Convert Pixel to Alpha

[pspballer07]

I use graphics.h for this. I do not want to open Jasc Paint Shop Pro every time I need to make the bg of an image alpha transparent. My question is:
Is there a code I can use that will convert all the pixels of a certain color to the Alpha pixel?

[Raphael]

sceGuEnable(GU_COLOR_TEST);
sceGuColorFunc(GU_NOTEQUAL,color,0xFFFFFFFF);

[SG57]

In the graphics library that comes with yeldarb's tutorials, no.

You can, however use other graphic libraries to do just that. Old School Library, for example, you do this when loading the image:

oslSetTransparentColor(RGB(255,255,255));
// load image
oslDisableTransparentColor();

That makes any image loaded with any '255,255,255' / white pixels transparent when rendered.

[flatmush]

Or you can just edit graphics.c to incorporate this, I did it when I first started it's pretty obvious. I would however advise against prolonged use of graphic.c as it is very slow.
I made an improved version of graphics.c on psp-programming which includes this function, but as I said it's very slow.

[pspballer07]

How would I edit it?

How would the RGB be defined because I have a couple different RGB macros?

[flatmush]

This is how I did it, keep in mind that some of the member variables of the struct are named differently to the graphics.c as I did a full edit, also Raphael's method is much better if you know how to implement it.

void imageReplaceColor(Color inReplace, Color inColor, Image* inImage) {
     int i, j;
     for(j = 0; j < inImage->imageHeight; j++) {
          for(i = 0; i < inImage->imageWidth; i++) {
               if((inImage->imageData[(j * inImage->imageTextureWidth) + i] & 0x00FFFFFF) == (inReplace & 0x00FFFFFF))
                    inImage->imageData[(j * inImage->imageTextureWidth) + i] = inColor;
          }
     }
}

[pspballer07]

that is very similar to my attempt except my if check was different and didn't work, thanks

[Raphael]

Originally Posted by SG57

In the graphics library that comes with yeldarb's tutorials, no.

You can, however use other graphic libraries to do just that. Old School Library, for example, you do this when loading the image:

oslSetTransparentColor(RGB(255,255,255));
// load image
oslDisableTransparentColor();

That makes any image loaded with any '255,255,255' / white pixels transparent when rendered.

That's the same as my code, only with osl naming scheme which he doesn't use. So it's not that helpfull. Also, you CAN use color testing with graphics.c, but only together with the blitAlphaImageToScreen function.

@pspballer7: Try adding my two lines after a guStart() and before drawing an image with blitAlphaImageToScreen(). You can use the GU_COLOR() or equivalent macro to set the source color.
If you want to disable color-keying, just call sceGuDisable(GU_COLOR_TES T);

[pspballer07]

one more question...what is the proper RGB macro in my case?

[PSPJunkie_]

GU_RGBA(r, g, b, a)

[pspballer07]

What would I set alpha(a) to?

[PSPJunkie_]

Fully visible red.

GU_RGBA(255, 0, 0, 255)

About half visible red.

GU_RGBA(255, 0, 0, 130)

[Raphael]

The alpha component doesn't matter, since the color test only tests the RGB channels. So you can set it to 0 or 255.

[pspballer07]

Thanks guys, it works!