image/color: when color.ModelFunc.Convert,it seems no need pre rendering #5854
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Not sure I understand the issue. Assigning to Nigel to test it out. Labels changed: added priority-soon, packagebug, removed priority-triage. Owner changed to @nigeltao. Status changed to Accepted. |
I also don't fully understand the issue. Do you think there's a problem based solely on
looking at the code, or do you have an actual PNG image file where you've ended up with
a different palette? The image/png reader code generates RGBA palette colors if they're
opaque, otherwise NRGBA colors.
For fully opaque colors, the transformation between RGBA and NRGBA should be lossless;
the program below does not print anything. For transparent colors, there is no
transformation, as we're always in NRGBA.
--------
package main
import (
"fmt"
"image/color"
)
func main() {
for r := 0; r < 256; r++ {
c0 := color.RGBA{uint8(r), 0, 0, 0xff}
c1 := color.NRGBAModel.Convert(c0).(color.NRGBA)
c2 := color.RGBAModel.Convert(c1).(color.RGBA)
if c0.R != c1.R || c0.R != c2.R {
fmt.Printf("%02x\n", c0.R)
}
}
for r := 0; r < 65536; r++ {
c0 := color.RGBA64{uint16(r), 0, 0, 0xffff}
c1 := color.NRGBA64Model.Convert(c0).(color.NRGBA64)
c2 := color.RGBA64Model.Convert(c1).(color.RGBA64)
if c0.R != c1.R || c0.R != c2.R {
fmt.Printf("%04x\n", c0.R)
}
}
}
--------
Status changed to WaitingForReply. |
Who did you send that mail to? I don't think that I got it. Can you point your browser to https://golang.org/issue/5854 and upload that sample to the issue page instead? |
I replied from my gmail to the account named go@googlecode.com.This is my first time use googlecode,I thought it could to the correct address. there are "sample.png" and "result.png". you can find the different form "different.jpg" easily. By the way,sample.png is a illeag file,in my country,it not a big matter,but maybe you must delete it...(becouse I can't find another fit image in my harddisk,after all we use RGBA PNG or jpeg in daily life,and these format can't trigger this bug) Attachments:
|
Both sample.png and result.png have the same PLTE chunk. The difference is that the tRNS chunk is 256 bytes long for sample.png and 254 bytes long for result.png. The final two tRNS bytes for sample.png are 0xff and 0xff. The PNG spec at http://www.w3.org/TR/PNG/#11tRNS says that "The tRNS chunk shall not contain more alpha values than there are palette entries, but a tRNS chunk may contain fewer values than there are palette entries. In this case, the alpha value for all remaining palette entries is assumed to be 255." It seems to me that Go's image/png package is writing a valid (shorter) tRNS chunk but your viewing program (I don't know what it is called but it is what's seen in different.jpg) is not following the PNG specification. The bug lies in that program, not in Go. Status changed to WontFix. |
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by augustheart:
I found these code at "func nrgbaModel(c Color) Color" and "func nrgba64Model(c Color) Color" in "image/color" " // Since Color.RGBA returns a alpha-premultiplied color, we should have r <= a && g <= a && b <= a. r = (r * 0xffff) / a g = (g * 0xffff) / a b = (b * 0xffff) / a " It seems no need for precess a image.And when we process a paletted png,these code will distory the palette. example: f,_ := os.Open("sample.png") defer f.Close() img,_ := png.Decode(f) out,_ := os.Create("result.png") defer out.Close() png.Encode(out,img) If the sample.png is a paletted png with alpha,these code will generate a different pLTEThe text was updated successfully, but these errors were encountered: