[runtime_image] Share one scale_boundary() between draw() and the JPEG RGB565 tables; test uneven scales

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

esphome/components/runtime_image/image_decoder.cpp

@@ -2,7 +2,6 @@
 #include "runtime_image.h"
 #include "esphome/core/log.h"
 #include <algorithm>
-#include <cmath>
 
 namespace esphome::runtime_image {
 
@@ -11,8 +10,8 @@ ESPHOME_LOG_TAG(TAG, "image_decoder");
 bool ImageDecoder::set_size(int width, int height) {
   bool success = this->image_->resize(width, height) > 0;
   this->size_valid_ = success;
-  this->x_scale_ = static_cast<double>(this->image_->get_buffer_width()) / width;
-  this->y_scale_ = static_cast<double>(this->image_->get_buffer_height()) / height;
+  this->draw_width_ = width;
+  this->draw_height_ = height;
   return success;
 }
 
@@ -20,10 +19,24 @@ void ImageDecoder::draw(int x, int y, int w, int h, const Color &color) {
   if (!this->size_valid_) {
     return;
   }
-  auto width = std::min(this->image_->get_buffer_width(), static_cast<int>(std::ceil((x + w) * this->x_scale_)));
-  auto height = std::min(this->image_->get_buffer_height(), static_cast<int>(std::ceil((y + h) * this->y_scale_)));
-  for (int i = x * this->x_scale_; i < width; i++) {
-    for (int j = y * this->y_scale_; j < height; j++) {
+  // When shrinking, each target pixel is painted once, by the first source pixel that reaches it, rather than by every
+  // pixel that lands on it. Integer maths: the S3 has no double FPU.
+  const int buffer_width = this->image_->get_buffer_width();
+  const int buffer_height = this->image_->get_buffer_height();
+  int x1 = x;
+  int y1 = y;
+  int x2 = x + w;
+  int y2 = y + h;
+  if (buffer_width != this->draw_width_ || buffer_height != this->draw_height_) {
+    x1 = scale_boundary(x, this->draw_width_, buffer_width);
+    y1 = scale_boundary(y, this->draw_height_, buffer_height);
+    x2 = scale_boundary(x + w, this->draw_width_, buffer_width);
+    y2 = scale_boundary(y + h, this->draw_height_, buffer_height);
+  }
+  x2 = std::min(buffer_width, x2);
+  y2 = std::min(buffer_height, y2);
+  for (int i = x1; i < x2; i++) {
+    for (int j = y1; j < y2; j++) {
       this->image_->draw_pixel(i, j, color);
     }
   }

esphome/components/runtime_image/image_decoder.h

@@ -6,6 +6,15 @@
 
 namespace esphome::runtime_image {
 
+/// The first of `target` pixels that pixel n of `source` paints, rounded up: ceil(n * target / source). The spans of
+/// neighbouring pixels meet without overlapping, so a shrunk image paints each target pixel once.
+inline int scale_boundary(int n, int source, int target) {
+  if (source <= 0)
+    return 0;
+  return static_cast<int>((static_cast<uint32_t>(n) * static_cast<uint32_t>(target) + source - 1) /
+                          static_cast<uint32_t>(source));
+}
+
 enum DecodeError : int {
   DECODE_ERROR_INVALID_TYPE = -1,
   DECODE_ERROR_UNSUPPORTED_FORMAT = -2,
@@ -83,8 +92,8 @@ class ImageDecoder {
     this->expected_size_ = 0;
     this->decoded_bytes_ = 0;
     this->size_valid_ = true;
-    this->x_scale_ = 1.0;
-    this->y_scale_ = 1.0;
+    this->draw_width_ = 0;
+    this->draw_height_ = 0;
   }
 
   /**
@@ -184,8 +193,9 @@ class ImageDecoder {
   uint8_t *image_buffer_() const;
   bool image_is_big_endian_() const;
 
-  double x_scale_ = 1.0;
-  double y_scale_ = 1.0;
+  // Size of the image as the decoder draws it, which draw() spreads over the buffer
+  int draw_width_ = 0;
+  int draw_height_ = 0;
   RuntimeImage *image_;
   size_t expected_size_ = 0;  // Expected data size (0 if unknown)
   size_t decoded_bytes_ = 0;  // Bytes processed so far

