08226: artwork: Color overflow on disk element with some value of alpha

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
08226	Core	Minor	Always	Feb 21, 2022, 12:52	Feb 28, 2022, 12:29

Tester	hap	View Status	Public	Platform
Assigned To	hap	Resolution	Fixed	OS
Status [?]	Resolved			Driver
Version	0.240	Fixed in Version	0.242	Build
		Fixed in Git Commit	fba39f5	Github Pull Request #

Summary	08226: artwork: Color overflow on disk element with some value of alpha
Description	It appears color channels may overflow depending on its color alpha value when using the disk element in internal artwork. See screenshot and .lay file example.
Steps To Reproduce	copy merlin.zip to artwork folder mame.exe merlin
Additional Information
Github Commit

Flags
Regression Version
Affected Sets / Systems

Attached Files	merlin.zip (438 bytes) Feb 21, 2022, 12:52 Uploaded by hap
Attached Files	blue_disk.png (31,904 bytes) Feb 21, 2022, 12:53 Uploaded by hap

No.19860 hap Developer Feb 28, 2022, 10:30	class layout_element::disk_component draw(running_machine &machine, bitmap_argb32 &dest, int state) override u32 const a(c.a * 255.0F); 0.5255 = 127.5 round down to 127 u32 const r(c.r c.a * (255.0F * 255.0F * 255.0F)); 1.00.5255255255 = 8290687.5 round down to 8290687, hex = 0x7E817F u32 const inva(255 - a); 128 rendlay.cpp inline void alpha_blend(u32 &dest, u32 a, u32 r, u32 g, u32 b, u32 inva) u32 const finala((a * 255) + (da * inva)); 127255 + 0128 = 32385, hex = 0x7E81 if ((finalr / finala)>255) printf("%X = %X / %X -- %X %X %X\n",(finalr / finala), finalr, finala, a, inva, r); says: 100 = 7E817F / 7E81 -- 7F 80 7E817F palette.h rgb_t(u8, u8, u8, u8) gets a 0x100 in one of the colors and truncates to 0 thus red is gone in the merlin.lay example

No.19860

hap

Developer

Feb 28, 2022, 10:30

class layout_element::disk_component
draw(running_machine &machine, bitmap_argb32 &dest, int state) override

u32 const a(c.a * 255.0F);
0.5*255 = 127.5
round down to 127

u32 const r(c.r * c.a * (255.0F * 255.0F * 255.0F));
1.0*0.5*255*255*255 = 8290687.5
round down to 8290687, hex = 0x7E817F

u32 const inva(255 - a);
128

rendlay.cpp inline void alpha_blend(u32 &dest, u32 a, u32 r, u32 g, u32 b, u32 inva)
u32 const finala((a * 255) + (da * inva));
127*255 + 0*128 = 32385, hex = 0x7E81

if ((finalr / finala)>255) printf("%X = %X / %X -- %X %X %X\n",(finalr / finala), finalr, finala, a, inva, r);

says:
100 = 7E817F / 7E81 -- 7F 80 7E817F

palette.h rgb_t(u8, u8, u8, u8) gets a 0x100 in one of the colors and truncates to 0
thus red is gone in the merlin.lay example