Yes, that explains it. The simple summary is that I forgot the r,g,b,a are 16bit and was working with them as 8bit. This is why the image.RGBA worked, but the png decoded image did not - image.RGBA returned c|c<<8 for each channel, so the error was masked by having the low byte mimic the high byte.
thanks On Tue, 2016-06-14 at 11:15 +1000, Nigel Tao wrote: > On Mon, Jun 13, 2016 at 9:10 PM, Dan Kortschak > <dan.kortsc...@adelaide.edu.au> wrote: > > Though doing the direct round trip of an image through an RGB565 gets > > back the pixels in a state that you would expect, shown here in the > > playground snippet: > > > > https://play.golang.org/p/UCwE4YJk21 > > Here's your color model function: > > func rgb565Model(c color.Color) color.Color { > if _, ok := c.(Pixel565); ok { > return c > } > r, g, b, _ := c.RGBA() > r >>= (bytewid - rwid) > g >>= (bytewid - gwid) > b >>= (bytewid - bwid) > return Pixel565((r&rmask)<<roff | (g&gmask)<<goff | b&bmask) > } > > The > r, g, b, _ := c.RGBA() > line gives a 16-bit red value, in the range [0x0000, 0xffff]. The > value's type is uint32, but the effective range is 16 bits. (See the > "three important subtleties" paragraph at > https://blog.golang.org/go-image-package). The next line is: > r >>= (bytewid - rwid) > or equivalently, > r >>= (8 - 5) > so that r is effectively an 13 bit value. The final line says: > r&rmask > but rmask is 0x1f, so you're masking off all but the low 5 bytes, > instead of the high 5 bytes. I suspect that it suffices to replace > r >>= (bytewid - rwid) > with > r >>= (8 + bytewid - rwid) > and the same for g and b, obviously. > > Here's a quick test: > fmt.Printf("0x%04x\n", RGB565Model.Convert(color.RGBA64{ > R: 0x0000, > G: 0x0000, > B: 0x8000, > A: 0xffff, > })) > should print a Pixel565 value that's 50-ish% blue: 0x0010 instead of 0x0000. -- Omnes mundum facimus. Dan Kortschak <dan.kortsc...@adelaide.edu.au> F9B3 3810 C4DD E214 347C B8DA D879 B7A7 EECC 5A40 10C7 EEF4 A467 89C9 CA00 70DF C18F 3421 A744 607C -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
