Bug on face drawing code in lesson 1

[tristancalderbank]

Please correct if this is wrong here but I think there's a bug in the lesson one code when we draw the obj mesh.

Specifically here:

tinyrenderer/main.cpp

Line 49 in f6fecb7

int x0 = (v0.x+1.)*width/2.;

If a vertex has a x/y value equal to 1.0 or -1.0, then x or y becomes equal to width/height.

The problem is the data variable in TGAImage has a max width/height of (width - 1)/(height - 1) because its zero indexed.

What is hiding this bug is the fact that we just exit TGAImage::set if the x/y is out of bounds:

tinyrenderer/tgaimage.cpp

Line 252 in f6fecb7

if (!data || x<0 || y<0 || x>=width || y>=height) {

I think we're effectively cutting off the top/right edge of the image in the current lesson 1 code.

I think the solution here would be to do something like this?

int x0 = (v0.x+1.)*(width - 1)/2.;
int y0 = (v0.y+1.)*(height - 1)/2.;
int x1 = (v1.x+1.)*(width - 1)/2.;
int y1 = (v1.y+1.)*(height - 1)/2.;

Happened to notice this because I implemented a difference image drawing class (for BMP instead of TGA), and I didn't have the bounds check in the set method.

As an easier way to confirm this, just comment out the bounds check code in TGAImage::set and you will get an out of bounds exception.

[ssloy]

Well, i would not call it a bug, it depends really what you want to do (and what part of the pixel corresponds to a given coordinage)...

[tristancalderbank]

Ok I think I see what you mean.

I think we're saying that for example 1.0 as an x value in obj coordinates corresponds to the right side of the right-most pixel in image coordinates which seems logical to not draw.