Converting float to unsigned char causes wrong values

Asked May 25 '15 at 17:01

Active May 25 '15 at 17:34

Viewed 282 times

I've created a function that creates a BMP image using RGB values.

The RGB values are stored as floats that range from 0.0 to 1.0.

When writing the values to the BMP file they need to range from 0 to 255.0 so I multiply the floats by 255.0

They also need to be unsigned chars. EDIT: Unless one of you can think of a better type.

So basically what I do is this

unsigned char pixel[3]
//BMP Expects BGR
pixel[0] = image.b*255.0;
pixel[1] = image.g*255.0;
pixel[2] = image.r*255.0;
fwrite(&pixel, 1, 3, file);

Where image.r is a float.

There seems to be some kind of loss of data in the conversion because some parts of the image are black when they shouldn't be.

The BMP image is set to 24 bits per pixel

I was going to post images but I don't have enough reputation.

Basically light blue parts become black.

If I multiply by 128 instead the image is darker but otherwise accurate. It starts getting weird around 180 ish

edited May 25 '15 at 17:34

asked May 25 '15 at 17:01

mcarlie

maybe you need 2 characters for each color, in order to support 8 bits, just like hex uses 2 digits – ericosg May 25 '15 at 17:04
1

@mcarlie If you want to add the images, you can just post them as links and a high-rep user can come along and add them for you. – Frxstrem May 25 '15 at 17:05
Alright, i've added links – mcarlie May 25 '15 at 17:18
What is `r`, please? – TonyK May 25 '15 at 17:22
A float. Ranges from 0 to 1 – mcarlie May 25 '15 at 17:23
I'm assuming this is just a typo but why are you not writing pixel to the file? fwrite(pixel, 1, 3, file); – nalyd88 May 25 '15 at 17:24
2

Have you checked that all r/g/b are between 0 and 1? What are values for pixels that turn black? – Niko May 25 '15 at 17:25
@nalyd88. Sorry, you're right. That's a typo. I tried writing 4 values instead but that doesn't work. Niko Yes, and I've also tried normalising them to make sure. I've also created functions that make sure that they are within range. – mcarlie May 25 '15 at 17:36
How did you set header of bmp file? – Jan Legner May 25 '15 at 18:04
http://stackoverflow.com/questions/2654480/writing-bmp-image-in-pure-c-c-without-other-libraries The implementation furthest down – mcarlie May 25 '15 at 18:08

0 Answers0