Then, there is the header that specifies the chunk type, which is also 4 bytes in length. With PNG, however, the stored gamma information is used to "correct" the RGB values prior to displaying the image, which means that they won't match other design elements when viewed in certain browsers. One way to make PNGs match other design elements, for a specific browser, is to apply an artificial gamma offset using a Levels adjustment. The easiest way is to use a MemoryStream. There's a lot here to deal with the length of the chunks. Cases where no edges are visible (using the corner guides as a reference) are considered "correct" for the browser within which this page is viewed. The standard at all the places I've worked is that all textures (except "data textures", like normal maps) are authored and stored in SRGB (~gamma 2.2). A png-compliant viewer is supposed to use the gamma value from the chunk, combined with the gamma of the display, to write the pixel intensity values properly on the display. gAMA 0.45455 cHRM White x = 0.3127 y = 0.329, Red x = 0.64 y = 0.33 Green x = 0.3 y = 0.6, Blue x = 0.15 y = 0.06 Both versions have the same pixel values stored in the file. IE Gamma corrects the PNG image when the information is available. This includes standalone graphics, as well as those with transparent or alpha-blended backgrounds. Note: Due to a bug in MacOS 10.3, Safari 1.3.x is incapable of displaying PNGs correctly, regardless of which ancillary chunks are present/removed. This is the length of the data field. Now, we come to the main part of the code. Slightly more accurate gamma correction. Or, if we find a gAMA, jumping it and writing out the rest of the bytes. Each chunk has four fields: uint32 length in big-endian. This is because PNG stores values in network byte order, or Big Endian. There shouldn't really be any surprises in Table 2. 4.1.1. 4. The lower case letter specifies that this is an ancillary chunk and is completely optional. The only thing to note at this point is that offset is already at eight because we've already dealt with the signature. Although it does it incorrectly, at least it is trying. If the image height is 2, the IDAT chunk contains 2 scanlines. png_set_gAMA() causes libpng to write a gAMA chunk in the output PNG file. Note: When designing images for the Web, it's a good idea to use sRGB as the working color space and a display-system gamma of 2.2. However, when both the sRGB and gAMA chunks are present, Internet Explorer uses a gamma of approximately 1.93, while Safari uses a gamma of 1.8 — regardless of the actual gamma — making the image appear slightly dark (as demonstrated in Method 1, above). This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL). I like png, but I don't like its gama ray that keeps pulling my hair off. Contrary to the CSS2 specification , which states that CSS color values refer to the sRGB color space , the majority of browsers simply display CSS colors using the system color space. Notes. Chunk struct There are however programs out there which ignore the gAMA chunk, so I tried to produce a PNG file with the usual gamma correction (1 / 2.2). Then, I've created a new stream and written out the first 8 bytes to it. chunks ¶ The first thing we have to do then is get our PNG image as bytes so we can mess with them. Given a Raw chunk, just rad, fills this chunk content override void CloneDataFromRead (PngChunk other) Override to make a copy (normally deep) from other chunk double GetGamma void SetGamma (double gamma) For browsers where Method 1 doesn't work (e.g., Safari 1.3), the only other recourse seems to be Method 2. 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