esphome/components/runtime_image/jpeg_decoder.cpp

@@ -28,16 +28,14 @@ bool JpegDecoder::prepare_rgb565_scaling_(int width, int height) {
   this->source_height_ = height;
   this->x_boundaries_.clear();
   this->y_boundaries_.clear();
-  const uint32_t target_width = this->image_->get_buffer_width();
-  const uint32_t target_height = this->image_->get_buffer_height();
-  // Boundary n is the first target pixel source pixel n covers, rounded up. When shrinking, the source
-  // pixel whose span lands on a target pixel paints it, so an 8 to 4 scale keeps columns 0, 2, 4 and 6;
-  // the generic draw() path rounds down and keeps the odd ones instead
-  for (uint32_t x = 0; x <= static_cast<uint32_t>(width); x++) {
-    this->x_boundaries_.push_back(static_cast<uint16_t>((x * target_width + width - 1) / width));
+  const int target_width = this->image_->get_buffer_width();
+  const int target_height = this->image_->get_buffer_height();
+  // The same boundaries as the generic draw() path, so an 8 to 4 scale keeps columns 0, 2, 4 and 6 in both
+  for (int x = 0; x <= width; x++) {
+    this->x_boundaries_.push_back(static_cast<uint16_t>(scale_boundary(x, width, target_width)));
   }
-  for (uint32_t y = 0; y <= static_cast<uint32_t>(height); y++) {
-    this->y_boundaries_.push_back(static_cast<uint16_t>((y * target_height + height - 1) / height));
+  for (int y = 0; y <= height; y++) {
+    this->y_boundaries_.push_back(static_cast<uint16_t>(scale_boundary(y, height, target_height)));
   }
   return true;
 }
@@ -181,8 +179,8 @@ int JpegDecoder::start_() {
   }
   const int decoded_width = (width + this->scale_ - 1) / this->scale_;
   const int decoded_height = (height + this->scale_ - 1) / this->scale_;
-  this->x_scale_ = static_cast<double>(buffer_width) / decoded_width;
-  this->y_scale_ = static_cast<double>(buffer_height) / decoded_height;
+  this->draw_width_ = decoded_width;
+  this->draw_height_ = decoded_height;
   if (this->scale_ != 1) {
     ESP_LOGD(TAG, "Decoding at 1/%d size, %d x %d", this->scale_, decoded_width, decoded_height);
   }

tests/components/runtime_image/test_decoder_reuse.cpp

@@ -365,6 +365,23 @@ static std::vector<uint8_t> pixel_bytes(TestableRuntimeImage &img) {
   return std::vector<uint8_t>(start, start + img.get_width_stride() * img.get_height());
 }
 
+// Shrinking a PNG keeps the first source pixel of each span, as the JPEG path does: the even rows and columns
+TEST(RuntimeImageDecoder, ShrunkPngKeepsTheFirstPixelOfEachSpan) {
+  TestableRuntimeImage img(PNG, image::IMAGE_TYPE_RGB, false, 2, 2);
+  ASSERT_TRUE(decode_all(img, PNG_RGB, sizeof(PNG_RGB), PNG));
+  ASSERT_EQ(img.get_width(), 2);
+  ASSERT_EQ(img.get_height(), 2);
+  for (int row = 0; row < 2; row++) {
+    for (int col = 0; col < 2; col++) {
+      const Color pixel = img.get_pixel(col, row);
+      const uint8_t *expected = PNG_RGB_EXPECTED[row * 2][col * 2];
+      EXPECT_EQ(pixel.r, expected[0]) << col << "," << row;
+      EXPECT_EQ(pixel.g, expected[1]) << col << "," << row;
+      EXPECT_EQ(pixel.b, expected[2]) << col << "," << row;
+    }
+  }
+}
+
 TEST(RuntimeImageDecoder, JpegDecoderStaysWarmAcrossDecodes) {
   TestableRuntimeImage img(JPEG);
 
@@ -456,6 +473,34 @@ TEST(RuntimeImageDecoder, JpegDecodesToScaledRgb) {
 // prepare_rgb565_scaling_ rounds: source n covers targets ceil(n * target / source) up to the next start
 static uint16_t scaled_boundary(int n, int source, int target) { return (n * target + source - 1) / source; }
 
+// Every target pixel of a PNG drawn at a size that is not a whole multiple: enlarged with spans 1 or 2 pixels wide, and
+// shrunk with spans of 0, 1 or 2 pixels
+TEST(RuntimeImageDecoder, ScaledPngPaintsEachSourcePixelOverItsSpan) {
+  // A fixed size keeps the aspect ratio, so both are square
+  for (auto [width, height] : {std::pair{6, 6}, std::pair{3, 3}}) {
+    TestableRuntimeImage img(PNG, image::IMAGE_TYPE_RGB, false, width, height);
+    ASSERT_TRUE(decode_all(img, PNG_RGB, sizeof(PNG_RGB), PNG));
+    ASSERT_EQ(img.get_width(), width);
+    ASSERT_EQ(img.get_height(), height);
+    int painted = 0;
+    for (int m = 0; m < 4; m++) {
+      for (int n = 0; n < 4; n++) {
+        const uint8_t *expected = PNG_RGB_EXPECTED[m][n];
+        for (int y = scaled_boundary(m, 4, height); y < scaled_boundary(m + 1, 4, height); y++) {
+          for (int x = scaled_boundary(n, 4, width); x < scaled_boundary(n + 1, 4, width); x++) {
+            const Color pixel = img.get_pixel(x, y);
+            EXPECT_EQ(pixel.r, expected[0]) << width << "x" << height << " at " << x << "," << y;
+            EXPECT_EQ(pixel.g, expected[1]) << width << "x" << height << " at " << x << "," << y;
+            EXPECT_EQ(pixel.b, expected[2]) << width << "x" << height << " at " << x << "," << y;
+            painted++;
+          }
+        }
+      }
+    }
+    EXPECT_EQ(painted, width * height);
+  }
+}
+
 // JPEGDEC decodes at the smallest of full, 1/2, 1/4 and 1/8 size that still covers the buffer
 TEST(RuntimeImageDecoder, JpegDecodesAtTheSmallestSizeThatCoversTheBuffer) {
   for (auto [size, scale] :