256 color PNG doesn't save as 8 bit per pixel file

Melonbread · March 8, 2025, 7:38pm

I’ve got Color Mode set to Indexed and my palette size is 256 colors. However, when I save as PNG or Export as PNG, the resultant file is saved as a 32 bit per pixel (32bpp) image … when I need it to be an 8 bit per pixel (8bpp) image. Any ideas? Thanks

Ethan_Buttazzi · March 8, 2025, 9:54pm

its 8 for red, 8 for green, 8 for blue, and since png supports transparancy so its 8 more for alpha, aka transparency which equals to 32bpp, bitmap might work but you will have to do further research i think.

behreandtjeremy · March 9, 2025, 4:55am

If this is still going on, providing more details might help. For example,

Aseprite version
export settings
sprite transparent color (index in the palette and RGBA channels)
whether the palette contains transparent and/or translucent color swatches
whether the sprite has a background layer
whether cel or layer opacity have been set to values other than 255 (100%)
whether layer blend modes other than “normal” are used

A screen shot or two of the entire Aseprite window can provide this info at a glance.

From what I could find so far, the Export for Twitter setting will change the png format to RGBA even when the sprite color mode is indexed. However, this only happened when I exported, not when I saved. So maybe there’s more than one way the format can be changed.

In the bottom left corner, the eye dropper shows that the alpha for white has been set to 254. The palette has been shortened by a few swatches.

Below are some links in the source code where I found this out:

github.com/aseprite/aseprite

src/app/file/png_format.h

main


      
          //
          // This program is distributed under the terms of
          // the End-User License Agreement for Aseprite.
          
          #ifndef APP_FILE_PNG_FORMAT_H_INCLUDED
          #define APP_FILE_PNG_FORMAT_H_INCLUDED
          #pragma once
          
          namespace app {
          
          // This can be used to save opaque png files with one pixel with
          // alpha=254, which is needed by Twitter to publish png images and
          // avoid automatic conversion to jpg files.
          class PngEncoderOneAlphaPixel {
          public:
            PngEncoderOneAlphaPixel(bool state);
            ~PngEncoderOneAlphaPixel();
          };
          
          } // namespace app

github.com/aseprite/aseprite

src/app/file/png_format.cpp

main


      
          
          switch (spec.colorMode()) {
            case ColorMode::RGB:
              color_type = (img->needAlpha() || fix_one_alpha_pixel ? PNG_COLOR_TYPE_RGB_ALPHA :
                                                                      PNG_COLOR_TYPE_RGB);
              break;
            case ColorMode::GRAYSCALE:
              color_type = (img->needAlpha() || fix_one_alpha_pixel ? PNG_COLOR_TYPE_GRAY_ALPHA :
                                                                      PNG_COLOR_TYPE_GRAY);
              break;
            case ColorMode::INDEXED:
              if (fix_one_alpha_pixel)
                color_type = PNG_COLOR_TYPE_RGB_ALPHA;
              else
                color_type = PNG_COLOR_TYPE_PALETTE;
              break;
          }
          
          const png_uint_32 width = spec.width();
          const png_uint_32 height = spec.height();

github.com/aseprite/aseprite

src/app/commands/cmd_save_file.cpp

main


      
            m_adjustFramesByTag = false;
          
            // Set other parameters
            params().aniDir(aniDirValue);
            if (!bounds.isEmpty())
              params().bounds(bounds);
            params().playSubtags(isPlaySubtags);
          
            // TODO This should be set as options for the specific encoder
            GifEncoderDurationFix fixGif(isForTwitter);
            PngEncoderOneAlphaPixel fixPng(isForTwitter);
          
            saveDocumentInBackground(context,
                                     doc,
                                     outputFilename,
                                     MarkAsSaved::Off,
                                     (resizeOnTheFly ? ResizeOnTheFly::On : ResizeOnTheFly::Off),
                                     scaleXY);
          }
          
          // Undo resize

Melonbread · March 9, 2025, 10:22am

Many thanks for your detailed reply.

It was (I think) a transparency issue so thanks to both. The issue was arising because I was using a 32 bit PNG as a base, then attempting to reduce the colour depth by changing colour mode to indexed, removing transparency etc, then Exporting or Save As.

I solved this by creating a new (blank) indexed image in Aseprite, then cutting and pasting the existing 32 bit PNG into that new image. Exporting or saving as PNG then worked as expected (8bpp rather than 32bpp).

Thanks again.