Transparent color should be offset by 1 for 8 bit TGA

[fafling]

Currently, the 8bits tga demo only enables transparent pixels on the Sonic sprite if its transparent color is set to 1 instead of 0.

If the bricks are drawn behind Sonic, they are completely overwritten if Sonic's transparent color is set at 0 :

The modified demo with Sonic transparent pixels working : demo - 8bits tga transparent color test.zip

This is due to 2 things in tga.c :

• jo_tga_get_pixel adds 1 to 8 bit TGA pixels.
• __jo_tga_read_contents doesn't take that offset in account when it compares transparent_color to the 8 bit pixels values returned by jo_tga_get_pixel. It should do it even if the transparent color is 0, since corresponding pixels returned by jo_tga_get_pixel are 1 in that case. jo_sprite_add_tga_tileset has the same issue.

[johannes-fetz]

This will be my next task in my todolist

[johannes-fetz]

I don't think there is an issue here. For my test I used the "8 bits TGA" demo and I put the brick in front of the Sonic image (more visual).

If we do : jo_sprite_add_tga("TEX", "BRK8.TGA", 0); // 0 means no transparency We have:

If we do : jo_sprite_add_tga("TEX", "BRK8.TGA", 1); // So the first color in the palette is transparent We have:

If we do : jo_sprite_add_tga("TEX", "BRK8.TGA", 2); // So the second color in the palette is transparent We have:

[johannes-fetz]

@fafling Hi, if you agree i will close this issue

[fafling]

Sorry but I don't agree that there's no issue.

In Sonic TGA texture, the area that surrounds Sonic uses index 0. Yet, you have to set the transparent_color parameter of jo_sprite_add_tga to 1 to make that TGA index 0 transparent. How can a user guess that transparent index has to be increased by 1 for that parameter ?

Where is it stated that "0 means no transparency" in jo_sprite_add_tga ? If that is the case, it is unintuitive to put such a flag in the transparent_color parameter, especially when 0 is the transparent index in Saturn.

Why does jo_tga_get_pixel always add 1 to 8 bit TGA pixels ? What does happen if the TGA texture uses index 255 for a non transparent color ? Does it become 0, hence being considered transparent by Saturn ?

If all this aims at allowing the use of a transparent TGA index different than 0, I think that when reading the TGA texture (and its palette), the transparent index and index 0 should be swapped, without modifying the other indexes.

[johannes-fetz]

Sorry but I don't agree that there's no issue.

In Sonic TGA texture, the area that surrounds Sonic uses index 0. Yet, you have to set the transparent_color parameter of jo_sprite_add_tga to 1 to make that TGA index 0 transparent. How can a user guess that transparent index has to be increased by 1 for that parameter ?

There are comments in examples, but I will add some info in the documentation Note: The value of JO_COLOR_Transparent is 0.

Where is it stated that "0 means no transparency" in jo_sprite_add_tga ? If that is the case, it is unintuitive to put such a flag in the transparent_color parameter, especially when 0 is the transparent index in Saturn.

From the beginning, if we put JO_COLOR_Transparent as transparent color for an image, it means that the image has no alpha component. When I start to implement 8 bits images with the same method (jo_sprite_add_tga) I used the same mecanics with JO_COLOR_Transparent, but because of its value, you have to add 1 to the index. Otherwise, I would have had to split the code in half with new methods for 8-bit images just because of that.

Why does jo_tga_get_pixel always add 1 to 8 bit TGA pixels ? What does happen if the TGA texture uses index 255 for a non transparent color ? Does it become 0, hence being considered transparent by Saturn ?

Yes, 0 means transparent by the saturn. that's why. In the image data, 1 means index 0 in the palette. You can use 255 as transparent color, but 8 bits TGA must be 255 colors max.

If all this aims at allowing the use of a transparent TGA index different than 0, I think that when reading the TGA texture (and its palette), the transparent index and index 0 should be swapped, without modifying the other indexes.

[johannes-fetz]

https://jo-engine.org/doxygen/tga_8h.html

[fafling]

As it is, that documentation is incorrect :

• "The first index of the palette is 1 not 0" : the 1st index of 8 bit TGA is 0. That index only becomes 1 when it is loaded by Jo engine.
• "0 (or JO_COLOR_Transparent) means no transparency" : it actually means that TGA index 255 is the only transparent index, since jo_tga_get_pixel transforms TGA index 255 into 0. Setting the transparent_color parameter of jo_sprite_add_tga to a value different of 0 adds a 2nd transparent TGA index which is (transparent_color - 1).
• "The palette of 8 bits TGA must be 255 colors max not 256" : maybe you should state that TGA color palette index 255 is not loaded (if I understand correctly __jo_tga_read_header), so TGA index 255 can only be used for transparency.