Fix an issue where a (cached) shader program could be used from multiple

[movit] / resample_effect.frag

diff --git a/resample_effect.frag b/resample_effect.frag
index d71b5f5128d60e8656b544c476a58b7ed634dfda..26838dcfbbad3f1a3df25cc18d44475c2cc998d9 100644 (file)
--- a/resample_effect.frag
+++ b/resample_effect.frag
@@ -1,12 +1,33 @@
 // DIRECTION_VERTICAL will be #defined to 1 if we are scaling vertically,
 // and 0 otherwise.
 
-uniform sampler2D PREFIX(sample_tex);
-uniform int PREFIX(num_samples);
-uniform float PREFIX(num_loops);
-uniform float PREFIX(sample_x_scale);
-uniform float PREFIX(sample_x_offset);
-uniform float PREFIX(slice_height);
+// Implicit uniforms:
+// uniform sampler2D PREFIX(sample_tex);
+// uniform int PREFIX(num_samples);
+// uniform float PREFIX(num_loops);
+// uniform float PREFIX(sample_x_scale);
+// uniform float PREFIX(sample_x_offset);
+// uniform float PREFIX(slice_height);
+
+// We put the fractional part of the offset (-0.5 to 0.5 pixels) in the weights
+// because we have to (otherwise they'd do nothing). However, the support texture
+// has limited numerical precision; we'd need as much of it as we can for
+// getting the subpixel sampling right, and adding a large constant to each value
+// will reduce the precision further. Thus, the non-fractional part of the offset
+// is sent in through a uniform that we simply add in. (It should be said that
+// for high values of (dst_size/num_loop), we're pretty much hosed anyway wrt.
+// this accuracy.)
+//
+// Unfortunately, we cannot just do it at the beginning of the shader,
+// since the texcoord value is used to index into the support texture,
+// and if zoom != 1, the support texture will not wrap properly, causing
+// us to read the wrong texels. (Also remember that whole_pixel_offset is
+// measured in _input_ pixels and tc is in _output_ pixels, although we could
+// compensate for that.) However, the shader should be mostly bandwidth bound
+// and not ALU bound, so an extra add per sample shouldn't be too hopeless.
+//
+// Implicitly declared:
+// uniform float PREFIX(whole_pixel_offset);
 
 // Sample a single weight. First fetch information about where to sample
 // and the weight from sample_tex, and then read the pixel itself.
@@ -22,9 +43,9 @@ vec4 PREFIX(do_sample)(vec2 tc, int i)
        vec2 sample = tex2D(PREFIX(sample_tex), sample_tc).rg;
 
 #if DIRECTION_VERTICAL
-       tc.y = sample.g + floor(sample_tc.y) * PREFIX(slice_height);
+       tc.y = sample.g + (floor(sample_tc.y) * PREFIX(slice_height) + PREFIX(whole_pixel_offset));
 #else
-       tc.x = sample.g + floor(sample_tc.y) * PREFIX(slice_height);
+       tc.x = sample.g + (floor(sample_tc.y) * PREFIX(slice_height) + PREFIX(whole_pixel_offset));
 #endif
        return vec4(sample.r) * INPUT(tc);
 }