Special treatment for 1-class segmentation category mask output on GPU.

PiperOrigin-RevId: 523271622

mediapipe/tasks/cc/vision/image_segmenter/calculators/segmentation_postprocessor_gl.cc

@@ -185,6 +185,28 @@ void main() {
     }
 })";
 
+// Special argmax shader for N=1 classes. We don't need to worry about softmax
+// activation (it is assumed softmax requires N > 1 classes), but this should
+// occur after SIGMOID activation if specified. Instead of a true argmax, we
+// simply use 0.5 as the cutoff, assigning 1 (foreground) or 0 (background)
+// based on whether the confidence value reaches this cutoff or not,
+// respectively.
+static constexpr char kArgmaxOneClassShader[] = R"(
+DEFAULT_PRECISION(mediump, float)
+in vec2 sample_coordinate;
+uniform sampler2D input_texture;
+
+void main() {
+  float input_val = texture2D(input_texture, sample_coordinate).x;
+  // Category is just value rounded to nearest integer; then we map to either
+  // 0 or 1/255 accordingly. If the input has been activated properly, then the
+  // values should always be in the range [0, 1]. But just in case it hasn't, to
+  // avoid category overflow issues when the activation function is not properly
+  // chosen, we add an extra clamp here, as performance hit is minimal.
+  float category = clamp(floor(input_val + 0.5), 0.0, 1.0);
+  gl_FragColor = vec4(category / 255.0, 0.0, 0.0, 1.0);
+})";
+
 // Softmax is in 3 steps:
 // - First we find max over all masks
 // - Then we transform all masks to be exp(val - maxval), and also add to
@@ -377,11 +399,14 @@ absl::Status SegmentationPostprocessorGl::GlInit() {
         "softmax normalization", kNormalizationShader,
         {"sum_texture", "current_chunk"}, &softmax_normalization_shader_));
 
-    // Category mask shaders (Argmax)
+    // Category mask shaders (Argmax and special 1-class fg/bg argmax)
     MP_RETURN_IF_ERROR(CreateBasicFragmentShaderProgram(
         "argmax", kArgmaxShader,
         {"prev_max_texture", "current_chunk", "num_channels", "argmax_offset"},
         &argmax_shader_));
+    MP_RETURN_IF_ERROR(CreateBasicFragmentShaderProgram(
+        "one-class argmax", kArgmaxOneClassShader, {"input_texture"},
+        &argmax_one_class_shader_));
 
     // Split shader. This is created separately since it uses a custom vertex
     // shader. TODO: Refactor so this shares common init code as well.
@@ -646,10 +671,25 @@ SegmentationPostprocessorGl::GetSegmentationResultGpu(const Shape& input_shape,
 
     std::vector<GlTexture> outputs;
     if (is_category_mask) {
-      // Step 3: For CATEGORY, apply argmax shader iteratively with each chunk
+      // Step 3, N = 1: For CATEGORY with 1 class, use special FG/BG argmax
-      // to get a 2-channel texture representing "combined maxval" and "argmax",
+      // shader instead of our usual N-class system.
-      // and then slice off the second channel for the category mask output,
+      if (num_outputs == 1) {
-      // using our usual channel_select program.
+        outputs.push_back(helper_.CreateDestinationTexture(
+            output_width, output_height, final_output_format));
+        helper_.BindFramebuffer(outputs.back());
+        glUseProgram(argmax_one_class_shader_.program);
+        glUniform1i(argmax_one_class_shader_.uniforms["input_texture"], 1);
+        glActiveTexture(GL_TEXTURE1);
+        // Only one chunk, and softmax cannot be applied to 1-class models
+        // anyways.
+        glBindTexture(GL_TEXTURE_2D, chunks[0].name());
+        glClear(GL_COLOR_BUFFER_BIT);
+        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+      } else {
+        // Step 3, N > 1: For CATEGORY with N classes, apply argmax shader
+        // iteratively with each chunk to get a 2-channel texture representing
+        // "combined maxval" and "argmax", and then slice off the second channel
+        // for the category mask output, using our usual channel_select program.
         glUseProgram(argmax_shader_.program);
         glUniform1i(argmax_shader_.uniforms["current_chunk"], 1);
         glUniform1i(argmax_shader_.uniforms["prev_max_texture"], 2);
@@ -702,6 +742,7 @@ SegmentationPostprocessorGl::GetSegmentationResultGpu(const Shape& input_shape,
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
         glClear(GL_COLOR_BUFFER_BIT);
         glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+      }
     } else {
       // Step 3: For CONFIDENCE, apply channel-select repeatedly to extract
       // final textures.
@@ -760,6 +801,7 @@ SegmentationPostprocessorGl::~SegmentationPostprocessorGl() {
 
     glDeleteProgram(activation_shader_.program);
     glDeleteProgram(argmax_shader_.program);
+    glDeleteProgram(argmax_one_class_shader_.program);
     glDeleteProgram(channel_select_shader_.program);
     glDeleteProgram(softmax_max_shader_.program);
     glDeleteProgram(softmax_transform_and_sum_shader_.program);

mediapipe/tasks/cc/vision/image_segmenter/calculators/segmentation_postprocessor_gl.h

@@ -63,6 +63,7 @@ class SegmentationPostprocessorGl {
 
   GlShader activation_shader_;
   GlShader argmax_shader_;
+  GlShader argmax_one_class_shader_;
   GlShader channel_select_shader_;
   GlShader softmax_max_shader_;
   GlShader softmax_transform_and_sum_shader_;