Round explicitly after dithering, for GPUs that don't do it properly themselves.

[movit] / init.cpp

diff --git a/init.cpp b/init.cpp
index f6dd9043751572ec03f4b64dd215ad8e3a217d69..396cd8b68106d5855e88ff6f67ee46f6d982f9b3 100644 (file)
--- a/init.cpp
+++ b/init.cpp
@@ -11,6 +11,7 @@ bool movit_initialized = false;
 MovitDebugLevel movit_debug_level = MOVIT_DEBUG_ON;
 float movit_texel_subpixel_precision;
 bool movit_srgb_textures_supported;
+int movit_num_wrongly_rounded;
 
 // The rules for objects with nontrivial constructors in static scope
 // are somewhat convoluted, and easy to mess up. We simply have a
@@ -131,6 +132,122 @@ void measure_texel_subpixel_precision()
        check_error();
 }
 
+void measure_roundoff_problems()
+{
+       // Generate a destination texture to render to, and an FBO.
+       GLuint dst_texnum, fbo;
+
+       glGenTextures(1, &dst_texnum);
+       check_error();
+       glBindTexture(GL_TEXTURE_2D, dst_texnum);
+       check_error();
+       glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 512, 1, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+       check_error();
+
+       glGenFramebuffers(1, &fbo);
+       check_error();
+       glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+       check_error();
+       glFramebufferTexture2D(
+               GL_FRAMEBUFFER,
+               GL_COLOR_ATTACHMENT0,
+               GL_TEXTURE_2D,
+               dst_texnum,
+               0);
+       check_error();
+
+       // Now generate a texture where every value except the last should be
+       // rounded up to the next one. However, there are cards (in highly
+       // common use) that can't do this right, for unknown reasons.
+       GLuint src_texnum;
+       float texdata[512];
+       for (int i = 0; i < 256; ++i) {
+               texdata[i * 2 + 0] = (i + 0.48) / 255.0;
+               texdata[i * 2 + 1] = (i + 0.52) / 255.0;
+       }
+       glGenTextures(1, &src_texnum);
+       check_error();
+       glBindTexture(GL_TEXTURE_1D, src_texnum);
+       check_error();
+       glTexParameteri(GL_TEXTURE_1D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+       check_error();
+       glTexParameteri(GL_TEXTURE_1D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+       check_error();
+       glTexImage1D(GL_TEXTURE_1D, 0, GL_LUMINANCE32F_ARB, 512, 0, GL_LUMINANCE, GL_FLOAT, texdata);
+       check_error();
+       glEnable(GL_TEXTURE_1D);
+       check_error();
+
+       // Basic state.
+       glDisable(GL_BLEND);
+       check_error();
+       glDisable(GL_DEPTH_TEST);
+       check_error();
+       glDepthMask(GL_FALSE);
+       check_error();
+
+       glViewport(0, 0, 512, 1);
+
+       glMatrixMode(GL_PROJECTION);
+       glLoadIdentity();
+       glOrtho(0.0, 1.0, 0.0, 1.0, 0.0, 1.0);
+
+       glMatrixMode(GL_MODELVIEW);
+       glLoadIdentity();
+       check_error();
+
+       // Draw the texture stretched over a long quad, interpolating it out.
+       glBegin(GL_QUADS);
+
+       glTexCoord1f(0.0f);
+       glVertex2f(0.0f, 0.0f);
+
+       glTexCoord1f(1.0f);
+       glVertex2f(1.0f, 0.0f);
+
+       glTexCoord1f(1.0f);
+       glVertex2f(1.0f, 1.0f);
+
+       glTexCoord1f(0.0f);
+       glVertex2f(0.0f, 1.0f);
+
+       glEnd();
+       check_error();
+
+       glDisable(GL_TEXTURE_1D);
+       check_error();
+
+       // Now read the data back and see what the card did. (Ignore the last value.)
+       // (We only look at the red channel; the others will surely be the same.)
+       unsigned char out_data[512];
+       glReadPixels(0, 0, 512, 1, GL_RED, GL_UNSIGNED_BYTE, out_data);
+       check_error();
+
+       int wrongly_rounded = 0;
+       for (unsigned i = 0; i < 255; ++i) {
+               if (out_data[i * 2 + 0] != i) {
+                       ++wrongly_rounded;
+               }
+               if (out_data[i * 2 + 1] != i + 1) {
+                       ++wrongly_rounded;
+               }
+       }
+
+       movit_num_wrongly_rounded = wrongly_rounded;
+
+       // Clean up.
+       glBindTexture(GL_TEXTURE_1D, 0);
+       check_error();
+       glBindFramebuffer(GL_FRAMEBUFFER, 0);
+       check_error();
+       glDeleteFramebuffers(1, &fbo);
+       check_error();
+       glDeleteTextures(1, &dst_texnum);
+       check_error();
+       glDeleteTextures(1, &src_texnum);
+       check_error();
+}
+
 void check_extensions()
 {
        // We fundamentally need FBOs and floating-point textures.
@@ -174,6 +291,7 @@ void init_movit(const std::string& data_directory, MovitDebugLevel debug_level)
        glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
 
        measure_texel_subpixel_precision();
+       measure_roundoff_problems();
        check_extensions();
 
        movit_initialized = true;