+
+static void analyze_image(image_info &imageInfo, png_colorp rgbPalette, png_bytep alphaPalette,
+ int *paletteEntries, bool *hasTransparency, int *colorType,
+ png_bytepp outRows)
+{
+ int w = imageInfo.width;
+ int h = imageInfo.height;
+ bool trans = imageInfo.hasTransparency;
+ int i, j, rr, gg, bb, aa, idx;
+ uint32_t colors[256], col;
+ int num_colors = 0;
+
+ bool isOpaque = true;
+ bool isPalette = true;
+ bool isGrayscale = true;
+
+ // Scan the entire image and determine if:
+ // 1. Every pixel has R == G == B (grayscale)
+ // 2. Every pixel has A == 255 (opaque)
+ // 3. There are no more than 256 distinct RGBA colors
+ for (j = 0; j < h; j++) {
+ png_bytep row = imageInfo.rows[j];
+ png_bytep out = outRows[j];
+ for (i = 0; i < w; i++) {
+ rr = *row++;
+ gg = *row++;
+ bb = *row++;
+ aa = *row++;
+ if (!trans) {
+ // Ignore the actually byte and assume alpha == 255
+ aa = 0xff;
+ }
+
+ // Check if image is really grayscale
+ if (isGrayscale) {
+ if (rr != gg || rr != bb) {
+ isGrayscale = false;
+ }
+ }
+
+ // Check if image is really opaque
+ if (isOpaque) {
+ if (aa != 0xff) {
+ isOpaque = false;
+ }
+ }
+
+ // Check if image is really <= 256 colors
+ if (isPalette) {
+ col = (uint32_t) ((rr << 24) | (gg << 16) | (bb << 8) | aa);
+ bool match = false;
+ for (idx = 0; idx < num_colors; idx++) {
+ if (colors[idx] == col) {
+ match = true;
+ break;
+ }
+ }
+
+ // Write the palette index for the pixel to outRows optimistically
+ // We might overwrite it later if we decide to encode as gray or
+ // gray + alpha
+ *out++ = idx;
+ if (!match) {
+ if (num_colors == 256) {
+ isPalette = false;
+ } else {
+ colors[num_colors++] = col;
+ }
+ }
+ }
+ }
+ }
+
+ *paletteEntries = 0;
+ *hasTransparency = !isOpaque;
+ int bpp = isOpaque ? 3 : 4;
+ int paletteSize = w * h + bpp * num_colors;
+
+ // Choose the best color type for the image.
+ // 1. Opaque gray - use COLOR_TYPE_GRAY at 1 byte/pixel
+ // 2. Gray + alpha - use COLOR_TYPE_PALETTE if the number of distinct combinations
+ // is sufficiently small, otherwise use COLOR_TYPE_GRAY_ALPHA
+ // 3. RGB(A) - use COLOR_TYPE_PALETTE if the number of distinct colors is sufficiently
+ // small, otherwise use COLOR_TYPE_RGB{_ALPHA}
+ if (isGrayscale) {
+ if (isOpaque) {
+ *colorType = PNG_COLOR_TYPE_GRAY; // 1 byte/pixel
+ } else {
+ // Use a simple heuristic to determine whether using a palette will
+ // save space versus using gray + alpha for each pixel.
+ // This doesn't take into account chunk overhead, filtering, LZ
+ // compression, etc.
+ if (isPalette && (paletteSize < 2 * w * h)) {
+ *colorType = PNG_COLOR_TYPE_PALETTE; // 1 byte/pixel + 4 bytes/color
+ } else {
+ *colorType = PNG_COLOR_TYPE_GRAY_ALPHA; // 2 bytes per pixel
+ }
+ }
+ } else if (isPalette && (paletteSize < bpp * w * h)) {
+ *colorType = PNG_COLOR_TYPE_PALETTE;
+ } else {
+ *colorType = isOpaque ? PNG_COLOR_TYPE_RGB : PNG_COLOR_TYPE_RGB_ALPHA;
+ }
+
+ // Perform postprocessing of the image or palette data based on the final
+ // color type chosen
+
+ if (*colorType == PNG_COLOR_TYPE_PALETTE) {
+ // Create separate RGB and Alpha palettes and set the number of colors
+ *paletteEntries = num_colors;
+
+ // Create the RGB and alpha palettes
+ for (int idx = 0; idx < num_colors; idx++) {
+ col = colors[idx];
+ rgbPalette[idx].red = (png_byte) ((col >> 24) & 0xff);
+ rgbPalette[idx].green = (png_byte) ((col >> 16) & 0xff);
+ rgbPalette[idx].blue = (png_byte) ((col >> 8) & 0xff);
+ alphaPalette[idx] = (png_byte) (col & 0xff);
+ }
+ } else if (*colorType == PNG_COLOR_TYPE_GRAY || *colorType == PNG_COLOR_TYPE_GRAY_ALPHA) {
+ // If the image is gray or gray + alpha, compact the pixels into outRows
+ for (j = 0; j < h; j++) {
+ png_bytep row = imageInfo.rows[j];
+ png_bytep out = outRows[j];
+ for (i = 0; i < w; i++) {
+ rr = *row++;
+ gg = *row++;
+ bb = *row++;
+ aa = *row++;
+
+ *out++ = rr;
+ if (!isOpaque) {
+ *out++ = aa;
+ }
+ }
+ }
+ }
+}
+
+
projects / android / aapt.git / commitdiff
