Converting Jpeg images to Bmp - some images come out blue

Question

There are some Jpg images which Delphi doesn\'t seem to like. It appears to be specific with the files I\'m loading. And the procedure is simple - a) load Jpg image to

Answer 1

I figured out the issue. It's most likely a bug in Delphi.

The provided image is a peculiar format for a JPEG file called Adobe JPEG. Probably the most peculiar thing about an Adobe JPEG is that it allows storing the image in RGB format, though it also allows other formats. Most JPEGs are JFIF or EXIF format, which do not use RGB.

When copying the RGB data, whatever Delphi's doing, it's reversing the red and blue data when it's loading it onto the canvas. It's loading it as BGR instead of RGB. This may be because Windows (24-bit and 32-bit) DIBs (BMPs) are stored in BGR format.

I'm guessing that the bug will appear in Delphi for any RGB JPEG. Since most JPEGs do not use RGB, the incidence of the bug is low. The easy fix, if you have the source to the JPEG unit, is to reverse the order when loading an RGB JPEG.

If you don't have the source, then continue on.

The Adobe JPEG specifies the order of the colors in a format like this (in Hex) 43 11 00 47 11 00 42 11 00 that looks like this in a hex editor R..G..B. If you reverse the R and B here via a Hex editor, it shows wrong in Windows, and right in Delphi.

To recognize an Adobe JPEG, the first four bytes are either (in Hex) FF D8 FF ED or FF D8 FF EE, with the ED and EE being the differentiating bytes. All JPEG files start with FF D8 FF.

After those bytes are two bytes that represent the length of the type marker, followed by (In ASCII) Adobe, followed by six more bytes (representing the version, etc.) and finally, (the 18th byte) is the byte that specifies the format. 0 means RGB. So, check for those significant bytes, and then act accordingly.

You'll have to reverse the RGB order in the file header (to lie to Delphi), or copy it to a TBitmap and use ScanLine to reverse the RGB to the proper order.

The format details are from reading the libJPEG source in C.