Image texture pixel data conversions

[ahwatts]

I was working through the 2nd book, section 4.5, and I noticed that the rtw_image class is using stbi_loadf to load the image pixel data into floats, and converts them to unsigned chars in the convert_to_byes function by multiplying them by 256, and then in the image_texture class, it converts those unsigned chars to doubles by dividing them by 255. This seems like unnecessary work. Is there a reason it's done this way?

Also, it keeps around both copies of the pixel data, despite only using the unsigned int bdata array after construction. Is there some future direction where the book will use both fdata and bdata?

[hollasch]

• Bug? Colors are specified in linear color space #1033
• [The Next Week] Image texture is gamma compressed twice #842
• Load images in linear gamma #1450
• 6af7b47

[hollasch]

New issue #1723

[ahwatts]

I see why we're using stbi_loadf; we want the linear scale instead of the gamma-corrected scale that stbi_load would give us. But that doesn't answer why we're taking the result of stbi_loadf, which is a float with presumably the full dynamic range of the source image, and collapsing it all into 0-255, and then converting that back to a double. Couldn't we skip the convert_to_bytes step and just return the float (upcasted to a double), and then collapse to the 0-255 scale once when the final image is rendered?

(I suppose for non-HDR images the "full dynamic range" would be the same as 0-255 for each channel, just converted to a float between 0-1)

[hollasch]

Ah, I see what you're getting at. Yes, if the original image is a 16-bit or a floating-point image, then you could technical drop some precision on image load. I don't think it's worth worrying about, as most texture images will be regular 8-bit. In our specific use, we have a JPG image, which is already lossy to begin with. Given that, I think it makes sense just to keep things as they are.

Still, dropping the round-trip conversion in your own implementation doesn't sound like a bad idea at all.