative values
+    vec3 clampedColorWithGain = max(color.rgb * vec3(gain), vec3(0.0, 0.0, 0.0));
+
+    color.rgb = pow(pow(clampedColorWithGain, vec3(1.0 / gamma)), vec3(1.0 / 2.2));
+
     fragColor.r = color[int(channelOrder[0])];
     fragColor.g = color[int(channelOrder[1])];
     fragColor.b = color[int(channelOrder[2])];
     fragColor.a = int(channelOrder[3]) == -1 ? 1.0 : color[int(channelOrder[3])];
     fragColor.a *= qt_Opacity;
+
     if (distance(vec2(vTexCoord.x * aspectRatio, vTexCoord.y), vec2(fisheyeCircleCoord.x * aspectRatio, fisheyeCircleCoord.y)) > fisheyeCircleRadius && fisheyeCircleRadius > 0.0)
     {
        fragColor.a *= 0.001;